Constraints on couplings of a fast decaying axion

Abstract

Constraints are imposed on the couplings of an axion-like pseudoscalar particle φ that decays into e +e − and γγ. The electron g−2 sets a severe bound not only on the φe +e − coupling but also on the φγγ coupling. The most reliable bound on the φ coupling to the light quarks comes from the decay ∑ + → pe +e − which was measured in a hydrogen bubble-chamber experiment with good sensitivity and resolution covering the entire range of e +e − mass practically down to 2 m e. These upper bounds, together with a constraint from the decay K +→π +γγ, reinforce arguments against the particle-physics interpretation of the anomalous positrons in heavy-ion collisions, since the couplings are by orders of magnitude smaller than needed to produce e +e − pairs at the observed level by known mechanisms.