Cosmological Bound on the Decay $π^0\rightarrowγX$

Abstract

Using the upper bound on the effective number of light neutrino species during primordial nucleosynthesis and the cosmological pion-pole mechanism $γγ\rightarrow π^0\rightarrow γX$, we obtain an upper limit on the branching ratio for the decay BR$(π^0\rightarrow γX)<3\times 10^{-13}$, where $X$ is any long-lived weakly interacting neutral vector particle with mass smaller than the neutral pion mass.