Design studies for the beam-dump experiment DarkMESA

Abstract

At the Institute for Nuclear Physics in Mainz the new electron accelerator MESA will go into operation within the next years. In the extracted beam operation – with up to 155 MeV of beam energy and 150 μA of beam current – the P2 experiment will measure the weak mixing angle in electron-proton scattering. The high-power beam-dump of this experiment and the 20 000 hours of operation time are ideally suited for a parasitic dark sector experiment – DarkMESA. The experiment is designed for the detection of light dark matter. The model studied here is based on the coupling of dark matter to a massive vector particle, the dark photon γ'. It can potentially be produced in the P2 beam-dump by a process analogous to photon Bremsstrahlung and can then decay in dark matter particle pairs χ-χ-bar – if kinematically allowed. A fraction of them scatter off electrons or nuclei in the DarkMESA calorimeter, located in a well shielded area outside of the MESA hall. This thesis covers an evaluation of the accessible dark matter parameter space with a MadGraph and Geant4 simulation. For the calorimeter, high-density PbF₂ and lead glass SF5 Cherenkov radiators, and photomultipliers from previous experiments will be used. In Phase A a prototype with 25 PbF₂ crystals and in Phase B ∼ 2000 PbF₂ and SF5 crystals with a total active volume of 0.7 m³ are planned. The characterization of the crystals in laboratory and beam-time studies was part of this work, as well as the comparison of the experimental data with simulation studies. During a MAMI beam-time the suitability of these materials has been proven. For the prototype stage, a hermetic veto system with two layers of plastic scintillators and 1 cm of lead shielding is currently under development and a similar concept is foreseen for Phase B. In this thesis, the possibilities of background suppression with these concepts were investigated in simulation studies. The experimental setup was optimized and further concepts were investigated. An additional off-axis detector can further extend the accessible parameter range. This concept was evaluated in the context of this work as well as the possibilities of a negative-ion time projection chamber filled with CS₂ at low pressure serving as dark matter detector – DarkMESA-DRIFT. With the nuclear recoil threshold being in the keV range, the accessible parameter space can be extended to higher dark matter masses. The space to further increase the active volume of DarkMESA in a Phase C, based on traditional technologies or following an innovative concept, was also studied. The ability to perform experiments with different detector concepts and detection methods at one experimental site is a great advantage of the beam-dump experiment at MESA.