Search for long-lived neutral resonances in Bhabha scattering around 1.8 MeV/c2

A search has been carried out for long-lived neutral particles which could be created in resonant ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ scattering, with an invariant mass around 1.8 MeV/${\mathit{c}}^2$. A monoenergetic positron beam was scattered from a thin Be foil, and an active-shadow technique was used to suppress elastic ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ scattering. No evidence was found for neutral particles decaying dominantly into ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ pairs, in the lifetime region between 4.5×${10}^{\mathrm{-}13}$ and 7.5×${10}^{\mathrm{-}12}$ s (95% C.L.). This result, together with our previous work, eliminates recent theoretical models that have introduced new neutral particles in this mass region.