Paul Scherrer Inst, CH-5232 Villigen, Switzerland
; ETH Zurich, IPP, Inst Teilchenphys, CH-5232 Villigen, Switzerland
; Univ Virginia, Dept Phys, Charlottesville, VA 22901 USA
; Acad Sinica, Inst High Energy Phys, Beijing 100039, Peoples R China
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 less than or equal to 6.0 x 10(-10) of such a decay at a 95% confidence level.