Search for a new gauge boson in the $A'$ Experiment (APEX)

TL;DR

This paper reports a search for a new sub-GeV gauge boson in electron-nucleus scattering at Jefferson Laboratory, setting upper limits on its coupling and demonstrating the potential of fixed-target experiments to explore new force mediators.

Contribution

First search for a new gauge boson in the 175-250 MeV range using fixed-target data, establishing new constraints on its coupling to electrons.

Findings

No evidence for the $A'$ particle was observed.

Set an upper limit of $rac{ ext{alpha}'}{ ext{alpha}} oughly 10^{-6}$.

Demonstrated fixed-target experiments' capability to probe new sub-GeV forces.

Abstract

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.