Search for a new gauge boson in the $A^{\prime }$ Experiment (APEX)

Abstract

We present a search at Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling $\hat{I}{\pm }^{\prime }$ to electrons. Such a particle $A^{\prime }$ 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 $A^{\prime }\to e^{+}{e}^{-}$ reaction, and set an upper limit of $\hat{I}{\pm }^{\prime }/\hat{I}\pm \simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.

5 pages, 5 figures, references added