Probing the $L_\mu$-$L_\tau$ Gauge Boson at the MUonE Experiment

TL;DR

This paper explores the potential of the MUonE experiment to detect a light $L__$ gauge boson that could explain the muon g-2 anomaly, by analyzing specific scattering events and background suppression methods.

Contribution

It demonstrates the feasibility of probing the $L__$ gauge boson at MUonE without additional equipment, focusing on event signatures and background reduction strategies.

Findings

Approximately 1000 signal events expected in the relevant parameter space.

Background processes can be effectively suppressed with kinematic cuts and photon veto.

Detection of the $Z'$ is feasible within the muon g-2 motivated parameter region.

Abstract

We discuss the prospects of probing the $L_\mu - L_\tau$ gauge boson at the MUonE experiment. The $L_\mu - L_\tau$ gauge boson $Z^\prime$ with a mass of $\lesssim 200$ MeV, which can explain the discrepancy between the measured value of the muon $g-2$ and the value calculated in the Standard Model, can be produced at the MUonE experiment through the process $\mu e \to \mu e Z^\prime$. The $Z^\prime$ in the final state decays into a pair of neutrinos, and therefore we cannot observe the decay of $Z^\prime$ directly. It is, however, still possible to probe this signature by searching for events with a large scattering angle of muon and a less energetic final-state electron. The background events coming from the elastic scattering $\mu e \to \mu e$ as well as radiative process $\mu e \to \mu e \gamma$ can be removed by the kinematical cuts on the muon scattering angle and the electron energy, in addition to a photon veto. The background events from the electroweak process $\mu e \to \mu e \nu \bar{\nu}$ are negligible. With our selection criteria, the number of signal events $\mu e \to \mu e Z^\prime$ is found to be as large as $\sim 10^3$ in the parameter region motivated by the muon $g-2$ discrepancy. It is, therefore, quite feasible to probe the $L_\mu - L_\tau$ gauge boson at the MUonE experiment -- without introducing additional devices -- and we strongly recommend recording the events relevant to this $Z^\prime$ production process.