Prospects in the search for a new light $Z'$ boson with the NA64$\mu$ experiment at the CERN SPS

Abstract

A light $Z'$ vector boson coupled to the second and third lepton generations through $L_{\mu}-L_{\tau}$ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed $(g-2)_\mu$ anomaly. This boson can be produced in the bremsstrahlung reaction $\mu N \rightarrow \mu N Z'$ after a high energy muon beam collides with a target. NA64$\mu$ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for the $Z'$ production and its subsequent decays, $Z'\rightarrow invisible$. In this paper, we present the study of the NA64$\mu$ sensitivity to search for such a $Z'$. This includes a realistic beam simulation, the detailed detectors description and a discussion about the main potential background sources. A pilot run is scheduled in order to validate the simulation results. If those are confirmed, NA64$\mu$ will be able to explore all the remaining phase space which could provide an explanation for the $g-2$ muon anomaly.