Single-muon rate reduction for beam dump operation of the K12 beam line at CERN

The Physics Beyond Colliders study investigates the feasibility of future projects and experiments using CERN facilities. In the scope of this study, a future operation of the NA62 experiment using the existing K12 beam line operated in beam dump mode is discussed. Such a setup allows for a search for dark sector particles, e.g. heavy neutral leptons, dark photons and axions. Production of these hypothetical particles requires the 400GeV/c proton beam extracted from the Super Proton Synchrotron (SPS) to be dumped on a massive dump collimator located in the first part of the K12 beam line. The decay of these dark sector particles, for example, into muons could be detected by the NA62 experimental setup. However, muons created in particle interactions with material and from in-flight decays of secondary hadrons pose a critical background for this kind of experiment. Magnetic sweeping serves as an efficient tool for reducing this background. This paper discusses the simulation studies conducted with the software framework G4beamline to reduce the expected single-muon rate for this experiment. The required beam line modifications, in particular the removal of the beryllium target and repositioning of the dump, have been implemented into this model. A comparison of simulated muon distributions and spectra available experimental data, already taken in test runs, has been made in order to validate the beam line simulation. Finally, additional beam line modifications to further suppress the muon background are elaborated upon. These studies show a potential further reduction of a factor four of the total single-muon rate relative to the nominal setup over the full momentum range, and up to a factor twenty considering only muons with momenta p > 15GeV/c.