1.
Particle
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A particle is a minute fragment or quantity of matter. In the physical sciences, a particle is a small localized object to which can be ascribed several physical or chemical properties such as volume or mass. Particles can also be used to create models of even larger objects depending on their density. The term particle is rather general in meaning, and is refined as needed by various scientific fields, something that is composed of particles may be referred to as being particulate. However, the particulate is most frequently used to refer to pollutants in the Earths atmosphere. The concept of particles is particularly useful when modelling nature, as the treatment of many phenomena can be complex. It can be used to make simplifying assumptions concerning the processes involved, francis Sears and Mark Zemansky, in University Physics, give the example of calculating the landing location and speed of a baseball thrown in the air. The treatment of large numbers of particles is the realm of statistical physics, the term particle is usually applied differently to three classes of sizes. The term macroscopic particle, usually refers to particles much larger than atoms and these are usually abstracted as point-like particles, or even invisible. This is even though they have volumes, shapes, structures, examples of macroscopic particles would include powder, dust, sand, pieces of debris during a car accident, or even objects as big as the stars of a galaxy. Another type, microscopic particles usually refers to particles of sizes ranging from atoms to molecules, such as carbon dioxide, nanoparticles and these particles are studied in chemistry, as well as atomic and molecular physics. The smallest of particles are the particles, which refer to particles smaller than atoms. These particles are studied in particle physics, because of their extremely small size, the study of microscopic and subatomic particles fall in the realm of quantum mechanics. Particles can also be classified according to composition, composite particles refer to particles that have composition – that is particles which are made of other particles. For example, an atom is made of six protons, eight neutrons. By contrast, elementary particles refer to particles that are not made of other particles, according to our current understanding of the world, only a very small number of these exist, such as the leptons, quarks or gluons. However it is possible some of these might turn up to be composite particles after all. While composite particles can very often be considered point-like, elementary particles are truly punctual, both elementary and composite particles, are known to undergo particle decay
2.
Elementary particle
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In particle physics, an elementary particle or fundamental particle is a particle whose substructure is unknown, thus, it is unknown whether it is composed of other particles. A particle containing two or more elementary particles is a composite particle, soon, subatomic constituents of the atom were identified. As the 1930s opened, the electron and the proton had been observed, along with the photon, via quantum theory, protons and neutrons were found to contain quarks—up quarks and down quarks—now considered elementary particles. And within a molecule, the three degrees of freedom can separate via wavefunction into three quasiparticles. Yet a free electron—which, not orbiting a nucleus, lacks orbital motion—appears unsplittable. Meanwhile, an elementary boson mediating gravitation—the graviton—remains hypothetical, all elementary particles are—depending on their spin—either bosons or fermions. These are differentiated via the theorem of quantum statistics. Particles of half-integer spin exhibit Fermi–Dirac statistics and are fermions, Particles of integer spin, in other words full-integer, exhibit Bose–Einstein statistics and are bosons. In the Standard Model, elementary particles are represented for predictive utility as point particles, though extremely successful, the Standard Model is limited to the microcosm by its omission of gravitation and has some parameters arbitrarily added but unexplained. According to the current models of big bang nucleosynthesis, the composition of visible matter of the universe should be about 75% hydrogen. Neutrons are made up of one up and two down quark, while protons are made of two up and one down quark. Since the other elementary particles are so light or so rare when compared to atomic nuclei. Therefore, one can conclude that most of the mass of the universe consists of protons and neutrons. Some estimates imply that there are roughly 1080 baryons in the observable universe, the number of protons in the observable universe is called the Eddington number. Other estimates imply that roughly 1097 elementary particles exist in the universe, mostly photons, gravitons. However, the Standard Model is widely considered to be a theory rather than a truly fundamental one. The 12 fundamental fermionic flavours are divided into three generations of four particles each, six of the particles are quarks. The remaining six are leptons, three of which are neutrinos, and the three of which have an electric charge of −1, the electron and its two cousins, the muon and the tau
3.
Particle statistics
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Particle statistics is a particular description of multiple particles in statistical mechanics. Its core concept is an ensemble that emphasizes properties of a large system as a whole at the expense of knowledge about parameters of separate particles. When an ensemble consists of particles with similar properties, their number is called the particle number, in classical mechanics, all particles in the system are considered distinguishable. This means that particles in a system can be tracked. As a consequence, changing the position of any two particles in the leads to a completely different configuration of the entire system. Furthermore, there is no restriction on placing more than one particle in any given state accessible to the system and these characteristics of classical positions are called Maxwell–Boltzmann statistics. The fundamental feature of quantum mechanics that distinguishes it from classical mechanics is that particles of a type are indistinguishable from one another. This means that in an assembly consisting of particles, interchanging any two particles does not lead to a new configuration of the system. In the case of a system consisting of particles of different kinds, all quantum particles, such as leptons and baryons, in the universe have three translational motion degrees of freedom and one discrete degree of freedom, known as spin. Thats why quantum statistics is useful when one considers, say, helium liquid or ammonia gas, the spin–statistics theorem binds two particular kinds of combinatorial symmetry with two particular kinds of spin symmetry, namely bosons and fermions. In Bose–Einstein statistics interchanging any two particles of the leaves the resultant system in a symmetric state. That is, the function of the system before interchanging equals the wave function of the system after interchanging. It is important to emphasize that the function of the system has not changed itself. This has very important consequences on the state of the system and it is found that the particles that obey Bose–Einstein statistics are the ones which have integer spins, which are therefore called bosons. Examples of bosons include photons and helium-4, one type of system obeying B–E statistics is the Bose–Einstein condensate where all particles of the assembly exist in the same state. In Fermi–Dirac statistics interchanging any two particles of the leaves the resultant system in an antisymmetric state. That is, the function of the system before interchanging is the wave function of the system after interchanging. Again, the function of the system itself does not change
4.
Fermion
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In particle physics, a fermion is any subatomic particle characterized by Fermi–Dirac statistics. These particles obey the Pauli exclusion principle, fermions include all quarks and leptons, as well as any composite particle made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose–Einstein statistics, a fermion can be an elementary particle, such as the electron, or it can be a composite particle, such as the proton. According to the theorem in any reasonable relativistic quantum field theory, particles with integer spin are bosons. Besides this spin characteristic, fermions have another specific property, they possess conserved baryon or lepton quantum numbers, therefore, what is usually referred to as the spin statistics relation is in fact a spin statistics-quantum number relation. As a consequence of the Pauli exclusion principle, only one fermion can occupy a quantum state at any given time. If multiple fermions have the same probability distribution, then at least one property of each fermion, such as its spin. Weakly interacting fermions can also display bosonic behavior under extreme conditions, at low temperature fermions show superfluidity for uncharged particles and superconductivity for charged particles. Composite fermions, such as protons and neutrons, are the key building blocks of everyday matter, the Standard Model recognizes two types of elementary fermions, quarks and leptons. In all, the model distinguishes 24 different fermions, there are six quarks, and six leptons, along with the corresponding antiparticle of each of these. Mathematically, fermions come in three types - Weyl fermions, Dirac fermions, and Majorana fermions, most Standard Model fermions are believed to be Dirac fermions, although it is unknown at this time whether the neutrinos are Dirac or Majorana fermions. Dirac fermions can be treated as a combination of two Weyl fermions, in July 2015, Weyl fermions have been experimentally realized in Weyl semimetals. Composite particles can be bosons or fermions depending on their constituents, more precisely, because of the relation between spin and statistics, a particle containing an odd number of fermions is itself a fermion. Examples include the following, A baryon, such as the proton or neutron, the nucleus of a carbon-13 atom contains six protons and seven neutrons and is therefore a fermion. The atom helium-3 is made of two protons, one neutron, and two electrons, and therefore it is a fermion. The number of bosons within a composite made up of simple particles bound with a potential has no effect on whether it is a boson or a fermion. Fermionic or bosonic behavior of a particle is only seen at large distances. At proximity, where spatial structure begins to be important, a composite particle behaves according to its constituent makeup, fermions can exhibit bosonic behavior when they become loosely bound in pairs
5.
Fundamental interaction
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In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four conventionally accepted fundamental interactions—gravitational, electromagnetic, strong, each one is described mathematically as a field. The gravitational force is modelled as a classical field. The other three, part of the Standard Model of particle physics, are described as discrete quantum fields, and their interactions are carried by a quantum. The strong and weak interactions have short ranges, producing forces at minuscule, subatomic distances, the strong interaction, which is carried by the gluon particle, is responsible for the binding of quarks together to form hadrons, such as protons and neutrons. As a residual effect, it creates the force that binds the latter particles to form atomic nuclei. The weak interaction, which is carried by the W and Z particles, also acts on the nucleus, the other two, electromagnetism and gravity, produce significant forces at macroscopic scales where the effects can be seen directly in everyday life. The electromagnetic force, carried by the photon, creates electric and magnetic fields, Electromagnetic forces tend to cancel each other out when large collections of objects are considered, so over the largest distances, gravity tends to be the dominant force. Other theorists seek to unite the electroweak and strong fields within a Grand Unified Theory, some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything. A few researchers have interpreted various anomalous observations in physics as evidence for a fifth force, inferring that all objects bearing mass approach at a constant rate, but collide by impact proportional to their masses, Newton inferred that matter exhibits an attractive force. Thus Newtons theory violated the first principle of mechanical philosophy, as stated by Descartes, conversely, during the 1820s, when explaining magnetism, Michael Faraday inferred a field filling space and transmitting that force. Faraday conjectured that ultimately, all unified into one. In the early 1870s, James Clerk Maxwell unified electricity and magnetism as effects of a field whose third consequence was light. The Standard Model of particle physics was developed throughout the half of the 20th century. In the Standard Model, the electromagnetic, strong, and weak interactions associate with elementary particles, for predictive success with QMs probabilistic outcomes, particle physics conventionally models QM events across a field set to special relativity, altogether relativistic quantum field theory. Force particles, called gauge bosons—force carriers or messenger particles of underlying fields—interact with matter particles, everyday matter is atoms, composed of three fermion types, up-quarks and down-quarks constituting, as well as electrons orbiting, the atoms nucleus. The electromagnetic interaction was modelled with the interaction, whose force carriers are W and Z bosons, traversing the minuscule distance. Electroweak interaction would operate at high temperatures as soon after the presumed Big Bang
6.
Invariant mass
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More precisely, it is a characteristic of the systems total energy and momentum that is the same in all frames of reference related by Lorentz transformations. If a center of momentum frame exists for the system, then the invariant mass of a system is equal to its mass in that rest frame. In other reference frames, where the momentum is nonzero, the total mass of the system is greater than the invariant mass. Due to mass-energy equivalence, the rest energy of the system is simply the invariant mass times the speed of light squared, similarly, the total energy of the system is its total mass times the speed of light squared. Systems whose four-momentum is a null vector have zero invariant mass, a physical object or particle moving faster than the speed of light would have space-like four-momenta, and these do not appear to exist. Any time-like four-momentum possesses a frame where the momentum is zero. In this case, invariant mass is positive and is referred to as the rest mass, if objects within a system are in relative motion, then the invariant mass of the whole system will differ from the sum of the objects rest masses. This is also equal to the energy of the system divided by c2. See mass–energy equivalence for a discussion of definitions of mass, for example, a scale would measure the kinetic energy of the molecules in a bottle of gas to be part of invariant mass of the bottle, and thus also its rest mass. The same is true for massless particles in such system, which add invariant mass and also rest mass to systems, for an isolated massive system, the center of mass of the system moves in a straight line with a steady sub-luminal velocity. Thus, an observer can always be placed to move along with it. In this frame, which is the center of momentum frame, the momentum is zero. In this frame, which exists under these assumptions, the invariant mass of the system is equal to the system energy divided by c2. This total energy in the center of momentum frame, is the energy which the system may be observed to have. Note that for reasons above, such a rest frame does not exist for single photons, when two or more photons move in different directions, however, a center of mass frame exists. Thus, the mass of a system of several photons moving in different directions is positive, for example, rest mass and invariant mass are zero for individual photons even though they may add mass to the invariant mass of systems. For this reason, invariant mass is in not an additive quantity. Consider the simple case of system, where object A is moving towards another object B which is initially at rest
7.
Electric charge
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Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of charges, positive and negative. Like charges repel and unlike attract, an absence of net charge is referred to as neutral. An object is charged if it has an excess of electrons. The SI derived unit of charge is the coulomb. In electrical engineering, it is common to use the ampere-hour. The symbol Q often denotes charge, early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that dont require consideration of quantum effects. The electric charge is a conserved property of some subatomic particles. Electrically charged matter is influenced by, and produces, electromagnetic fields, the interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces. 602×10−19 coulombs. The proton has a charge of +e, and the electron has a charge of −e, the study of charged particles, and how their interactions are mediated by photons, is called quantum electrodynamics. Charge is the property of forms of matter that exhibit electrostatic attraction or repulsion in the presence of other matter. Electric charge is a property of many subatomic particles. The charges of free-standing particles are integer multiples of the charge e. Michael Faraday, in his electrolysis experiments, was the first to note the discrete nature of electric charge, robert Millikans oil drop experiment demonstrated this fact directly, and measured the elementary charge. By convention, the charge of an electron is −1, while that of a proton is +1, charged particles whose charges have the same sign repel one another, and particles whose charges have different signs attract. The charge of an antiparticle equals that of the corresponding particle, quarks have fractional charges of either −1/3 or +2/3, but free-standing quarks have never been observed. The electric charge of an object is the sum of the electric charges of the particles that make it up. An ion is an atom that has lost one or more electrons, giving it a net charge, or that has gained one or more electrons
8.
Color charge
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Color charge is a property of quarks and gluons that is related to the particles strong interactions in the theory of quantum chromodynamics. The color charge of quarks and gluons is completely unrelated to visual perception of color, another color scheme is red, yellow, and blue, using paint as the perceptible analogy. Particle physicists call these antired, antigreen, and antiblue, all three colors mixed together, or any one of these colors and its complement, is colorless or white and has a net color charge of zero. This color charge differs from electromagnetic charges since electromagnetic charges have only one kind of value, positive and negative electrical charges are the same kind of charge as they only differ by the sign. The theory of quantum chromodynamics has been under development since the 1970s, in quantum chromodynamics, a quarks color can take one of three values or charges, red, green, and blue. An antiquark can take one of three anticolors, called antired, antigreen, and antiblue, gluons are mixtures of two colors, such as red and antigreen, which constitutes their color charge. QCD considers eight gluons of the possible nine color–anticolor combinations to be unique, however, the color field lines do not arc outwards from one charge to another as much, because they are pulled together tightly by gluons. This effect confines quarks within hadrons, in a quantum field theory, a coupling constant and a charge are different but related notions. The coupling constant sets the magnitude of the force of interaction, for example, in quantum electrodynamics, the charge in a gauge theory has to do with the way a particle transforms under the gauge symmetry, i. e. its representation under the gauge group. For example, the electron has charge −1 and the positron has charge +1, since QCD is a non-abelian theory, the representations, and hence the color charges, are more complicated. They are dealt with in the next section, in QCD the gauge group is the non-abelian group SU. The running coupling is usually denoted by αs, each flavor of quark belongs to the fundamental representation and contains a triplet of fields together denoted by ψ. The antiquark field belongs to the conjugate representation and also contains a triplet of fields. We can write ψ = and ψ ¯ =, the gluon contains an octet of fields, and belongs to the adjoint representation, and can be written using the Gell-Mann matrices as A μ = A μ a λ a. All other particles belong to the representation of color SU. The color charge of each of these fields is fully specified by the representations, quarks have a color charge of red, green or blue and antiquarks have a color charge of antired, antigreen or antiblue. Gluons have a combination of two charges in a superposition of states which are given by the Gell-Mann matrices. All other particles have zero color charge, mathematically speaking, the color charge of a particle is the value of a certain quadratic Casimir operator in the representation of the particle
9.
Spin (physics)
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In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles, and atomic nuclei. Spin is one of two types of angular momentum in mechanics, the other being orbital angular momentum. In some ways, spin is like a vector quantity, it has a definite magnitude, all elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. The SI unit of spin is the or, just as with classical angular momentum, very often, the spin quantum number is simply called spin leaving its meaning as the unitless spin quantum number to be inferred from context. When combined with the theorem, the spin of electrons results in the Pauli exclusion principle. Wolfgang Pauli was the first to propose the concept of spin, in 1925, Ralph Kronig, George Uhlenbeck and Samuel Goudsmit at Leiden University suggested an physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927, when Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it. As the name suggests, spin was originally conceived as the rotation of a particle around some axis and this picture is correct so far as spin obeys the same mathematical laws as quantized angular momenta do. On the other hand, spin has some properties that distinguish it from orbital angular momenta. Although the direction of its spin can be changed, a particle cannot be made to spin faster or slower. The spin of a particle is associated with a magnetic dipole moment with a g-factor differing from 1. This could only occur if the internal charge of the particle were distributed differently from its mass. The conventional definition of the quantum number, s, is s = n/2. Hence the allowed values of s are 0, 1/2,1, 3/2,2, the value of s for an elementary particle depends only on the type of particle, and cannot be altered in any known way. The spin angular momentum, S, of any system is quantized. The allowed values of S are S = ℏ s = h 4 π n, in contrast, orbital angular momentum can only take on integer values of s, i. e. even-numbered values of n. Those particles with half-integer spins, such as 1/2, 3/2, 5/2, are known as fermions, while particles with integer spins. The two families of particles obey different rules and broadly have different roles in the world around us, a key distinction between the two families is that fermions obey the Pauli exclusion principle, that is, there cannot be two identical fermions simultaneously having the same quantum numbers
10.
Chirality (physics)
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A chiral phenomenon is one that is not identical to its mirror image. The spin of a particle may be used to define a handedness, or helicity, for that particle, a symmetry transformation between the two is called parity. Invariance under parity by a Dirac fermion is called chiral symmetry, an experiment on the weak decay of cobalt-60 nuclei carried out by Chien-Shiung Wu and collaborators in 1957 demonstrated that parity is not a symmetry of the universe. The helicity of a particle is right-handed if the direction of its spin is the same as the direction of its motion and it is left-handed if the directions of spin and motion are opposite. By convention for rotation, a clock, with its spin vector defined by the rotation of its hands. Mathematically, helicity is the sign of the projection of the vector onto the momentum vector, left is negative. The chirality of a particle is more abstract and it is determined by whether the particle transforms in a right- or left-handed representation of the Poincaré group. For massive particles—such as electrons, quarks, and neutrinos—chirality and helicity must be distinguished, with the discovery of neutrino oscillation, which implies that neutrinos have mass, the only observed massless particle is the photon. The gluon is also expected to be massless, although the assumption that it is has not been conclusively tested, hence, these are the only two particles now known for which helicity could be identical to chirality, and only one of them has been confirmed by measurement. All other observed particles have mass and thus may have different helicities in different reference frames and it is still possible that as-yet unobserved particles, like the graviton, might be massless, and hence have invariant helicity like the photon. Only left-handed fermions and right-handed antifermions interact with the weak interaction, Chirality for a Dirac fermion ψ is defined through the operator γ5, which has eigenvalues ±1. Any Dirac field can thus be projected into its left- or right-handed component by acting with the projection operators /2 or /2 on ψ, the coupling of the charged weak interaction to fermions is proportional to the first projection operator, which is responsible for this interactions parity symmetry violation. A common source of confusion is due to conflating this operator with the helicity operator, since the helicity of massive particles is frame-dependent, it might seem that the same particle would interact with the weak force according to one frame of reference, but not another. The resolution to this paradox is that the chirality operator is equivalent to helicity for massless fields only. A theory that is asymmetric with respect to chiralities is called a chiral theory, many pieces of the Standard Model of physics are non-chiral, which is traceable to anomaly cancellation in chiral theories. Quantum chromodynamics is an example of a theory, since both chiralities of all quarks appear in the theory, and couple to gluons in the same way. The electroweak theory, developed in the mid 20th century, is an example of a chiral theory, originally, it assumed that neutrinos were massless, and only assumed the existence of left-handed neutrinos. However, it is still a chiral theory, as it does not respect parity symmetry, vector gauge theories with massless Dirac fermion fields ψ exhibit chiral symmetry, i. e. rotating the left-handed and the right-handed components independently makes no difference to the theory
11.
Lepton
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A lepton is an elementary, half-integer spin particle that does not undergo strong interactions. Two main classes of leptons exist, charged leptons, and neutral leptons, the best known of all leptons is the electron. There are six types of leptons, known as flavours, forming three generations, electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay, thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions. Leptons have various properties, including electric charge, spin. Unlike quarks however, leptons are not subject to the strong interaction, for every lepton flavor there is a corresponding type of antiparticle, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to theories, neutrinos may be their own antiparticle. The first charged lepton, the electron, was theorized in the century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, after investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of leptons as a family of particle to be proposed, the first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay. It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan, the muon neutrino was discovered in 1962 by Leon M. The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery, Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons, exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium. The name lepton comes from the Greek λεπτός leptós, fine, small, thin, Lepton was first used by physicist Léon Rosenfeld in 1948, Following a suggestion of Prof. C. Møller, I adopt—as a pendant to nucleon—the denomination lepton to denote a particle of small mass, the etymology incorrectly implies that all the leptons are of small mass. However, the mass of the tau is nearly twice that of the proton, the first lepton identified was the electron, discovered by J. J. Thomson and his team of British physicists in 1897, then in 1930 Wolfgang Pauli postulated the electron neutrino to preserve conservation of energy, conservation of momentum, and conservation of angular momentum in beta decay. Pauli theorized that a particle was carrying away the difference between the energy, momentum, and angular momentum of the initial and observed final particles
12.
Neutrino
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A neutrino is a fermion that interacts only via the weak subatomic force and gravity. The mass of the neutrino is much smaller than that of the known elementary particles. The neutrino is so named because it is neutral and because its rest mass is so small that it was originally thought to be zero. The weak force has a short range, gravity is extremely weak on the subatomic scale. Thus, neutrinos pass through normal matter unimpeded and undetected. Weak interactions create neutrinos in one of three flavors, electron neutrinos, muon neutrinos, or tau neutrinos, in association with the corresponding charged lepton. Although neutrinos were long believed to be massless, it is now known there are three discrete neutrino masses with different tiny values, but they dont correspond uniquely to the three flavors. A neutrino created with a specific flavor is in a specific quantum superposition of all three mass states. Neutrinos oscillate between different flavors in flight, for example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. For each neutrino, there exists a corresponding antiparticle, called an antineutrino. They are distinguished from the neutrinos by having opposite signs of lepton number, the majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. About 65 billion solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth, neutrinos are also finding practical applications, such as tomography of the interior of the earth. The neutrino was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum and he considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay. James Chadwick discovered a more massive nuclear particle in 1932 and also named it a neutron. However, the journal Nature rejected Fermis paper, saying that the theory was too remote from reality. He submitted the paper to an Italian journal, which accepted it, however, by 1934 there was experimental evidence against Bohrs idea that energy conservation is invalid for beta decay. Such a limit is not expected if the conservation of energy is invalid, Pauli made use of the occasion to publicly emphasize that the still-undetected neutrino must be an actual particle. In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos, in the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, F. B
13.
Gravity
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Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, on Earth, gravity gives weight to physical objects and causes the ocean tides. Gravity has a range, although its effects become increasingly weaker on farther objects. The most extreme example of this curvature of spacetime is a hole, from which nothing can escape once past its event horizon. More gravity results in time dilation, where time lapses more slowly at a lower gravitational potential. Gravity is the weakest of the four fundamental interactions of nature, the gravitational attraction is approximately 1038 times weaker than the strong force,1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, gravity has an influence on the behavior of subatomic particles. On the other hand, gravity is the dominant interaction at the macroscopic scale, for this reason, in part, pursuit of a theory of everything, the merging of the general theory of relativity and quantum mechanics into quantum gravity, has become an area of research. While the modern European thinkers are credited with development of gravitational theory, some of the earliest descriptions came from early mathematician-astronomers, such as Aryabhata, who had identified the force of gravity to explain why objects do not fall out when the Earth rotates. Later, the works of Brahmagupta referred to the presence of force, described it as an attractive force. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and this was a major departure from Aristotles belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere, galileos work set the stage for the formulation of Newtons theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. Newtons theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the position of the planet. A discrepancy in Mercurys orbit pointed out flaws in Newtons theory, the issue was resolved in 1915 by Albert Einsteins new theory of general relativity, which accounted for the small discrepancy in Mercurys orbit. The simplest way to test the equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the rate when other forces are negligible
14.
Standard Model
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The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong interactions, as well as classifying all the elementary particles known. It was developed throughout the half of the 20th century. The current formulation was finalized in the mid-1970s upon experimental confirmation of the existence of quarks, since then, discoveries of the top quark, the tau neutrino, and the Higgs boson have given further credence to the Standard Model. Because of its success in explaining a wide variety of experimental results and it does not incorporate the full theory of gravitation as described by general relativity, or account for the accelerating expansion of the Universe. The model does not contain any viable dark matter particle that all of the required properties deduced from observational cosmology. It also does not incorporate neutrino oscillations, the development of the Standard Model was driven by theoretical and experimental particle physicists alike. For theorists, the Standard Model is a paradigm of a field theory. The first step towards the Standard Model was Sheldon Glashows discovery in 1961 of a way to combine the electromagnetic, in 1967 Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into Glashows electroweak interaction, giving it its modern form. The Higgs mechanism is believed to rise to the masses of all the elementary particles in the Standard Model. This includes the masses of the W and Z bosons, the W± and Z0 bosons were discovered experimentally in 1983, and the ratio of their masses was found to be as the Standard Model predicted. The theory of the interaction, to which many contributed, acquired its modern form around 1973–74. At present, matter and energy are best understood in terms of the kinematics, to date, physics has reduced the laws governing the behavior and interaction of all known forms of matter and energy to a small set of fundamental laws and theories. The Standard Model includes members of classes of elementary particles. All particles can be summarized as follows, The Standard Model includes 12 elementary particles of spin known as fermions. According to the theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle, the fermions of the Standard Model are classified according to how they interact. There are six quarks, and six leptons, pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior. The defining property of the quarks is that they carry color charge, a phenomenon called color confinement results in quarks being very strongly bound to one another, forming color-neutral composite particles containing either a quark and an antiquark or three quarks
15.
Weak interaction
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In particle physics, the weak interaction is one of the four known fundamental interactions of nature, alongside the strong interaction, electromagnetism, and gravitation. The weak interaction is responsible for radioactive decay, which plays an role in nuclear fission. The theory of the interaction is sometimes called quantum flavourdynamics, in analogy with the terms QCD dealing with the strong interaction. However the term QFD is rarely used because the force is best understood in terms of electro-weak theory. The Standard Model of particle physics, which does not address gravity, provides a framework for understanding how the electromagnetic, weak. An interaction occurs when two particles, typically but not necessarily half-integer spin fermions, exchange integer-spin, force-carrying bosons, the fermions involved in such exchanges can be either elementary or composite, although at the deepest levels, all weak interactions ultimately are between elementary particles. In the case of the interaction, fermions can exchange three distinct types of force carriers known as the W+, W−, and Z bosons. The mass of each of these bosons is far greater than the mass of a proton or neutron, the force is in fact termed weak because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force or electromagnetic force. During the quark epoch of the universe, the electroweak force separated into the electromagnetic. Important examples of the weak interaction include beta decay, and the fusion of hydrogen into deuterium that powers the Suns thermonuclear process, most fermions will decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium illumination, quarks, which make up composite particles like neutrons and protons, come in six flavours – up, down, strange, charm, top and bottom – which give those composite particles their properties. The weak interaction is unique in that it allows for quarks to swap their flavour for another, the swapping of those properties is mediated by the force carrier bosons. Also, the interaction is the only fundamental interaction that breaks parity-symmetry, and similarly. In 1933, Enrico Fermi proposed the first theory of the weak interaction and he suggested that beta decay could be explained by a four-fermion interaction, involving a contact force with no range. However, it is described as a non-contact force field having a finite range. The existence of the W and Z bosons was not directly confirmed until 1983, the weak interaction is unique in a number of respects, It is the only interaction capable of changing the flavour of quarks. It is the interaction that violates P or parity-symmetry
16.
Chirality
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Chirality /kaɪˈrælɪtiː/ is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek, χειρ, hand, an object or a system is chiral if it is distinguishable from its mirror image, that is, it cannot be superposed onto it. Conversely, an image of an achiral object, such as a sphere. A chiral object and its image are called enantiomorphs or. A non-chiral object is called achiral and can be superposed on its mirror image, human hands are perhaps the most universally recognized example of chirality. The left hand is a mirror image of the right hand. This difference in symmetry becomes obvious if someone attempts to shake the hand of a person using their left hand. In mathematics, chirality is the property of a figure that is not identical to its mirror image, in mathematics, a figure is chiral if it cannot be mapped to its mirror image by rotations and translations alone. For example, a shoe is different from a left shoe. A chiral object and its image are said to be enantiomorphs. The word enantiomorph stems from the Greek ἐναντίος opposite + μορφή form, a non-chiral figure is called achiral or amphichiral. The helix and Möbius strip are chiral two dimensional objects in three dimensional ambient space, the J, L, S and Z-shaped tetrominoes of the popular video game Tetris also exhibit chirality, but only in a two dimensional space. Many other familiar objects exhibit the same symmetry of the human body, such as gloves, glasses. A similar notion of chirality is considered in theory, as explained below. Some chiral three dimensional objects, such as the helix, can be assigned a right or left handedness, in geometry a figure is achiral if and only if its symmetry group contains at least one orientation-reversing isometry. In two dimensions, every figure that possesses an axis of symmetry is achiral, and it can be shown that every bounded achiral figure must have an axis of symmetry, in three dimensions, every figure that possesses a plane of symmetry or a center of symmetry is achiral. There are, however, achiral figures lacking both plane and center of symmetry, in terms of point groups, all chiral figures lack an improper axis of rotation. This means that they contain a center of inversion or a mirror plane
17.
Neutrino mass
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A neutrino is a fermion that interacts only via the weak subatomic force and gravity. The mass of the neutrino is much smaller than that of the known elementary particles. The neutrino is so named because it is neutral and because its rest mass is so small that it was originally thought to be zero. The weak force has a short range, gravity is extremely weak on the subatomic scale. Thus, neutrinos pass through normal matter unimpeded and undetected. Weak interactions create neutrinos in one of three flavors, electron neutrinos, muon neutrinos, or tau neutrinos, in association with the corresponding charged lepton. Although neutrinos were long believed to be massless, it is now known there are three discrete neutrino masses with different tiny values, but they dont correspond uniquely to the three flavors. A neutrino created with a specific flavor is in a specific quantum superposition of all three mass states. Neutrinos oscillate between different flavors in flight, for example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. For each neutrino, there exists a corresponding antiparticle, called an antineutrino. They are distinguished from the neutrinos by having opposite signs of lepton number, the majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. About 65 billion solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth, neutrinos are also finding practical applications, such as tomography of the interior of the earth. The neutrino was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum and he considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay. James Chadwick discovered a more massive nuclear particle in 1932 and also named it a neutron. However, the journal Nature rejected Fermis paper, saying that the theory was too remote from reality. He submitted the paper to an Italian journal, which accepted it, however, by 1934 there was experimental evidence against Bohrs idea that energy conservation is invalid for beta decay. Such a limit is not expected if the conservation of energy is invalid, Pauli made use of the occasion to publicly emphasize that the still-undetected neutrino must be an actual particle. In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos, in the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, F. B
18.
Electronvolt
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In physics, the electronvolt is a unit of energy equal to approximately 1. 6×10−19 joules. By definition, it is the amount of energy gained by the charge of an electron moving across an electric potential difference of one volt. Thus it is 1 volt multiplied by the elementary charge, therefore, one electronvolt is equal to 6981160217662079999♠1. 6021766208×10−19 J. The electronvolt is not a SI unit, and its definition is empirical, like the elementary charge on which it is based, it is not an independent quantity but is equal to 1 J/C √2hα / μ0c0. It is a unit of energy within physics, widely used in solid state, atomic, nuclear. It is commonly used with the metric prefixes milli-, kilo-, in some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion electronvolts, it is equivalent to the GeV. By mass–energy equivalence, the electronvolt is also a unit of mass and it is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c2, where c is the speed of light in vacuum. It is common to express mass in terms of eV as a unit of mass. The mass equivalent of 1 eV/c2 is 1 eV / c 2 = ⋅1 V2 =1.783 ×10 −36 kg. For example, an electron and a positron, each with a mass of 0.511 MeV/c2, the proton has a mass of 0.938 GeV/c2. In general, the masses of all hadrons are of the order of 1 GeV/c2, the unified atomic mass unit,1 gram divided by Avogadros number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula,1 u =931.4941 MeV/c2 =0.9314941 GeV/c2, in high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy and this gives rise to usage of eV as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of units are LMT−1. The dimensions of units are L2MT−2. Then, dividing the units of energy by a constant that has units of velocity. In the field of particle physics, the fundamental velocity unit is the speed of light in vacuum c. Thus, dividing energy in eV by the speed of light, the fundamental velocity constant c is often dropped from the units of momentum by way of defining units of length such that the value of c is unity
19.
Electroweak interaction
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In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature, electromagnetism and the weak interaction. Although these two forces appear very different at low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 246 GeV, thus, if the universe is hot enough, then the electromagnetic force and weak force merge into a combined electroweak force. During the electroweak epoch, the electroweak force separated from the strong force, during the quark epoch, the electroweak force split into the electromagnetic and weak force. In 1999, Gerardus t Hooft and Martinus Veltman were awarded the Nobel prize for showing that the theory is renormalizable. Mathematically, the unification is accomplished under an SU × U gauge group, the corresponding gauge bosons are the three W bosons of weak isospin from SU, and the B boson of weak hypercharge from U, respectively, all of which are massless. In the Standard Model, the W± and Z0 bosons, UY and Uem are different copies of U, the generator of Uem is given by Q = Y/2 + I3, where Y is the generator of UY, and I3 is one of the SU generators. The spontaneous symmetry breaking makes the W3 and B bosons coalesce into two different bosons – the Z0 boson, and the photon, = Where θW is the mixing angle. The axes representing the particles have essentially just been rotated, in the plane and this also introduces a mismatch between the mass of the Z0 and the mass of the W± particles, M Z = M W cos θ W. The W1 and W2 bosons, in turn, combine to give massive charged bosons W ± =12, the Lagrangian for the electroweak interactions is divided into four parts before electroweak symmetry breaking L E W = L g + L f + L h + L y. The L g term describes the interaction between the three W particles and the B particle, L f is the kinetic term for the Standard Model fermions. The interaction of the bosons and the fermions are through the gauge covariant derivative. The h term describes the Higgs field F. L h = | D μ h |2 − λ2 The y term gives the Yukawa interaction that generates the masses after the Higgs acquires a nonzero vacuum expectation value. The Lagrangian reorganizes itself after the Higgs boson acquires a vacuum expectation value, due to its complexity, this Lagrangian is best described by breaking it up into several parts as follows. The neutral current L N and charged current L C components of the Lagrangian contain the interactions between the fermions and gauge bosons, the charged current part of the Lagrangian is given by L C = − g 2 W μ + + h. c. L H contains the Higgs three-point and four-point self interaction terms, L H = − g m H24 m W H3 − g 2 m H232 m W2 H4 L H V contains the Higgs interactions with gauge vector bosons. L H V = L W W V contains the gauge three-point self interactions. L Y = − ∑ f g m f 2 m W f ¯ f H Note the 1 − γ52 factors in the weak couplings and this is why electroweak theory is commonly said to be a chiral theory
20.
Particle physics
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Particle physics is the branch of physics that studies the nature of the particles that constitute matter and radiation. By our current understanding, these particles are excitations of the quantum fields that also govern their interactions. The currently dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model, in more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. All particles and their interactions observed to date can be described almost entirely by a field theory called the Standard Model. The Standard Model, as formulated, has 61 elementary particles. Those elementary particles can combine to form composite particles, accounting for the hundreds of species of particles that have been discovered since the 1960s. The Standard Model has been found to agree with almost all the tests conducted to date. However, most particle physicists believe that it is a description of nature. In recent years, measurements of mass have provided the first experimental deviations from the Standard Model. The idea that all matter is composed of elementary particles dates from at least the 6th century BC, in the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature was composed of a single, unique type of particle. Throughout the 1950s and 1960s, a variety of particles were found in collisions of particles from increasingly high-energy beams. It was referred to informally as the particle zoo, the current state of the classification of all elementary particles is explained by the Standard Model. It describes the strong, weak, and electromagnetic fundamental interactions, the species of gauge bosons are the gluons, W−, W+ and Z bosons, and the photons. The Standard Model also contains 24 fundamental particles, which are the constituents of all matter, finally, the Standard Model also predicted the existence of a type of boson known as the Higgs boson. Early in the morning on 4 July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson, the worlds major particle physics laboratories are, Brookhaven National Laboratory. Its main facility is the Relativistic Heavy Ion Collider, which collides heavy ions such as gold ions and it is the worlds first heavy ion collider, and the worlds only polarized proton collider. Its main projects are now the electron-positron colliders VEPP-2000, operated since 2006 and its main project is now the Large Hadron Collider, which had its first beam circulation on 10 September 2008, and is now the worlds most energetic collider of protons. It also became the most energetic collider of heavy ions after it began colliding lead ions and its main facility is the Hadron Elektron Ring Anlage, which collides electrons and positrons with protons
21.
Neutrino oscillations
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Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates through space, first predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by a multitude of experiments in several different contexts. Notably, the existence of neutrino oscillation resolved the solar neutrino problem. Neutrino oscillation is of theoretical and experimental interest, as the precise properties of the process can shed light on several properties of the neutrino. In particular, it implies that the neutrino has a non-zero mass, a great deal of evidence for neutrino oscillation has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies. The 2015 Nobel Prize in Physics was shared by Takaaki Kajita, mcDonald for their early pioneering observations of these oscillations. Neutrino oscillation is a function of the ratio L/E, where L is the distance traveled and E is the neutrinos energy. Neutrino sources and detectors are far too large to move, but all available sources produce a range of energies, the preferred distance depends on the most common energy, but the exact distance is not critical as long as it is known. The limiting factor in measurements is the accuracy with which the energy of each observed neutrino can be measured, because current detectors have energy uncertainties of a few percent, it is satisfactory to know the distance to within 1%. This gave rise to the Solar neutrino problem, Solar neutrinos have energies below 20 MeV. Following the theories that were proposed in the 1970s suggesting unification of weak, strong and Electromagnetic forces, large detectors such as IMB, MACRO, and Kamiokande II have observed a deficit in the ratio of the flux of muon to electron flavor atmospheric neutrinos. Many experiments have searched for oscillation of electron anti-neutrinos produced at nuclear reactors, such oscillations give the value of the parameter θ13. Neutrinos produced in nuclear reactors have energies similar to solar neutrinos, the baselines of these experiments have ranged from tens of meters to over 100 km. In 2012, the Daya Bay experiment announced a discovery that θ13 ≠0 with a significance of 5. 2σ, Neutrino beams produced at a particle accelerator offer the greatest control over the neutrinos being studied. Many experiments have taken place that study the same oscillations as in atmospheric neutrino oscillation using neutrinos with a few GeV of energy, the MINOS, K2K, and Super-K experiments have all independently observed muon neutrino disappearance over such long baselines. Data from the LSND experiment appear to be in conflict with the parameters measured in other experiments. Results from the MiniBooNE appeared in Spring 2007 and contradicted the results from LSND, although they could support the existence of a fourth neutrino type, the sterile neutrino. In 2010, the INFN and CERN announced the observation of a tau particle in a neutrino beam in the OPERA detector located at Gran Sasso,730 km away from the source in Geneva
22.
Physical cosmology
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Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate. These advances made it possible to speculate about the origin of the universe, a few researchers still advocate a handful of alternative cosmologies, however, most cosmologists agree that the Big Bang theory explains the observations better. Cosmology draws heavily on the work of many areas of research in theoretical. Areas relevant to include particle physics experiments and theory, theoretical and observational astrophysics, general relativity, quantum mechanics. Modern cosmology developed along tandem tracks of theory and observation, in 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a universe. This is because masses distributed throughout the universe gravitationally attract, however, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his equations in order to force them to model a static universe. However, this so-called Einstein model is unstable to small perturbations—it will eventually start to expand or contract, the Einstein model describes a static universe, space is finite and unbounded. It was later realized that Einsteins model was just one of a set of possibilities, all of which were consistent with general relativity. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s and his equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed. In the 1910s, Vesto Slipher interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth, however, it is difficult to determine the distance to astronomical objects. One way is to compare the size of an object to its angular size. Another method is to measure the brightness of an object and assume an intrinsic luminosity, due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1929, Edwin Hubble provided a basis for Lemaîtres theory. Hubble showed that the nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance and he interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance. Given the cosmological principle, Hubbles law suggested that the universe was expanding, two primary explanations were proposed for the expansion
23.
Astrophysics
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Astrophysics is the branch of astronomy that employs the principles of physics and chemistry to ascertain the nature of the heavenly bodies, rather than their positions or motions in space. Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. In practice, modern astronomical research often involves an amount of work in the realms of theoretical and observational physics. Although astronomy is as ancient as recorded history itself, it was separated from the study of terrestrial physics. Their challenge was that the tools had not yet been invented with which to prove these assertions, for much of the nineteenth century, astronomical research was focused on the routine work of measuring the positions and computing the motions of astronomical objects. Kirchhoff deduced that the lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere. In this way it was proved that the elements found in the Sun. Among those who extended the study of solar and stellar spectra was Norman Lockyer and he thus claimed the line represented a new element, which was called helium, after the Greek Helios, the Sun personified. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types, most significantly, she discovered that hydrogen and helium were the principal components of stars. This discovery was so unexpected that her dissertation readers convinced her to modify the conclusion before publication, however, later research confirmed her discovery. By the end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray and it is the practice of observing celestial objects by using telescopes and other astronomical apparatus. The majority of observations are made using the electromagnetic spectrum. Radio astronomy studies radiation with a greater than a few millimeters. The study of these waves requires very large radio telescopes, infrared astronomy studies radiation with a wavelength that is too long to be visible to the naked eye but is shorter than radio waves. Infrared observations are made with telescopes similar to the familiar optical telescopes. Objects colder than stars are studied at infrared frequencies. Optical astronomy is the oldest kind of astronomy, telescopes paired with a charge-coupled device or spectroscopes are the most common instruments used. The Earths atmosphere interferes somewhat with optical observations, so adaptive optics, in this wavelength range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies and nebulae
24.
Dark matter
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Dark matter is an unidentified type of matter distinct from dark energy, baryonic matter, and neutrinos whose existence would explain a number of otherwise puzzling astronomical observations. The name refers to the fact that it does not emit or interact with radiation, such as light. The standard model of cosmology indicates that the total mass–energy of the universe contains 4. 9% ordinary matter,26. 8% dark matter and 68. 3% dark energy. Thus, dark matter constitutes 84. 5% of total mass, the most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles that interact only through gravity and the weak force. All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals. Many experiments to detect proposed dark matter particles through non-gravitational means are under way, however, no dark matter particle has been conclusively identified. ”In 1906 Henri Poincaré in the “The Milky Way and Theory of Gases” used dark matter, ” or “matière obscure” in French in discussing Kelvins work. The first to suggest the existence of matter was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of matter in 1932. In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, Zwicky applied the virial theorem to the Coma galaxy cluster and obtained evidence of unseen mass that he called dunkle Materie dark matter. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and he estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the galaxies was far too small for such fast orbits. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and this was the first formal inference about the existence of dark matter. However, Zwicky did correctly infer that the bulk of the matter was dark, the first robust indications that the mass to light ratio was anything other than unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula and he attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to missing matter. Vera Rubin and Kent Ford in the 1960s–1970s provided further strong evidence, Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. This result was confirmed in 1978, an influential paper presented Rubins results in 1980. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle, the search for this particle, by a variety of means, is one of the major efforts in particle physics. In cosmology, the CMB is explained as relic radiation which has travelled freely since the era of recombination, the CMB’s anisotropies are explained as the result of small primordial density fluctuations, and subsequent acoustic oscillations in the photon-baryon plasma whose restoring force is gravity
25.
Dark radiation
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Dark radiation is a postulated type of radiation that mediates interactions of dark matter. By analogy to the way photons mediate electromagnetic interactions between particles in the Standard Model, dark radiation is proposed to mediate interactions between matter particles. Similar to dark matter particles, the hypothetical dark radiation does not interact with Standard Model particles, there has been no notable evidence for the existence of such radiation, but since baryonic matter contains multiple interacting particle types, it is reasonable to suppose that dark matter does also. This extra degree of freedom could arise from having a non-trivial amount of radiation in the universe. One possible candidate for dark radiation is the sterile neutrino
26.
MiniBooNE
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MiniBooNE is an experiment at Fermilab designed to observe neutrino oscillations. A neutrino beam consisting primarily of muon neutrinos is directed at a detector filled with 800 tons of mineral oil, an excess of electron neutrino events in the detector would support the neutrino oscillation interpretation of the LSND result. It started collecting data in 2002 and was running in 2012. Experimental observation of neutrinos and atmospheric neutrinos provided evidence for neutrino oscillations. Either there must be an extension to the Standard Model, or one of the results must have a different explanation. Moreover, the KARMEN experiment in Karlsruhe examined a region similar to the LSND experiment and this experiment was less sensitive than LSND, and both could be right. Cosmological data can provide an indirect but rather model-dependent bound to the mass of sterile neutrinos, however, cosmological data can be accommodated within models with different assumptions, such as that by Gelmini et al. MiniBooNE was designed to verify or refute the LSND controversial result in a controlled environment. 2008 Some members of MiniBooNE have formed a new collaboration with outside scientists, miniBooNe first results press release and arXiv,0704
27.
Neutrino oscillation
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Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates through space, first predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by a multitude of experiments in several different contexts. Notably, the existence of neutrino oscillation resolved the solar neutrino problem. Neutrino oscillation is of theoretical and experimental interest, as the precise properties of the process can shed light on several properties of the neutrino. In particular, it implies that the neutrino has a non-zero mass, a great deal of evidence for neutrino oscillation has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies. The 2015 Nobel Prize in Physics was shared by Takaaki Kajita, mcDonald for their early pioneering observations of these oscillations. Neutrino oscillation is a function of the ratio L/E, where L is the distance traveled and E is the neutrinos energy. Neutrino sources and detectors are far too large to move, but all available sources produce a range of energies, the preferred distance depends on the most common energy, but the exact distance is not critical as long as it is known. The limiting factor in measurements is the accuracy with which the energy of each observed neutrino can be measured, because current detectors have energy uncertainties of a few percent, it is satisfactory to know the distance to within 1%. This gave rise to the Solar neutrino problem, Solar neutrinos have energies below 20 MeV. Following the theories that were proposed in the 1970s suggesting unification of weak, strong and Electromagnetic forces, large detectors such as IMB, MACRO, and Kamiokande II have observed a deficit in the ratio of the flux of muon to electron flavor atmospheric neutrinos. Many experiments have searched for oscillation of electron anti-neutrinos produced at nuclear reactors, such oscillations give the value of the parameter θ13. Neutrinos produced in nuclear reactors have energies similar to solar neutrinos, the baselines of these experiments have ranged from tens of meters to over 100 km. In 2012, the Daya Bay experiment announced a discovery that θ13 ≠0 with a significance of 5. 2σ, Neutrino beams produced at a particle accelerator offer the greatest control over the neutrinos being studied. Many experiments have taken place that study the same oscillations as in atmospheric neutrino oscillation using neutrinos with a few GeV of energy, the MINOS, K2K, and Super-K experiments have all independently observed muon neutrino disappearance over such long baselines. Data from the LSND experiment appear to be in conflict with the parameters measured in other experiments. Results from the MiniBooNE appeared in Spring 2007 and contradicted the results from LSND, although they could support the existence of a fourth neutrino type, the sterile neutrino. In 2010, the INFN and CERN announced the observation of a tau particle in a neutrino beam in the OPERA detector located at Gran Sasso,730 km away from the source in Geneva
28.
Momentum
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In classical mechanics, linear momentum, translational momentum, or simply momentum is the product of the mass and velocity of an object, quantified in kilogram-meters per second. It is dimensionally equivalent to impulse, the product of force and time, Newtons second law of motion states that the change in linear momentum of a body is equal to the net impulse acting on it. If the truck were lighter, or moving slowly, then it would have less momentum. Linear momentum is also a quantity, meaning that if a closed system is not affected by external forces. In classical mechanics, conservation of momentum is implied by Newtons laws. It also holds in special relativity and, with definitions, a linear momentum conservation law holds in electrodynamics, quantum mechanics, quantum field theory. It is ultimately an expression of one of the symmetries of space and time. Linear momentum depends on frame of reference, observers in different frames would find different values of linear momentum of a system. But each would observe that the value of linear momentum does not change with time, momentum has a direction as well as magnitude. Quantities that have both a magnitude and a direction are known as vector quantities, because momentum has a direction, it can be used to predict the resulting direction of objects after they collide, as well as their speeds. Below, the properties of momentum are described in one dimension. The vector equations are almost identical to the scalar equations, the momentum of a particle is traditionally represented by the letter p. It is the product of two quantities, the mass and velocity, p = m v, the units of momentum are the product of the units of mass and velocity. In SI units, if the mass is in kilograms and the velocity in meters per second then the momentum is in kilogram meters/second, in cgs units, if the mass is in grams and the velocity in centimeters per second, then the momentum is in gram centimeters/second. Being a vector, momentum has magnitude and direction, for example, a 1 kg model airplane, traveling due north at 1 m/s in straight and level flight, has a momentum of 1 kg m/s due north measured from the ground. The momentum of a system of particles is the sum of their momenta, if two particles have masses m1 and m2, and velocities v1 and v2, the total momentum is p = p 1 + p 2 = m 1 v 1 + m 2 v 2. If all the particles are moving, the center of mass will generally be moving as well, if the center of mass is moving at velocity vcm, the momentum is, p = m v cm. This is known as Eulers first law, if a force F is applied to a particle for a time interval Δt, the momentum of the particle changes by an amount Δ p = F Δ t
29.
Relativistic invariant
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In a relativistic theory of physics, a Lorentz scalar is a scalar which is invariant under a Lorentz transformation. A Lorentz scalar may be generated from multiplication of vectors or tensors, while the components of vectors and tensors are in general altered by Lorentz transformations, scalars remain unchanged. A Lorentz scalar is not a scalar in the mathematical sense, for example, the determinant of the matrix of basis vectors is a number that is invariant under Lorentz transformations, but it is not invariant under any basis transformation. Often the alternate signature of the Minkowski metric is used in which the signs of the ones are reversed. In the Minkowski metric the space-like interval s is defined as x μ x μ = η μ ν x μ x ν = x ⋅ x −2 = d e f s 2 and we use the space-like Minkowski metric in the rest of this article. The velocity in spacetime is defined as v μ = d e f d x μ d τ = = = γ where γ = d e f 11 − v ⋅ v c 2. The magnitude of the 4-velocity is a Lorentz scalar, v μ v μ = − c 2, hence, c is a Lorentz scalar. The 4-acceleration is given by a μ = d e f d v μ d τ, the 4-acceleration is always perpendicular to the 4-velocity 0 =12 d d τ = d v μ d τ v μ = a μ v μ. Therefore, we can regard acceleration in spacetime as simply a rotation of the 4-velocity, the inner product of the acceleration and the velocity is a Lorentz scalar and is zero. This rotation is simply an expression of energy conservation, d E d τ = F ⋅ v where E is the energy of a particle and F is the 3-force on the particle. The 4-momentum of a particle is p μ = m v μ = = = where m is the particle rest mass, p is the momentum in 3-space, consider a second particle with 4-velocity u and a 3-velocity u 2. In the rest frame of the particle the inner product of u with p is proportional to the energy of the first particle p μ u μ = − E1 where the subscript 1 indicates the first particle. Since the relationship is true in the rest frame of the second particle, E1, the energy of the first particle in the frame of the second particle, is a Lorentz scalar. In the rest frame of the particle the inner product of the momentum is p μ p μ = −2, therefore, the rest mass is a Lorentz scalar. The relationship remains true independent of the frame in which the product is calculated. The square of the magnitude of the 3-momentum of the particle as measured in the frame of the particle is a Lorentz scalar. The 3-speed, in the frame of the particle, can be constructed from two Lorentz scalars v 12 = v 1 ⋅ v 1 = p 1 ⋅ p 1 c 4 E12. Scalars may also be constructed from the tensors and vectors, from the contraction of tensors, misner, Charles, Thorne, Kip S. & Wheeler, John Archibald
30.
Strong interaction
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At the range of 10−15 m, the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction and 1038 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as proton and neutron, in addition, the strong force binds neutrons and protons to create atomic nuclei. Most of the mass of a proton or neutron is the result of the strong force field energy. The strong interaction is observable at two ranges, on a scale, it is the force that binds protons and neutrons together to form the nucleus of an atom. On the smaller scale, it is the force that holds together to form protons, neutrons. In the latter context, it is known as the color force. The strong force inherently has such a strength that hadrons bound by the strong force can produce new massive particles. Thus, if hadrons are struck by particles, they give rise to new hadrons instead of emitting freely moving radiation. This property of the force is called color confinement, and it prevents the free emission of the strong force, instead, in practice. In the context of binding protons and neutrons together to form atomic nuclei, in this case, it is the residuum of the strong interaction between the quarks that make up the protons and neutrons. As such, the strong interaction obeys a quite different distance-dependent behavior between nucleons, from when it is acting to bind quarks within nucleons. The binding energy that is released on the breakup of a nucleus is related to the residual strong force and is harnessed as fission energy in nuclear power. The strong interaction is mediated by the exchange of particles called gluons that act between quarks, antiquarks, and other gluons. Gluons are thought to interact with quarks and other gluons by way of a type of charge called color charge. Color charge is analogous to electromagnetic charge, but it comes in three rather than one, which results in a different type of force, with different rules of behavior. These rules are detailed in the theory of quantum chromodynamics, which is the theory of quark-gluon interactions, after the Big Bang and during the electroweak epoch of the universe, the electroweak force separated from the strong force. A Grand Unified Theory is hypothesized to exist to describe this, but no theory has yet been successfully formulated. Before the 1970s, physicists were uncertain as to how the nucleus was bound together
31.
SO(10) (physics)
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In particle physics, one of the grand unified theories is based on the SO Lie group. It has the rules to /Z5. 45 →240 ⊕10 −4 ⊕10 ¯4 ⊕1016 →101 ⊕5 ¯ −3 ⊕15. The superpotential may then include renormalizable terms of the form Tr, Tr,10 ⋅45 ⋅10,10 ⋅ 16* ⋅16 and 16* ⋅16. The first three are responsible to the symmetry breaking at low energies and give the Higgs mass. There is another possible branching, under which the hypercharge is a combination of an SU generator. This is known as flipped SU, another important subgroup is either /Z2 or Z2 ⋊ /Z2 depending upon whether or not the left-right symmetry is broken, yielding the Pati–Salam model, whose branching rule is 16 → ⊕. The symmetry breaking of SO is usually done with a combination of, lets say we choose a 54H. When this Higgs field acquires a GUT scale VEV, we have a symmetry breaking to Z2 ⋊ /Z2, if we have a 45H instead, this Higgs field can acquire any VEV in a two dimensional subspace without breaking the standard model. The choice diag is called the Dimopoulos-Wilczek mechanism aka the missing VEV mechanism, the choice of a 16H and a 16 ¯ H breaks the gauge group down to the Georgi–Glashow SU. The same comment applies to the choice of a 126H and a 126 ¯ H and it is the combination of BOTH a 45/54 and a 16/16 ¯ or 126/126 ¯ which breaks SO down to the Standard Model. The electroweak Higgs doublets come from an SO 10H, unfortunately, this same 10 also contains triplets. Among the solutions for it is the Dimopoulos-Wilczek mechanism, or the choice of diag of <45>, unfortunately, this is not stable once the 16/16 ¯ or 126/126 ¯ sector interacts with the 45 sector. The matter representations come in three copies of the 16 representation, the Yukawa coupling is 10H 16f 16f. These graphics refer to the X bosons and Higgs bosons, note that SO contains both the Georgi–Glashow SU and flipped SU
32.
Grand Unified Theory
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This unified interaction is characterized by one larger gauge symmetry and thus several force carriers, but one unified coupling constant. If Grand Unification is realized in nature, there is the possibility of a grand unification epoch in the universe in which the fundamental forces are not yet distinct. Unifying gravity with the other three interactions would provide a theory of everything, rather than a GUT, nevertheless, GUTs are often seen as an intermediate step towards a TOE. Some GUTs, such as the Pati-Salam model, predict the existence of magnetic monopoles, the main reason for this complexity lies in the difficulty of reproducing the observed fermion masses and mixing angles. Due to this difficulty, and due to the lack of any observed effect of grand unification so far, historically, the first true GUT which was based on the simple Lie group SU, was proposed by Howard Georgi and Sheldon Glashow in 1974. The Georgi–Glashow model was preceded by the semisimple Lie algebra Pati–Salam model by Abdus Salam and Jogesh Pati, nanopoulos later that year was the first to use the acronym in a paper. This would automatically predict the nature and values of all elementary particle charges. For an elementary introduction to how Lie algebras are related to physics, see the article Particle physics. The smallest simple Lie group which contains the model. Such group symmetries allow the reinterpretation of several known particles as different states of a particle field. However, it is not obvious that the simplest possible choices for the extended Grand Unified symmetry should yield the correct inventory of elementary particles, the two smallest irreducible representations of SU are 5 and 10. This scheme has to be replicated for each of the three generations of matter. It is notable that the theory is anomaly free with this matter content, the next simple Lie group which contains the standard model is S O ⊃ S U ⊃ S U × S U × U. This is already the largest simple group which achieves the unification of matter in a scheme involving only the already known matter particles, some of these mass relations hold approximately, but most dont. The boson matrix for SO is found by taking the 15 ×15 matrix from the 10 +5 representation of SU and adding an extra row and column for the right handed neutrino. The bosons are found by adding a partner to each of the 20 charged bosons, the boson matrix will have a boson or its new partner in each row and column. These pairs combine to create the familiar 16D Dirac spinor matrices of SO, assuming 4 generations of fermions instead of 3 makes a total of 64 types of particles. These can be put into 64 =8 +56 representations of SU and this can be divided into SU × SUF × U which is the SU theory together with some heavy bosons which act on the generation number
33.
Electromagnetism
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Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually exhibits electromagnetic fields such as fields, magnetic fields. The other three fundamental interactions are the interaction, the weak interaction, and gravitation. The word electromagnetism is a form of two Greek terms, ἤλεκτρον, ēlektron, amber, and μαγνῆτις λίθος magnētis lithos, which means magnesian stone. The electromagnetic force plays a role in determining the internal properties of most objects encountered in daily life. Ordinary matter takes its form as a result of forces between individual atoms and molecules in matter, and is a manifestation of the electromagnetic force. Electrons are bound by the force to atomic nuclei, and their orbital shapes. The electromagnetic force governs the processes involved in chemistry, which arise from interactions between the electrons of neighboring atoms, there are numerous mathematical descriptions of the electromagnetic field. In classical electrodynamics, electric fields are described as electric potential, although electromagnetism is considered one of the four fundamental forces, at high energy the weak force and electromagnetic force are unified as a single electroweak force. In the history of the universe, during the epoch the unified force broke into the two separate forces as the universe cooled. Originally, electricity and magnetism were considered to be two separate forces, Magnetic poles attract or repel one another in a manner similar to positive and negative charges and always exist as pairs, every north pole is yoked to a south pole. An electric current inside a wire creates a corresponding magnetic field outside the wire. Its direction depends on the direction of the current in the wire. A current is induced in a loop of wire when it is moved toward or away from a field, or a magnet is moved towards or away from it. While preparing for a lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation. As he was setting up his materials, he noticed a compass needle deflected away from north when the electric current from the battery he was using was switched on. At the time of discovery, Ørsted did not suggest any explanation of the phenomenon. However, three later he began more intensive investigations
34.
Higgs boson
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The Higgs boson is an elementary particle in the Standard Model of particle physics. It is the excitation of the Higgs field, a fundamental field of crucial importance to particle physics theory first suspected to exist in the 1960s. Unlike other known fields such as the field, it has a non-zero constant value in vacuum. The question of the Higgs fields existence became the last unverified part of the Standard Model of particle physics and it also resolves several other long-standing puzzles, such as the reason for the weak forces extremely short range. Although the Higgs field is believed to permeate the entire Universe, in principle, it can be proved to exist by detecting its excitations, which manifest as Higgs particles, but these are extremely difficult to produce and to detect. On 4 July 2012, the discovery of a new particle with a mass between 125 and 127 GeV/c2 was announced, physicists suspected that it was the Higgs boson and this also means it is the first elementary scalar particle discovered in nature. The Higgs boson is named after Peter Higgs, one of six physicists who, in the 1964 PRL symmetry breaking papers, on December 10,2013, two of them, Peter Higgs and François Englert, were awarded the Nobel Prize in Physics for their work and prediction. Although Higgss name has come to be associated with this theory, in the Standard Model, the Higgs particle is a boson with no spin, electric charge, or colour charge. It is also unstable, decaying into other particles almost immediately. It is an excitation of one of the four components of the Higgs field. The latter constitutes a field, with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU symmetry. The Higgs field is tachyonic, which does not refer to faster-than-light speeds, the Higgs field has a Mexican hat shaped potential with nonzero strength everywhere, which in its vacuum state breaks the weak isospin symmetry of the electroweak interaction. When this happens, three components of the Higgs field are absorbed by the SU and U gauge bosons to become the longitudinal components of the now-massive W and Z bosons of the weak force. The remaining electrically neutral component either manifests as a Higgs particle, or can couple separately to other known as fermions. Some versions of the theory predicted more than one kind of Higgs fields, alternative Higgsless models might have been considered if the Higgs boson had not been discovered. In this model, the forces in nature arise from properties of our universe called gauge invariance. The forces themselves are transmitted by particles known as gauge bosons, field theories had been used with great success in understanding the electromagnetic field and the strong force. The problem was that the requirements in gauge theory predicted that both electromagnetisms gauge boson and the weak forces gauge bosons should have zero mass
35.
Higgs mechanism
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In the Standard Model of particle physics, the Higgs mechanism is essential to explain the generation mechanism of the property mass for gauge bosons. Without the Higgs mechanism, all bosons would be massless, but measurements show that the W+, W−, the Higgs field resolves this conundrum. The simplest description of the mechanism adds a quantum field that permeates all space, below some extremely high temperature, the field causes spontaneous symmetry breaking during interactions. The breaking of symmetry triggers the Higgs mechanism, causing the bosons it interacts with to have mass, in the Standard Model, the phrase Higgs mechanism refers specifically to the generation of masses for the W±, and Z weak gauge bosons through electroweak symmetry breaking. The Higgs mechanism was incorporated into modern particle physics by Steven Weinberg and Abdus Salam, in the standard model, at temperatures high enough that electroweak symmetry is unbroken, all elementary particles are massless. At a critical temperature, the Higgs field becomes tachyonic, the symmetry is broken by condensation. Fermions, such as the leptons and quarks in the Standard Model, can acquire mass as a result of their interaction with the Higgs field. In the standard model, the Higgs field is an SU doublet, the Higgs field, through the interactions specified by its potential, induces spontaneous breaking of three out of the four generators of the gauge group U. This is often written as SU × U, because the phase factor also acts on other fields in particular quarks. Three out of its four components would ordinarily amount to Goldstone bosons, the gauge group of the electroweak part of the standard model is SU × U. The group SU is the group of all 2-by-2 unitary matrices with unit determinant, rotating the coordinates so that the second basis vector points in the direction of the Higgs boson makes the vacuum expectation value of H the spinor. The generators for rotations about the x, y, and z axes are by half the Pauli matrices σx, σy, while the Tx and Ty generators mix up the top and bottom components of the spinor, the Tz rotations only multiply each by opposite phases. This phase can be undone by a U rotation of angle 1/2θ, consequently, under both an SU Tz-rotation and a U rotation by an amount 1/2θ, the vacuum is invariant. This combination of generators preserves the vacuum, and defines the unbroken gauge group in the standard model, the part of the gauge field in this direction stays massless, and amounts to the physical photon. In spite of the introduction of spontaneous symmetry breaking, the mass terms preclude chiral gauge invariance, for these fields the mass terms should always be replaced by a gauge-invariant Higgs mechanism. The quantities γμ are the Dirac matrices, and Gψ is the already-mentioned Yukawa coupling parameter, already now the mass-generation follows the same principle as above, namely from the existence of a finite expectation value | ⟨ ϕ ⟩ |, as described above. Again, this is crucial for the existence of the property mass, spontaneous symmetry breaking offered a framework to introduce bosons into relativistic quantum field theories. However, according to Goldstones theorem, these bosons should be massless, the only observed particles which could be approximately interpreted as Goldstone bosons were the pions, which Yoichiro Nambu related to chiral symmetry breaking
36.
Cold dark matter
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It is believed that approximately 84. 54% of matter in the Universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets and living organisms. An influential review article in 1984 by Blumenthal, Sandra Moore Faber, Primack, predictions of the cold dark matter paradigm are in general agreement with astronomical observations. Dark matter is detected through its interactions with ordinary matter. As such, it is difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories, Axions are very light particles with a type of self-interaction that makes them a suitable CDM candidate. Axions have the advantage that their existence solves the Strong CP problem in QCD. MACHOs or Massive Compact Halo Objects are large, condensed objects such as holes, neutron stars, white dwarfs, very faint stars. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies, most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate. WIMPs, Dark matter is composed of Weakly Interacting Massive Particles, there is no currently known particle with the required properties, but many extensions of the standard model of particle physics predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, WIMPs are generally regarded as the most promising dark matter candidates. The missing satellites problem, cold dark matter simulations predict much larger numbers of small galaxies than are observed around galaxies like the Milky Way. Some of these problems have proposed solutions but it unclear whether they can be solved without abandoning the CDM paradigm. Fuzzy cold dark matter Meta-cold dark matter Dark matter Hot dark matter Warm dark matter Self-interacting dark matter Lambda-CDM model Modified Newtonian dynamics Bertone, particle Dark Matter, Observations, Models and Searches
37.
Warm dark matter
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The most common WDM candidates are sterile neutrinos and gravitinos. The WIMPs, when produced non-thermally could be candidates for dark matter. In general, however the thermally produced WIMPs are cold dark matter candidates and these particles are considered inert because they only have suppressed interactions with the Z boson. Sterile neutrinos with masses of a few keV are possible candidates for keVins, at temperatures below the electroweak scale their only interactions with standard model particles are weak interactions due to their mixing with ordinary neutrinos. Due to the smallness of the angle they are not overproduced because they freeze out before reaching thermal equilibrium. Their properties are consistent with astrophysical bounds coming from structure formation, in February 2014, different analyses have extracted from the spectrum of X-ray emissions observed by XMM-Newton, a monochromatic signal around 3.5 keV. This signal is coming from different galaxy clusters and several scenarios of dark matter can justify such a line. We can cite, for example, a 3.5 keV candidate annihilating into 2 photons, or a 7 keV dark matter particle decaying into a photon, IOP Science, Journal of Cosmology and Astroparticle Physics,1208016. The first star formation in WDM Universe W. B. Lin, D. H. Huang, X. Zhang, R. Brandenberger, Non-Thermal Production of WIMPs, particle Dark Matter, Observations, Models and Searches
38.
Gauge boson
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In particle physics, a gauge boson is a force carrier, a bosonic particle that carries any of the fundamental interactions of nature, commonly called forces. Elementary particles, whose interactions are described by a gauge theory, all known gauge bosons have a spin of 1. Therefore, all known gauge bosons are vector bosons, isolated gluons do not occur because they are color-charged and subject to color confinement. In a quantized gauge theory, gauge bosons are quanta of the gauge fields, consequently, there are as many gauge bosons as there are generators of the gauge field. In quantum electrodynamics, the group is U, in this simple case. In quantum chromodynamics, the more complicated group SU has eight generators, the three W and Z bosons correspond to the three generators of SU in GWS theory. For technical reasons involving gauge invariance, gauge bosons are described mathematically by field equations for massless particles, therefore, at a naïve theoretical level all gauge bosons are required to be massless, and the forces that they describe are required to be long-ranged. The conflict between this idea and experimental evidence that the weak and strong interactions have a short range requires further theoretical insight. According to the Standard Model, the W and Z bosons gain mass via the Higgs mechanism, in the Higgs mechanism, the four gauge bosons of the unified electroweak interaction couple to a Higgs field. This field undergoes spontaneous symmetry breaking due to the shape of its interaction potential, as a result, the universe is permeated by a nonzero Higgs vacuum expectation value. This VEV couples to three of the gauge bosons, giving them mass, the remaining gauge boson remains massless. This theory also predicts the existence of a scalar Higgs boson, the Georgi-Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons direct interactions between quarks and leptons, hence violating conservation of number and causing proton decay. Such bosons would be more massive than W and Z bosons due to symmetry breaking. Analysis of data collected from sources as the Super-Kamiokande neutrino detector has yielded no evidence of X and Y bosons. The fourth fundamental interaction, gravity, may also be carried by a boson, in the absence of experimental evidence and a mathematically coherent theory of quantum gravity, it is unknown whether this would be a gauge boson or not. The role of gauge invariance in general relativity is played by a similar symmetry, W and Z bosons refer to hypothetical new gauge bosons. 1964 PRL symmetry breaking papers Boson Glueball Quantum chromodynamics Quantum electrodynamics Explanation of gauge boson and gauge fields by Christopher T
. Bibcode:2013IJMPE..2230019D. doi:10.1142/S0218301313300191.
