Proposed test of charge-conjugation invariance in positronium decay

Abstract

If the electromagnetic interactions are invariant under charge conjugation, C, then singlet positronium (Ps) can only decay into an even number of photons, while triplet Ps can only decay into an odd number of photons. In this paper we consider a model Hamiltonian which violates C invariance but conserves parity. If λ is the coupling constant in the proposed C-noninvariant interaction, then the branching ratio of the forbidden decay, 1³S₁→4λ, to the allowed decay, 1³S¹→3λ, has been found to be F_T^4λ=1.2×10^-5λ². An experimental upper limit on F_T^4λ would provide a novel check of C invariance in electromagnetic interactions, and would also yield new information on the e^- eγγγγ interaction. At present, indirect limits on F_T^4λ show it to be less than about 10^-2. We are now working on an experiment which is designed to reduce this upper limit to 10^-5-10^-6. If successful, it will subject the proposed interaction, or any similar interaction leading to 1³S₁→4γ, to a stringent test. An outline of the experiment is included in this paper.