The KARMEN Time Anomaly: Search for a Neutral Particle of Mass 33.9 MeV in Pion Decay

TL;DR

This study searched for a hypothetical neutral particle of 33.9 MeV mass in pion decay to explain a time anomaly, but found no evidence and set a strict upper limit on its occurrence.

Contribution

The paper provides the first experimental search for a neutral particle of this specific mass in pion decay, constraining its possible branching fraction.

Findings

No evidence for the proposed decay process was observed.

An upper limit on the decay's branching fraction was established as 6.0 x 10^{-10}.

Results challenge the hypothesis that this particle explains the KARMEN anomaly.

Abstract

We have searched for the pion decay pi^+ --> mu^+ X, where X is a neutral particle of mass 33.905 MeV. This process was suggested by the KARMEN Collaboration to explain an anomaly in their observed time distribution of neutrino induced reactions. Having measured the muon momentum spectrum of charged pions decaying in flight, we find no evidence for this process and place an upper limit on the branching fraction eta leq 6.0 * 10^{-10} of such a decay at a 95% confidence level.