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

TL;DR

This study searches for a hypothetical light gauge boson produced in neutral pion decays, setting new upper limits on its production rate based on data from CERN SPS neutrino experiments.

Contribution

It provides the first experimental constraints on a new gauge boson in pion decays using high-energy proton interactions and neutrino detector data.

Findings

No evidence of the gauge boson was observed.

Set new upper limits on the branching ratio of pion decay involving the gauge boson.

Excluded certain parameter spaces for the gauge boson mass and coupling.

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 $1.45\times10^{18}$ 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 $Br(\pi^{0}\to\gamma + X)< (3.3 to 1.9) \times10^{-5}$ for $X$ masses ranging from 0 to 120 MeV/c^2 is obtained.