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

We present a search at 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{sup +}e{sup -} 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' {yields} e{sup +}e{sup -} reaction, and set an upper limit of {alpha}'/{alpha} {approx} 10{sup -6}. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.