Search For A New Light Gauge Boson In Decays Of Pi(O) And Eta

We have searched for new light gauge bosons produced in pi0 and eta decays by studying the kinematically well-constrained reactions ppBAR --> pi0pi0pi0 and ppBAR --> pi0pi0eta, where one pi0 or the eta decays through the emission of a single photon recoiling against a missing state X (where X is a long-lived weakly interacting particle or X --> nunuBAR). No signal has been observed and branching ratio upper limits of 6 x 10(-5) at 90 % C.L. have been obtained for masses of the gauge boson lying between approximately 65 MeV and 125 MeV (pi0 decay), and 6 x 10(-5) at 90 % C.L., for X masses between approximately 200 MeV and 525 MeV (eta decay). The pi0-decay limit represents a factor of 4 to 8 improvement when compared to the existing limit, whereas the eta decay limit has been measured for the first time, thereby extending the m(X) range from 130 MeV up to 525 MeV.