Search for a new gauge boson in ${\pi }^0$ decays

Abstract

A search was made for a new light gauge boson $X$ which might be produced in ${\pi }^0\to \gamma +X$ decay from neutral pions generated by 450-GeV protons in the CERN SPS neutrino target. The X's would penetrate the downstream shielding and be observed in the NOMAD detector via the Primakoff effect, in the process of $X\to {\pi }^0$ conversion in the external Coulomb field of a nucleus. With $1.45\times 10^{18}$ protons on target, 20 candidate events with energy between 8 and 140 GeV were found from the analysis of neutrino data. This number is in agreement with the expectation of 18.1$\pm$2.8 background events from standard neutrino processes. A new 90% C.L. upper limit on the branching ratio $Br({\pi }^0\to \gamma +X)<(3.3to1.9)\times 10^{-5}$ for $X$ masses ranging from 0 to 120 MeV/c^2 is obtained.

15 pages, LaTex, 6 eps figures included, submitted to Physics Letters B