Search for a New Gauge Boson in Electron-Nucleus Fixed-Target Scattering by the APEX Experiment

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling α′ to electrons. Such a particle A′ 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 A′→e[superscript +]e[superscript -] reaction, and set an upper limit of α′/α≃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.