Positronium
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Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. 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 energy levels of the two particles are similar to that of the hydrogen atom (which is a bound state of a proton and an electron). However, because of the reduced mass, the frequencies of the spectral lines are less than half of those for the corresponding hydrogen lines.
StatesEdit
The mass of positronium is 1.022 MeV, which is twice the electron mass minus the binding energy of a few eV. The lowest energy orbital state of positronium is 1S, and like with hydrogen, it has a hyperfine structure arising from the relative orientations of the spins of the electron and the positron.
The singlet state, Template:SubatomicParticle, with antiparallel spins (S = 0, Ms = 0) is known as para-positronium (p-Ps). It has a mean lifetime of Template:Val and decays preferentially into two gamma rays with energy of Template:Val each (in the center-of-mass frame). Para-positronium can decay into any even number of photons (2, 4, 6, ...), but the probability quickly decreases with the number: the branching ratio for decay into 4 photons is Template:Val.<ref name="hep-ph0310099"> Template:Cite journal</ref>
Para-positronium lifetime in vacuum is approximately<ref name="hep-ph0310099"/> <math display="block">t_0 = \frac{2 \hbar}{m_\mathrm{e} c^2 \alpha^5} = 0.1244 ~\mathrm{ns}.</math>
The triplet states, 3S1, with parallel spins (S = 1, Ms = −1, 0, 1) are known as ortho-positronium (o-Ps), and have an energy that is approximately 0.001 eV higher than the singlet.<ref name="hep-ph0310099"/> These states have a mean lifetime of Template:Val,<ref name="Badertscher"> Template:Cite journal</ref> and the leading decay is three gammas. Other modes of decay are negligible; for instance, the five-photons mode has branching ratio of ≈Template:Val.<ref name="hep-ph9911410"> Template:Cite book</ref>
Ortho-positronium lifetime in vacuum can be calculated approximately as:<ref name="hep-ph0310099"/> <math display="block">t_1 = \frac{\frac{1}{2} 9 h}{2 m_\mathrm{e} c^2 \alpha^6 (\pi^2 - 9)} = 138.6 ~\mathrm{ns}.</math>
However more accurate calculations with corrections to O(α2) yield a value of Template:Val−1 for the decay rate, corresponding to a lifetime of Template:Val.<ref name=Kat/><ref name=adk>Template:Cite journal</ref>
Positronium in the 2S state is metastable having a lifetime of Template:Val against annihilation.<ref name="atom-ph150305755"> Template:Cite journal</ref> The positronium created in such an excited state will quickly cascade down to the ground state, where annihilation will occur more quickly.
MeasurementsEdit
Measurements of these lifetimes and energy levels have been used in precision tests of quantum electrodynamics, confirming quantum electrodynamics (QED) predictions to high precision.<ref name="hep-ph0310099" /><ref> Template:Cite journal</ref><ref> Template:Cite journal</ref>
Annihilation can proceed via a number of channels, each producing gamma rays with total energy of Template:Val (sum of the electron and positron mass-energy), usually 2 or 3, with up to 5 gamma ray photons recorded from a single annihilation.
The annihilation into a neutrino–antineutrino pair is also possible, but the probability is predicted to be negligible. The branching ratio for o-Ps decay for this channel is Template:Val (electron neutrino–antineutrino pair) and Template:Val (for other flavour)<ref name="hep-ph9911410" /> in predictions based on the Standard Model, but it can be increased by non-standard neutrino properties, like relatively high magnetic moment. The experimental upper limits on branching ratio for this decay (as well as for a decay into any "invisible" particles) are <Template:Val for p-Ps and <Template:Val for o-Ps.<ref name="Badertscher" />
Energy levelsEdit
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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, μ, in the energy equation (see electron energy levels for a derivation): <math display="block">E_n = -\frac{\mu q_\mathrm{e}^4}{8 h^2 \varepsilon_0^2} \frac{1}{n^2},</math> where:
- Template:Math is the charge magnitude of the electron (same as the positron),
- Template:Mvar is the Planck constant,
- Template:Math is the electric constant (otherwise known as the permittivity of free space),
- Template:Mvar is the reduced mass: <math display="block">\mu = \frac{m_\mathrm{e} m_\mathrm{p}}{m_\mathrm{e} + m_\mathrm{p}} = \frac{m_\mathrm{e}^2}{2m_\mathrm{e}} = \frac{m_\mathrm{e}}{2},</math> where Template:Math and Template:Math are, respectively, the mass of the electron and the positron (which are the same by definition as antiparticles).
Thus, for positronium, its reduced mass only differs from the electron by a factor of 2. This causes the energy levels to also roughly be half of what they are for the hydrogen atom.
So finally, the energy levels of positronium are given by <math display="block"> E_n = -\frac{1}{2} \frac{m_\mathrm{e} q_\mathrm{e}^4}{8 h^2 \varepsilon_0^2} \frac{1}{n^2} = \frac{-6.8~\mathrm{eV}}{n^2}.</math>
The lowest energy level of positronium (Template:Math) is Template:Val. The next level is Template:Val. The negative sign is a convention that implies a bound state. Positronium can also be considered by a particular form of the two-body Dirac equation; Two particles with a Coulomb interaction can be exactly separated in the (relativistic) center-of-momentum frame and the resulting ground-state energy has been obtained very accurately using finite element methods of Janine Shertzer.<ref name="Shertzer"/> Their results lead to the discovery of anomalous states.<ref> Template:Cite journal</ref><ref> Template:Cite journal</ref> The Dirac equation whose Hamiltonian comprises two Dirac particles and a static Coulomb potential is not relativistically invariant. But if one adds the Template:Math (or Template:Math, where Template:Mvar is the fine-structure constant) terms, where Template:Math, then the result is relativistically invariant. Only the leading term is included. The Template:Math contribution is the Breit term; workers rarely go to Template:Math because at Template:Math one has the Lamb shift, which requires quantum electrodynamics.<ref name="Shertzer"> Template:Cite journal</ref>
Formation and decay in materialsEdit
After a radioactive atom in a material undergoes a β+ decay (positron emission), the resulting high-energy positron slows down by colliding with atoms, and eventually annihilates with one of the many electrons in the material. It may however first form positronium before the annihilation event. The understanding of this process is of some importance in positron emission tomography. Approximately:<ref name="Harpen2003">Template:Cite journal</ref><ref name="pmid30641509">Template:Cite journal</ref>
- ~60% of positrons will directly annihilate with an electron without forming positronium. The annihilation usually results in two gamma rays. In most cases this direct annihilation occurs only after the positron has lost its excess kinetic energy and has thermalized with the material.
- ~10% of positrons form para-positronium, which then promptly (in ~0.12 ns) decays, usually into two gamma rays.
- ~30% of positrons form ortho-positronium but then annihilate within a few nanoseconds by 'picking off' another nearby electron with opposing spin. This usually produces two gamma rays. During this time, the very lightweight positronium atom exhibits a strong zero-point motion, that exerts a pressure and is able to push out a tiny nanometer-sized bubble in the medium.
- Only ~0.5% of positrons form ortho-positronium that self-decays (usually into three gamma rays). This natural decay rate of ortho-positronium is relatively slow (~140 ns decay lifetime), compared to the aforementioned pick-off process, which is why the three-gamma decay rarely occurs.
HistoryEdit
The Croatian physicist Stjepan Mohorovičić predicted the existence of positronium in a 1934 article published in Astronomische Nachrichten, in which he called it the "electrum".<ref> Template:Cite journal</ref> Other sources incorrectly credit Carl Anderson as having predicted its existence in 1932 while at Caltech.<ref name="DeutschObit"> Template:Cite press release</ref> It was experimentally discovered by Martin Deutsch at MIT in 1951 and became known as positronium.<ref name="DeutschObit"/> Many subsequent experiments have precisely measured its properties and verified predictions of quantum electrodynamics.
A discrepancy known as the ortho-positronium lifetime puzzle persisted for some time, but was resolved with further calculations and measurements.<ref>Template:Cite news</ref> Measurements were in error because of the lifetime measurement of unthermalised positronium, which was produced at only a small rate. This had yielded lifetimes that were too long. Also calculations using relativistic quantum electrodynamics are difficult, so they had been done to only the first order. Corrections that involved higher orders were then calculated in a non-relativistic quantum electrodynamics.<ref name="Kat">Template:Cite journal</ref>
In 2024, the AEgIS collaboration at CERN was the first to cool positronium by laser light, leaving it available for experimental use. The substance was brought to Template:Convert using laser cooling.<ref> Template:Cite journal</ref><ref>Template:Cite news</ref>
Exotic compoundsEdit
Molecular bonding was predicted for positronium.<ref> Template:Cite journal</ref> Molecules of positronium hydride (PsH) can be made.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Positronium can also form a cyanide and can form bonds with halogens or lithium.<ref name="Saito">Template:Cite journal</ref>
The first observation of di-positronium (Template:Chem2) molecules—molecules consisting of two positronium atoms—was reported on 12 September 2007 by David Cassidy and Allen Mills from University of California, Riverside.<ref> Template:Cite journal</ref><ref> Template:Cite journal</ref><ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Unlike muonium, positronium does not have a nucleus analogue, because the electron and the positron have equal masses.<ref name=barnabas/> Consequently, while muonium tends to behave like a light isotope of hydrogen,<ref>Template:Cite journal</ref> positronium shows large differences in size, polarisability, and binding energy from hydrogen.<ref name=barnabas>Template:Cite journal</ref>
Natural occurrenceEdit
The events in the early universe leading to baryon asymmetry predate the formation of atoms (including exotic varieties such as positronium) by around a third of a million years, so no positronium atoms occurred then.
Likewise, the naturally occurring positrons in the present day result from high-energy interactions such as in cosmic ray–atmosphere interactions, and so are too hot (thermally energetic) to form electrical bonds before annihilation.
See alsoEdit
- Breit equation
- Antiprotonic helium
- Di-positronium
- Exciton — solid-state analog
- Protonium
- Quantum electrodynamics
- Two-body Dirac equations
- Quarkonium