Search for a new gauge boson in π0 decays

Abstract

A search was made for a new light gauge boson X which might be produced in π0→γ+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→π0 conversion in the external Coulomb field of a nucleus. With 1.45×1018 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±2.8 background events from standard neutrino processes. A new 90 %C.L. upper limit on the branching ratio Br(π0→γ+X)< (3.3 to 1.9)×10−5 for X masses ranging from 0 to 120 MeV/c2 is obtained.