Search for weakly interacting neutral bosons produced in ${\mathrm{\pi }}^{\mathrm{-}}$p interactions at rest and decaying into ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs

We present the result of a search for weakly interacting neutral bosons (X) produced in ${\mathrm{\pi }}^{\mathrm{-}}$p interactions at rest and decaying into ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs. The data sample searched contains 98 400 ${\mathrm{\pi }}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$γ decays and 27 200 ${\mathrm{\pi }}^{\mathrm{-}}$p→${\mathit{ne}}^{+}$${\mathit{e}}^{\mathrm{-}}$ events, each with an ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ invariant mass between 25 and 139 MeV/${\mathit{c}}^2$. They were observed in the SINDRUM I magnetic spectrometer at the Paul Scherrer Institute (Switzerland). We measure upper limits for the branching ratios Γ(${\mathrm{\pi }}^0$→Xγ, X→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)/Γ(${\mathrm{\pi }}^0$→all) and Γ(${\mathrm{\pi }}^{\mathrm{-}}$p→Xn, X→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)/Γ(${\mathrm{\pi }}^{\mathrm{-}}$p→all) for X lifetimes between ${10}^{\mathrm{-}23}$ and ${10}^{\mathrm{-}11}$ s.