We report a new measurement of the positronium (Ps) 2 S 3 1→ 2 P 3 0 interval. Slow Ps
atoms, optically excited to the radiatively metastable 2 S 3 1 level, flew through a microwave
radiation field tuned to drive the transition to the short-lived 2 P 3 0 level, which was detected
via the time spectrum of subsequent ground state Ps annihilation radiation. After accounting
for Zeeman shifts we obtain a transition frequency ν 0= 18 501.02±0.61 MHz, which is not in
agreement with the theoretical value of ν 0= 18 498.25±0.08 MHz.

Since positronium (Ps) atoms are composed only of leptons they are, for all practical
purposes, pure QED systems and can therefore be used to perform rigorous tests of bound-
state QED theory. Moreover, since the theoretical description is limited only by the order of
the calculations performed, rather than unknown physical constants or incalculable terms,
any observed (and confirmed) disagreement with theory could indicate the existence
of``new physics''such as particles or fields not currently included in the Standard Model. In …