New results on a search for a 33.9 MeV/c2 neutral particle from ϖ+ decay in the NOMAD experiment

We report on a direct search in NOMAD for a new 33.9 MeV/c2 neutral particle (X) produced in pion decay 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 KARMEN experiment. From the analysis of the data collected during the 1996–1998 runs with 4.1 × 1019 protons on target, a single candidate event consistent with background expectations was found. The search is sensitive to a pion branching ratio BR(π →μX) > 3.7× 10−15, significantly smaller than previous experimental limits.