In physics, metastability is a stable state of a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope. Bowling pins show similar metastability by either wobbling for a moment or tipping over completely. A common example of metastability in science is isomerisation. Higher energy isomers are long lived as they are prevented from rearranging to their preferred ground state by barriers in the potential energy. During a metastable state of finite lifetime, all state-describing parameters reach and hold stationary values. In isolation: the state of least energy is the only one the system will inhabit for an indefinite length of time, until more external energy is added to the system; the metastability concept originated in the physics of first-order phase transitions. It acquired new meaning in the study of aggregated subatomic particles or in molecules, macromolecules or clusters of atoms and molecules.
It was borrowed for the study of decision-making and information transmission systems. Many complex natural and man-made systems can demonstrate metastability. Metastability is common in physics and chemistry – from an atom to statistical ensembles of molecules at molecular levels or as a whole; the abundance of states is more prevalent as the systems grow larger and/or if the forces of their mutual interaction are spatially less uniform or more diverse. In dynamic systems like electronic circuits, signal trafficking, decisional systems and neuroscience – the time-invariance of the active or reactive patterns with respect to the external influences defines stability and metastability. In these systems, the equivalent of thermal fluctuations in molecular systems is the "white noise" that affects signal propagation and the decision-making. Non-equilibrium thermodynamics is a branch of physics that studies the dynamics of statistical ensembles of molecules via unstable states. Being "stuck" in a thermodynamic trough without being at the lowest energy state is known as having kinetic stability or being kinetically persistent.
The particular motion or kinetics of the atoms involved has resulted in getting stuck, despite there being preferable alternatives. Metastable states of matter range from melting solids, boiling liquids and sublimating solids to supercooled liquids or superheated liquid-gas mixtures. Pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations or condensing seed doping initiates crystallization centers; this is a common situation for the droplets of atmospheric clouds. Metastable phases are common in condensed matter. For example, diamond is a metastable form of carbon at standard pressure, it can be converted to graphite, but only after overcoming an activation energy – an intervening hill. Martensite is a metastable phase used to control the hardness of most steel; the bonds between the building blocks of polymers such as DNA, RNA, proteins are metastable. Adenosine triphosphate is a metastable molecule, colloquially described as being "full of energy" that can be used in many ways in biology.
Metastable polymorphs of silica are observed. In some cases, such as in the allotropes of solid boron, acquiring a sample of the stable phase is difficult. Speaking, emulsions/colloidal systems and glasses are metastable. Sandpiles are one system which can exhibit metastability if tunnel is present. Sand grains form a pile due to friction, it is possible for an entire large sand pile to reach a point where it is stable, but the addition of a single grain causes large parts of it to collapse. The avalanche is a well-known problem with large piles of ice crystals on steep slopes. In dry conditions, snow slopes act to sandpiles. An entire mountainside of snow can slide due to the presence of a skier, or a loud noise or vibration. Aggregated systems of subatomic particles described by quantum mechanics are found to have many distinguishable states. Of these, one is indefinitely stable: global minimum. All other states besides the ground state have higher energies. Of all these other states, the metastable states are the ones having lifetimes lasting at least 102 to 103 times longer than the shortest lived states of the set.
A metastable state is long-lived but not eternal. Being excited – of an energy above the ground state – it will decay to a more stable state, releasing energy. Indeed, above absolute zero, all states of a system have a non-zero probability to decay. One mechanism for this to happen is through tunnelling; some energetic states of an atomic nucleus are much longer-lived than other (nuclear isomers of the same isoto
A neutrino is a fermion that interacts only via the weak subatomic force and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small that it was long thought to be zero; the mass of the neutrino is much smaller than that of the other known elementary particles. The weak force has a short range, the gravitational interaction is weak, neutrinos, as leptons, do not participate in the strong interaction. Thus, neutrinos pass through normal matter unimpeded and undetected. Weak interactions create neutrinos in one of three leptonic 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 that there are three discrete neutrino masses with different tiny values, but they do not correspond uniquely to the three flavors. A neutrino created with a specific flavor is in an associated specific quantum superposition of all three mass states.
As a result, 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. Although only differences of squares of the three mass values are known as of 2016, cosmological observations imply that the sum of the three masses must be less than one millionth that of the electron. For each neutrino, there exists a corresponding antiparticle, called an antineutrino, which has half-integer spin and no electric charge, they are distinguished from the neutrinos by having opposite signs of lepton chirality. To conserve total lepton number, in nuclear beta decay, electron neutrinos appear together with only positrons or electron-antineutrinos, electron antineutrinos with electrons or electron neutrinos. Neutrinos are created by various radioactive decays, including in beta decay of atomic nuclei or hadrons, nuclear reactions such as those that take place in the core of a star or artificially in nuclear reactors, nuclear bombs or particle accelerators, during a supernova, in the spin-down of a neutron star, or when accelerated particle beams or cosmic rays strike atoms.
The majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. In the vicinity of the Earth, about 65 billion solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun. For study, neutrinos can be created artificially with nuclear reactors and particle accelerators. There is intense research activity involving neutrinos, with goals that include the determination of the three neutrino mass values, the measurement of the degree of CP violation in the leptonic sector. Neutrinos can be used for 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 and angular momentum. In contrast to Niels Bohr, who proposed a statistical version of the conservation laws to explain the observed continuous energy spectra in beta decay, Pauli hypothesized an undetected particle that he called a "neutron", using the same -on ending employed for naming both the proton and the electron.
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 much more massive neutral nuclear particle in 1932 and named it a neutron leaving two kinds of particles with the same name. Earlier Pauli had used the term "neutron" for both the neutral particle that conserved energy in beta decay, a presumed neutral particle in the nucleus; the word "neutrino" entered the scientific vocabulary through Enrico Fermi, who used it during a conference in Paris in July 1932 and at the Solvay Conference in October 1933, where Pauli employed it. The name was jokingly coined by Edoardo Amaldi during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron. In Fermi's theory of beta decay, Chadwick's large neutral particle could decay to a proton and the smaller neutral particle: n0 → p+ + e− + νeFermi's paper, written in 1934, unified Pauli's neutrino with Paul Dirac's positron and Werner Heisenberg's neutron–proton model and gave a solid theoretical basis for future experimental work.
The journal Nature rejected Fermi's paper, saying that the theory was "too remote from reality". He submitted the paper to an Italian journal, which accepted it, but the general lack of interest in his theory at that early date caused him to switch to experimental physics. By 1934 there was experimental evidence against Bohr's idea that energy conservation is invalid for beta decay: At the Solvay conference of that year, measurements of the energy spectra of beta particles were reported, showing that there is a strict limit on the energy of electrons from each type of beta decay; such a limit is not expected if the conservation of energy is invalid, in which case any amount of energy would be statistically available in at least a few decays. The natural explanation of the beta decay spectrum as first measured in 1934 was that only a limited amount of en
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1 e, a spin of 1/2, has the same mass as an electron; when a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more gamma ray photons. Positrons can be created by positron emission radioactive decay, or by pair production from a sufficiently energetic photon, interacting with an atom in a material. In 1928, Paul Dirac published a paper proposing that electrons can have both a positive and negative charge; this paper introduced the Dirac equation, a unification of quantum mechanics, special relativity, the then-new concept of electron spin to explain the Zeeman effect. The paper did not explicitly predict a new particle but did allow for electrons having either positive or negative energy as solutions. Hermann Weyl published a paper discussing the mathematical implications of the negative energy solution.
The positive-energy solution explained experimental results, but Dirac was puzzled by the valid negative-energy solution that the mathematical model allowed. Quantum mechanics did not allow the negative energy solution to be ignored, as classical mechanics did in such equations. However, no such transition had yet been observed experimentally. Dirac wrote a follow-up paper in December 1929 that attempted to explain the unavoidable negative-energy solution for the relativistic electron, he argued that "... an electron with negative energy moves in an external field as though it carries a positive charge." He further asserted that all of space could be regarded as a "sea" of negative energy states that were filled, so as to prevent electrons jumping between positive energy states and negative energy states. The paper explored the possibility of the proton being an island in this sea, that it might be a negative-energy electron. Dirac acknowledged that the proton having a much greater mass than the electron was a problem, but expressed "hope" that a future theory would resolve the issue.
Robert Oppenheimer argued against the proton being the negative-energy electron solution to Dirac's equation. He asserted that if it were, the hydrogen atom would self-destruct. Persuaded by Oppenheimer's argument, Dirac published a paper in 1931 that predicted the existence of an as-yet-unobserved particle that he called an "anti-electron" that would have the same mass and the opposite charge as an electron and that would mutually annihilate upon contact with an electron. Feynman, earlier Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation. Electrons moving backward in time would have a positive electric charge. Wheeler invoked this concept to explain the identical properties shared by all electrons, suggesting that "they are all the same electron" with a complex, self-intersecting worldline. Yoichiro Nambu applied it to all production and annihilation of particle-antiparticle pairs, stating that "the eventual creation and annihilation of pairs that may occur now and is no creation or annihilation, but only a change of direction of moving particles, from the past to the future, or from the future to the past."
The backwards in time point of view is nowadays accepted as equivalent to other pictures, but it does not have anything to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description. Dmitri Skobeltsyn first observed the positron in 1929. While using a Wilson cloud chamber to try to detect gamma radiation in cosmic rays, Skobeltsyn detected particles that acted like electrons but curved in the opposite direction in an applied magnetic field. In 1929 Chung-Yao Chao, a graduate student at Caltech, noticed some anomalous results that indicated particles behaving like electrons, but with a positive charge, though the results were inconclusive and the phenomenon was not pursued. Carl David Anderson discovered the positron on 2 August 1932, for which he won the Nobel Prize for Physics in 1936. Anderson did not coin the term positron, but allowed it at the suggestion of the Physical Review journal editor to whom he submitted his discovery paper in late 1932.
The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge; the ion trail left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge ratio of an electron, but in a direction that showed its charge was positive. Anderson wrote in retrospect that the positron could have been discovered earlier based on Chung-Yao Chao's work, if only it had been followed up on. Frédéric and Irène Joliot-Curie in Paris had evidence of positrons in old photographs when Anderson's results came out, but they had dismissed them as protons; the positron had been contemporaneously discovered by Patrick Blackett and Giuseppe Occhialini at the Cavendish Laboratory in 1932. Blackett and Occhialini had delayed publication to obtain more solid evidence, so Anderson was able to publish the discovery first.
Positrons are produced in β+ decays of occurring radioactive isotopes and in interactions of gamma quanta with matter. Antineutrinos a
In the physical sciences, a particle is a small localized object to which can be ascribed several physical or chemical properties such as volume, density or mass. They vary in size or quantity, from subatomic particles like the electron, to microscopic particles like atoms and molecules, to macroscopic particles like powders and other granular materials. Particles can be used to create scientific models of larger objects depending on their density, such as humans moving in a crowd or celestial bodies in motion; the term'particle' is rather general in meaning, is refined as needed by various scientific fields. Something, composed of particles may be referred to as being particulate. However, the noun'particulate' is most used to refer to pollutants in the Earth's atmosphere, which are a suspension of unconnected particles, rather than a connected particle aggregation; the concept of particles is useful when modelling nature, as the full treatment of many phenomena can be complex and involve difficult computation.
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, they strip the baseball of most of its properties, by first idealizing it as a rigid smooth sphere by neglecting rotation and friction reducing the problem to the ballistics of a classical point particle. The treatment of large numbers of particles is the realm of statistical physics; the term "particle" is applied differently to three classes of sizes. The term macroscopic particle refers to particles much larger than atoms and molecules; these are abstracted as point-like particles though they have volumes, structures, etc. Examples of macroscopic particles would include powder, sand, pieces of debris during a car accident, or objects as big as the stars of a galaxy. Another type, microscopic particles refers to particles of sizes ranging from atoms to molecules, such as carbon dioxide and colloidal particles.
These particles are studied in chemistry, as well as molecular physics. The smallest of particles are the subatomic particles; these would include particles such as the constituents of atoms – protons and electrons – as well as other types of particles which can only be produced in particle accelerators or cosmic rays. These particles are studied in particle physics; because of their small size, the study of microscopic and subatomic particles fall in the realm of quantum mechanics. They will exhibit phenomena demonstrated in the particle in a box model, including wave–particle duality, whether particles can be considered distinct or identical is an important question in many situations. Particles can be classified according to composition. Composite particles refer to particles that have composition –, particles which are made of other particles. For example, a carbon-14 atom is made of six protons, eight neutrons, six electrons. By contrast, elementary particles refer to particles. According to our current understanding of the world, only a small number of these exist, such as leptons and gluons.
However it is possible that some of these might turn up to be composite particles after all, appear to be elementary for the moment. While composite particles can often be considered point-like, elementary particles are punctual. Both elementary and composite particles, are known to undergo particle decay; those that do not are called stable particles, such as a helium-4 nucleus. The lifetime of stable particles can be either infinite or large enough to hinder attempts to observe such decays. In the latter case, those particles are called "observationally stable". In general, a particle decays from a high-energy state to a lower-energy state by emitting some form of radiation, such as the emission of photons. In computational physics, N-body simulations are simulations of dynamical systems of particles under the influence of certain conditions, such as being subject to gravity; these simulations are common in cosmology and computational fluid dynamics. N refers to the number of particles considered.
As simulations with higher N are more computationally intensive, systems with large numbers of actual particles will be approximated to a smaller number of particles, simulation algorithms need to be optimized through various methods. Colloidal particles are the components of a colloid. A colloid is a substance microscopically; such colloidal system can be liquid, or gaseous. The dispersed-phase particles have a diameter of between 5 and 200 nanometers. Soluble particles smaller. Colloidal systems are the subject of colloid science. Suspended solids may be held in a liquid, while solid or liquid particles suspended in a gas together form an aerosol. Particles may be suspended in the form of atmospheric particulate matter, which may constitute air pollution. Larger particles can form marine debris or space debris. A conglomeration of discrete solid, macroscopic particles may be described as a granular material. Particles portal "What is a particle?". University of Florida, Particle Engineering Resea
The electron is a subatomic particle, symbol e− or β−, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, are thought to be elementary particles because they have no known components or substructure; the electron has a mass, 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant, ħ; as it is a fermion, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light; the wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy. Electrons play an essential role in numerous physical phenomena, such as electricity, magnetism and thermal conductivity, they participate in gravitational and weak interactions.
Since an electron has charge, it has a surrounding electric field, if that electron is moving relative to an observer, it will generate a magnetic field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law. Electrons absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields. Special telescopes can detect electron plasma in outer space. Electrons are involved in many applications such as electronics, cathode ray tubes, electron microscopes, radiation therapy, gaseous ionization detectors and particle accelerators. Interactions involving electrons with other subatomic particles are of interest in fields such as chemistry and nuclear physics; the Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms.
Ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the main cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of an indivisible quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge'electron' in 1891, J. J. Thomson and his team of British physicists identified it as a particle in 1897. Electrons can participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of radioactive isotopes and in high-energy collisions, for instance when cosmic rays enter the atmosphere; the antiparticle of the electron is called the positron. When an electron collides with a positron, both particles can be annihilated, producing gamma ray photons.
The ancient Greeks noticed. Along with lightning, this phenomenon is one of humanity's earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electrica, to refer to those substances with property similar to that of amber which attract small objects after being rubbed. Both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον. In the early 1700s, Francis Hauksbee and French chemist Charles François du Fay independently discovered what they believed were two kinds of frictional electricity—one generated from rubbing glass, the other from rubbing resin. From this, du Fay theorized that electricity consists of two electrical fluids and resinous, that are separated by friction, that neutralize each other when combined. American scientist Ebenezer Kinnersley also independently reached the same conclusion. A decade Benjamin Franklin proposed that electricity was not from different types of electrical fluid, but a single electrical fluid showing an excess or deficit.
He gave them the modern charge nomenclature of negative respectively. Franklin thought of the charge carrier as being positive, but he did not identify which situation was a surplus of the charge carrier, which situation was a deficit. Between 1838 and 1851, British natural philosopher Richard Laming developed the idea that an atom is composed of a core of matter surrounded by subatomic particles that had unit electric charges. Beginning in 1846, German physicist William Weber theorized that electricity was composed of positively and negatively charged fluids, their interaction was governed by the inverse square law. After studying the phenomenon of electrolysis in 1874, Irish physicist George Johnstone Stoney suggested that there existed a "single definite quantity of electricity", the charge of a monovalent ion, he was able to estimate the value of this elementary charge e by means of Faraday's laws of electrolysis. However, Stoney could not be removed. In 1881, German physicist Hermann von Helmholtz argued that both positive and negative charges were divided into elementary parts, each of which "behaves like atoms of electricity".
Stoney coined the term
A pentaquark is a subatomic particle consisting of four quarks and one antiquark bound together. As quarks have a baryon number of +1/3, antiquarks of −1/3, the pentaquark would have a total baryon number of 1, thus would be a baryon. Further, because it has five quarks instead of the usual three found in regular baryons, it would be classified as an exotic baryon; the name pentaquark was coined by Claude Gignoux et al. and Harry J. Lipkin in 1987. Although predicted for decades, pentaquarks proved difficult to discover and some physicists were beginning to suspect that an unknown law of nature prevented their production; the first claim of pentaquark discovery was recorded at LEPS in Japan in 2003, several experiments in the mid-2000s reported discoveries of other pentaquark states. Others were not able to replicate the LEPS results and the other pentaquark discoveries were not accepted because of poor data and statistical analysis. On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons.
Outside particle physics laboratories, pentaquarks could be produced by supernovae as part of the process of forming a neutron star. The scientific study of pentaquarks might offer insights into how these stars form, as well as allowing more thorough study of particle interactions and the strong force. A quark is a type of elementary particle that has mass, electric charge, colour charge, as well as an additional property called flavour, which describes what type of quark it is. Due to an effect known as colour confinement, quarks are never seen on their own. Instead, they form composite particles known as hadrons. Hadrons made of one quark and one antiquark are known as mesons, while those made of three quarks are known as baryons; these ` regular' hadrons are well characterized. A wide variety of pentaquarks are possible, with different quark combinations producing different particles. To identify which quarks compose a given pentaquark, physicists use the notation qqqqq, where q and q refer to any of the six flavours of quarks and antiquarks.
The symbols u, d, s, c, b, t stand for the up, strange, charm and top quarks with the symbols of u, d, s, c, b, t corresponding to the respective antiquarks. For instance a pentaquark made of two up quarks, one down quark, one charm quark, one charm antiquark would be denoted uudcc; the quarks are bound together by the strong force, which acts in such a way as to cancel the colour charges within the particle. In a meson, this means a quark is partnered with an antiquark with an opposite colour charge – blue and antiblue, for example – while in a baryon, the three quarks have between them all three colour charges – red and green. In a pentaquark, the colours need to cancel out, the only feasible combination is to have one quark with one colour, one quark with a second colour, two quarks with the third colour, one antiquark to counteract the surplus colour; the binding mechanism for pentaquarks is not yet clear. They may consist of five quarks bound together, but it is possible that they are more loosely bound and consist of a three-quark baryon and a two-quark meson interacting weakly with each other via pion exchange in a "meson-baryon molecule".
The requirement to include an antiquark means that many classes of pentaquark are hard to identify experimentally – if the flavour of the antiquark matches the flavour of any other quark in the quintuplet, it will cancel out and the particle will resemble its three-quark hadron cousin. For this reason, early pentaquark searches looked for particles. In the mid-2000s, several experiments claimed to reveal pentaquark states. In particular, a resonance with a mass of 1540 MeV/c2 was reported by LEPS in 2003, the Θ+; this coincided with a pentaquark state with a mass of 1530 MeV/c2 predicted in 1997. The proposed state was composed of two up quarks, two down quarks, one strange antiquark. Following this announcement, nine other independent experiments reported seeing narrow peaks from nK+ and pK0, with masses between 1522 MeV/c2 and 1555 MeV/c2, all above 4 σ. While concerns existed about the validity of these states, the Particle Data Group gave the Θ+ a 3-star rating in the 2004 Review of Particle Physics.
Two other pentaquark states were reported albeit with low statistical significance—the Φ−−, with a mass of 1860 MeV/c2 and the Θ0c, with a mass of 3099 MeV/c2. Both were found to be statistical effects rather than true resonances. Ten experiments looked for the Θ+, but came out empty-handed. Two in particular had nearly the same conditions as other experiments which claimed to have detected the Θ+; the 2006 Review of Particle Physics concluded: here has not been a high-statistics confirmation of any of the original experiments that claimed to see the Θ+.
Positronium is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom an onium. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states; the orbit and energy levels of the two particles are similar to that of the hydrogen atom. However, because of the reduced mass, the frequencies of the spectral lines are less than half of the corresponding hydrogen lines; the mass of positronium is 1.022 MeV, twice the electron mass minus the binding energy of a few eV. The ground state of positronium, like that of hydrogen, has two possible configurations depending on the relative orientations of the spins of the electron and the positron; the singlet state, 1S0, with antiparallel spins is known as para-positronium. It has a mean lifetime of 0.125 ns and decays preferentially into two gamma rays with energy of 511 keV each. By detecting these photons the position at which the decay occurred can be determined.
This process is used in positron-emission tomography. Para-positronium can decay into any number of photons, but the probability decreases with the number: the branching ratio for decay into 4 photons is 1.439×10−6. Para-positronium lifetime in vacuum is t 0 = 2 ℏ m e c 2 α 5 = 0.1244 n s. The triplet state, 3S1, with parallel spins is known as ortho-positronium, it has a mean lifetime of 142.05±0.02 ns, the leading decay is three gammas. Other modes of decay are negligible. Ortho-positronium lifetime in vacuum can be calculated as: t 1 = 1 2 9 h 2 m e c 2 α 6 = 138.6 n s. However more accurate calculations with corrections to order O yield a value of 7.040 μs−1 for the decay rate, corresponding to a lifetime of 142 ns. Positronium in the 2S state is metastable having a lifetime of 1100 ns against annihilation; the positronium created in such an excited state will cascade down to the ground state, where annihilation will occur more quickly. Measurements of these lifetimes and energy levels have been used in precision tests of quantum electrodynamics, confirming quantum electrodynamics predictions to high precision.
Annihilation can proceed via a number of channels, each producing gamma rays with total energy of 1022 keV 2 or 3, with up to 5 recorded. The annihilation into a neutrino–antineutrino pair is possible, but the probability is predicted to be negligible; the branching ratio for o-Ps decay for this channel is 6.2×10−18 and 9.5×10−21 in predictions based on the Standard Model, but it can be increased by non-standard neutrino properties, like high magnetic moment. The experimental upper limits on branching ratio for this decay are <4.3×10−7 for p-Ps and <4.2×10−7 for o-Ps. While precise calculation of positronium energy levels uses the Bethe–Salpeter equation or the Breit equation, the similarity between positronium and hydrogen allows a rough estimate. In this approximation, the energy levels are different because of a different effective mass, m*, in the energy equation: E n = − μ q e 4 8 h 2 ε 0 2 1 n 2, where: qe is the charge magnitude of the electron, h is Planck's constant, ε0 is the electric constant, μ is the reduced mass: μ = m e m p m e + m p = m e 2 2 m e = m e 2, where me and mp are the mass of the electron and the positron.
Thus, for positronium, its reduced mass only differs from the electron by a factor of 2. This causes the energy levels to roughly be half of what they are for the hydrogen atom. So the energy levels of positronium are given by E n = −