Studies and Optimization for Future Beam Dump Experiments in the K12 and P42 Beams at CERN

During the beam operation years 2021-2025 at CERN the NA62 experiment is expected to reach its goal of making a more precise measurement of the branching ratio of the decay $K^+ \rightarrow \pi^+ \nu \bar{\nu}$ . There is a number of potential future experiments under consideration which could utilize the beam lines P42 and K12 that currently provide the beam for NA62. One possibility would be to operate the K12 beam line in a beam dump mode. The experiment taking data in such a beam line setting is called NA62 beam dump (NA62-BD) and would be able to use the NA62 detectors to search for dark matter decays downstream the beam dump thereby looking for physics beyond the Standard Model. Additionally, a possible off-axis experiment alongside the K12 beam line called SHADOWS is considered, which would be able to focus on the search for feebly interacting particles. Prior to these experiments preliminary studies need to be performed to optimize the setup and to reduce the backgrounds that blur the signals of the detectors. Therefore, simulations in the Geant4-based software BDSIM have been performed to gain insight on the expected backgrounds and to evaluate the possibilities for reducing them. The BDSIM model of the K12 beam line has been validated in beam dump mode and the simulation data is in good agreement with measurement data provided by NA62. It was confirmed that in beam dump mode the muon background will be the main background and that it needs to be minimized. An optimization study for the magnetic field configuration of BEND1, a dipole configuration near the beam dump in the K12 beam line, showed that the muon background for NA62-BD can be reduced by a factor of 20 compared to the setting that is currently used in $K^+$ mode. Furthermore, the simulations pointed out that the magnetic field optimization of BEND1 is not sufficient to minimize the muon background for SHADOWS and that an off-axis muon sweeping system will be necessary. Several possibilities for such a system have been evaluated and a configuration was found that achieves a muon background reduction by a factor of 7 compared to the simulations without a SHADOWS dedicated muon sweeping system.