Positronium (Ps), the positron-electron bound state, is the lightest known atom, which is bounded and self-annihilates through the same, electromagnetic interaction. At the current level of experimental and theoretical precision this is the only interaction present in this system.
This feature has made positronium an ideal system for testing the accuracy of QED calculations for bound states, in particular for the triplet (1³S₁) state of Ps, orthopositronium (o-Ps). Due to the odd-parity under C-transformation o-Ps decays predominantly into three photons. As compared with the singlet (1¹S₀) state (parapositronium), the "slowness" of o-Ps decay rate, due to the phase-space and additional α suppression factors, gives an enhancement factor of about 10³, making it more sensitive to an admixture of new interactions which are not accommodated in the Standard Model.

Positronium was discovered experimentally by Deutsch in 1951, who observed its decay in different gases. Since then, a lot of focus has been set on the determination of its basic properties like decay life time, decay modes, spectroscopy, etc. In particular, the measurement of the o-Ps lifetime caught much attention.

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