Paper detail

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

A light $Z'$ vector boson coupled to the second and third lepton generations through $L_μ-L_τ$ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed $(g-2)_μ$ anomaly. This boson can be produced in the bremsstrahlung reaction $μN \rightarrow μN Z'$ after a high energy muon beam collides with a target. NA64$μ$ 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$μ$ 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$μ$ will be able to explore all the remaining phase space which could provide an explanation for the $g-2$ muon anomaly.