Charge-conjugation (C) invariance demands that ortho- and parapositronium decay into an odd number and an even number of photons, respectively. For the test of the C invariance, we searched for a C-violating process, i.e., four-photon decay from orthopositronium, using a multi-γ-ray spectrometer. No C-violating process was observed leading to the upper limit as 2.6×${10}^{\mathrm{-}6}$ at 90% confidence level for the branching ratio of four- to three-photon decays from orthopositronium. © 1996 The American Physical Society.

• Received 26 January 1996

DOI:https://doi.org/10.1103/PhysRevA.54.1952