Positronium

Onia Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically 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 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).

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, 1S0, with antiparallel spins (S = 0, Ms = 0) is known as para-positronium (p-Ps).

It has a mean lifetime of 0.12 ns and decays preferentially into two gamma rays with energy of 511 keV each (in the center-of-mass frame).

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.

[1] These states have a mean lifetime of 142.05±0.02 ns,[2] and the leading decay is three gammas.

However more accurate calculations with corrections to O(α2) yield a value of 7.040 μs−1 for the decay rate, corresponding to a lifetime of 142 ns.

Measurements of these lifetimes and energy levels have been used in precision tests of quantum electrodynamics, confirming quantum electrodynamics (QED) predictions to high precision.

[1][7][8] Annihilation can proceed via a number of channels, each producing gamma rays with total energy of 1022 keV (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 branching ratio for o-Ps decay for this channel is 6.2×10−18 (electron neutrino–antineutrino pair) and 9.5×10−21 (for other flavour)[3] in predictions based on the Standard Model, but it can be increased by non-standard neutrino properties, like relatively high magnetic moment.

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.

But if one adds the ⁠1/c2n⁠ (or α2n, where α is the fine-structure constant) terms, where n = 1,2..., then the result is relativistically invariant.

The α2 contribution is the Breit term; workers rarely go to α4 because at α3 one has the Lamb shift, which requires quantum electrodynamics.

[9] 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.

Approximately:[12][13] 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".

[15] Other sources incorrectly credit Carl Anderson as having predicted its existence in 1932 while at Caltech.

[16] 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.

Corrections that involved higher orders were then calculated in a non-relativistic quantum electrodynamics.

[4] In 2024, the AEgIS collaboration at CERN was the first to cool positronium by laser light, leaving it available for experimental use.

[22] The first observation of di-positronium (Ps2) molecules—molecules consisting of two positronium atoms—was reported on 12 September 2007 by David Cassidy and Allen Mills from University of California, Riverside.

[23][24][25] Unlike muonium, positronium does not have a nucleus analogue, because the electron and the positron have equal masses.

[26] 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.

An electron and positron orbiting around their common centre of mass . An s state has zero angular momentum, so orbiting around each other would mean going straight at each other until the pair of particles is either scattered or annihilated, whichever occurs first. This is a bound quantum state known as positronium .
The Positronium Beam at University College London , a lab used to study the properties of positronium. [ 14 ]