Search for a new gauge boson in the $A'$ Experiment (APEX)
  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 $e^+e^-$ 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 $A'\to e^+e^-$ reaction, and set an upper limit of $\alpha'/\alpha \simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces. 
S.Abrahamyan, Z.Ahmed, K.Allada, D.Anez, T.Averett, A.Barbieri,   K.Bartlett, J.Beacham, J.Bono, J.R.Boyce, P.Brindza, A.Cam...
hep-ex hep-ph nucl-ex