Search for short-lived axions in an electron-beam-dump experiment

We report results of an electron-beam-dump search for neutral particles with masses in the range 1 to 15 MeV and lifetimes \ensuremath{\tau} between ${10}^{\mathrm{\ensuremath{-}}14}$ and ${10}^{\mathrm{\ensuremath{-}}10}$ s. No evidence was found for such an object. We fule out the existence of any 1.8-MeV pseudoscalar boson with \ensuremath{\tau}>8.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ s and an absorption cross section in matter less than 1 mb per nucleon, and exclude \ensuremath{\tau}>1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ s were its cross section to equal 50 mb per nucleon. In conjunction with measurements of the electron's anomalous magnetic moment, this experiment shows that the narrow positron peaks observed in heavy-ion collisions at the Gessellschaft fu\ifmmode\ddot\else\textasciidieresis\fi{}r Schwerionenforschung are not due to an elementary pseudoscalar.