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 alpha' to electrons. Such a particle A' can be produced in electron-nucleus fixed-target scattering and then decay to an e(+)e(-) 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(+)e(-) reaction, and set an upper limit of alpha'/alpha similar or equal to 10(-6). Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.