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

Abstract

A light Z′ vector boson coupled to the second and third lepton generations through the Lμ-Lτ 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→μNZ′ after a high energy muon beam collides with a target. NA64μ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for Z′ production and its subsequent decays, Z′→invisible. In this paper, we present the study of the NA64μ 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, NA64μ will be able to explore all the remaining parameter space which could provide an explanation for the g-2 muon anomaly in the Lμ-Lτ model.