Probing hidden leptonic scalar portals using the NA64 experiment at CERN

In this study, we demonstrate the potential of the NA64 experiment at CERN SPS to search for New Physics processes involving e → μ transitions after the collision of 100 GeV electrons with target nuclei. A new Dark Sector leptonic portal in which a scalar boson ϕ could be produced in the lepton-flavor-changing bremsstrahlung-like reaction, eN → μNϕ, is used as benchmark process. In this work, we develop a realistic Monte Carlo simulation of the NA64 experimental setup implementing the differential and total production cross-section computed at exact tree-level and applying the Weiszäcker–Williams phase space approxima tion. Using this framework, we investigate the main back ground sources and calculate the expected sensitivity of the experiment. The results indicate that with minor setup opti mization, NA64 can probe a large fraction of the available parameter space compatible with the muon g − 2 anomaly and the Dark Matter relic predictions in the context of a new Dark Sector leptonic portal with 1011 EOT. This result paves the way to the exploration of lepton-flavour-changing transitions in NA64