The Mu2e electromagnetic calorimeter : design, qualification, assembly and physics studies

The Mu2e experiment at Fermi National Accelerator Laboratory will investigate Charged Lepton Flavour Violation (CLFV), providing a four order of magnitude improvement on the experimental limit for coherent µ≠ æ e≠ conversions in the field of Al nuclei. Although allowed, in the SM this conversion is strongly suppressed with an expected BR < 10≠54. Therefore, any direct observation of this process would be a clear evidence of New Physics. Mu2e will employ the most powerful, intense, and pure negative muon beam currently available. The muon beam will be produced, transported, and focused on the stopping target (ST) thanks to a state-of-the-art superconducting magnetic system. Particles produced in the interactions with the ST will be analysed by a high precision straw-tube tracker, in conjunction with an electromagnetic calorimeter, both operated, in high-vacuum, inside the magnetic system. This setup will allow the ecient identification of 105 MeV conversion electrons (CEs), which would be the signature of CLFV processes. Over three years of run time, more than 6 · 1017 negative muons will be stopped on the Mu2e target, allowing a single event sensitivity better than 3 · 10≠17. This result will guarantee the aforementioned four orders of magnitude improvement beyond the current best experimental limit, set by the SINDRUM II experiment. The Mu2e experiment is currently under commissioning, with first beams scheduled in 2025. Mu2e is a demanding experiment in the Intensity Frontier sector, requiring cutting-edge techniques in beam production and monitoring, along with highperformance detectors to ensure high sensitivity, power background rejection, and overall reliability in challenging operating scenarios, to ensure ecient identification of rare physics processes. Several dedicated studies were performed to achieve these goals, as detailed below. After an intense R&D phase, the final calorimeter design was extensively validated, allowing the Mu2e experiment to successfully move towards detector assembly and commissioning phases. Dedicated characterisation campaigns included tests with electron beams and cosmic rays performed on a large scale prototype. These tests were instrumental for the development of reconstruction, analysis and calibration techniques. The resulting performances were proved to be fully compliant with the Mu2e requirements. A calorimeter-based stopping rate monitor, employing high-performance crystals, was developed to provide a high-accuracy independent measurement of the experimental normalisation. The system was successfully validated for its operation in the Mu2e experiment. Dedicated studies on the radiative muon capture (RMC) processes were performed, providing several strategies to measure the spectrum of the outgoing photon. The relevance of RMC processes as a background for Mu2e conversion searches wasalso studied and discussed.