Prospects in the search for a new light Z′ boson with the NA64μ experiment at the CERN SPS

Abstract

A light ${Z}^{\ensuremath{'}}$ vector boson coupled to the second and third lepton generations through the ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed $(g\ensuremath{-}2{)}_{\ensuremath{\mu}}$ anomaly. This boson can be produced in the bremsstrahlung reaction $\ensuremath{\mu}N\ensuremath{\rightarrow}\ensuremath{\mu}N{Z}^{\ensuremath{'}}$ after a high energy muon beam collides with a target. $\mathrm{NA}64\ensuremath{\mu}$ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for ${Z}^{\ensuremath{'}}$ production and its subsequent decays, ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}\mathrm{invisible}$. In this paper, we present the study of the $\mathrm{NA}64\ensuremath{\mu}$ sensitivity to search for such a boson. This includes a realistic beam simulation, a detailed description of the detectors 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, $\mathrm{NA}64\ensuremath{\mu}$ will be able to explore all the remaining parameter space which could provide an explanation for the $g\ensuremath{-}2$ muon anomaly in the ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ model.