Editing Positronium (section)

=== Measurements ===
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>
{{cite journal
 |last1=Rubbia | first1=A.
 |date=2004
 |title=Positronium as a probe for new physics beyond the standard model
 |doi=10.1142/S0217751X0402021X
 |journal=International Journal of Modern Physics A
 |volume=19
 |issue=23
 |pages=3961–3985
 |arxiv=hep-ph/0402151
 |bibcode = 2004IJMPA..19.3961R | citeseerx=10.1.1.346.5173
 | s2cid=119442567
}}</ref><ref>
{{cite journal
 |last1=Vetter |first1=P.A. |last2=Freedman |first2=S.J.
 |date=2002
 |title=Branching-ratio measurements of multiphoton decays of positronium
 |journal=[[Physical Review A]]
 |volume=66 |pages=052505
 |doi=10.1103/PhysRevA.66.052505
 |bibcode = 2002PhRvA..66e2505V
 |issue=5 |osti=821022 |url=https://digital.library.unt.edu/ark:/67531/metadc736097/
}}</ref>

Annihilation can proceed via a number of channels, each producing [[gamma rays]] with total energy of {{val|1022|ul = 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 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 {{val|6.2|e=-18}} ([[electron neutrino]]–antineutrino pair) and {{val|9.5|e=-21}} (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 <{{val|4.3|e=-7}} for ''p''-Ps and <{{val|4.2|e=-7}} for ''o''-Ps.<ref name="Badertscher" />