Stringent limits on theπ0→γX,X→e+e−decay from neutrino experiments and constraints on new light gauge bosons

Abstract

We report new limits on the ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}X$ decay of the neutral pion into a photon and a new gauge boson $X$ followed by the decay $X\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$. If this process exists, one would expect a flux of high energy $X$'s produced from ${\ensuremath{\pi}}^{0}$'s generated by the proton beam in a neutrino target. The $X$'s would then penetrate the downstream shielding and be observed in a neutrino detector via their decays. Using bounds from the NOMAD and PS191 neutrino experiments at CERN that searched for an excess of ${e}^{+}{e}^{\ensuremath{-}}$ pairs from heavy neutrino decays, stringent limits on the branching ratio as small as $\mathrm{Br}({\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}X)\ensuremath{\lesssim}{10}^{\ensuremath{-}15}$ are obtained. These limits are several orders of magnitude smaller than the previous experimental and cosmological bounds. The obtained results are used to constrain models, where the $X$ interacts with quarks and leptons, or it is a new vector boson mixing with photons that transmits interaction between our world and hidden sectors consisting of $SU(3{)}_{C}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}U(1{)}_{Y}$ singlet fields.