Search for weakly interacting neutral bosons produced in π-p interactions at rest and decaying into e+e- pairs

We present the result of a search for weakly interacting neutral bosons (X) produced in π^-p interactions at rest and decaying into e⁺e^- pairs. The data sample searched contains 98 400 π⁰-->e⁺e^-γ decays and 27 200 π^-p-->ne⁺e^- events, each with an e⁺e^- invariant mass between 25 and 139 MeV/c². They were observed in the SINDRUM I magnetic spectrometer at the Paul Scherrer Institute (Switzerland). We measure upper limits for the branching ratios Γ(π⁰-->Xγ, X-->e⁺e^-)/Γ(π⁰-->all) and Γ(π^-p-->Xn, X-->e⁺e^-)/Γ(π^-p-->all) for X lifetimes between 10^-23 and 10^-11 s.