Search for a new gauge boson in electron-nucleus fixed-target scattering by the APEX experiment

Abstract

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling [alpha] 'to electrons. Such a particle [Alpha] can be produced in electron-nucleus fixed-target scattering and then decay to an e[superscript +] e[superscript -] pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175–250 MeV, found no evidence for an [Alpha] --> e[superscript +] e[superscript -] reaction, and set an upper limit of [alpha]/[alpha] = 10[superscript -6]. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.