Cosmological Bound on the Decay ${\pi }^0\to \gamma X$

Abstract

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

Comments: 7 pages, IP-ASTP-03-93