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

We have searched for the pion decay ${\pi }^{+}\to {\mu }^{+}X$, 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 $\eta \le 6.0\times {10}^{-10\mathrm{}}$ of such a decay at a 95% confidence level.