New results on a search for a 33.9-MeV/c**2 neutral particle from pi+ decay in the NOMAD experiment

Abstract We report on a direct search in NOMAD for a new 33.9 MeV/ c 2 neutral particle( X ) produced in piondecay in flight, π → μX followed by the decay X → νe + e − . Both decays are postulated to occur to explain the time anomaly observed by the KARMENexperiment. From the analysis of the data collected during the 1996–1998 runs with 4.1×10 19 protons on target, a single candidate event consistent with background expectations was found. The search is sensitive to a pionbranching ratio BR( π → μX )>3.7×10 −15 , significantly smaller than previous experimental limits.