Study of the calorimetric detection of the muon to electron conversion in the Mu2e experiment

The Mu2e experiment will search for Charged Lepton Flavor Violation (CLFV), looking at the coherent conversion of a muon into an electron in the field of an aluminum nucleus. The knowledge of such a CLFV reaction allows to indirectly probe new physics at energy scales up to thousands of TeV, inaccessible with direct searches at either present or planned high energy colliders. For this reason, Mu2e will measure the muon-to-electron conversion rate R_{\mu e} with an unprecedented accuracy, so to improve of a factor 10^4 the best current measurement and, in case of no observation, to constrain its value below 6 x 10^-17 at 90% of CL. To reach this ambitious sensitivity, about 10^18 muonic atom decays have to be observed: Mu2e is expected to use an intense pulsed muon beam, and rely on a detector system composed of a straw tube tracker and an electromagnetic calorimeter. The calorimeter is composed of 1348 un-doped CsI crystals, each coupled to two large area Silicon Photomultipliers (SiPMs). It plays a central role in the Mu2e measurement, providing particle identification capabilities that are necessary to reject the cosmic muons and antiprotons induced background. Moreover, the calorimeter has to help the tracker providing a seed for the pattern recognition and to provide a fast independ trigger. Having these experimental requests as pivotal reference, a set of Quality Assurance (QA) criteria for the calorimeter active components have been defined. Following the corresponding QA procedures, a first batch of crystals and photosensors has been characterized and used to assemble a medium scale prototype of the calorimeter (Module-0). The Module-0 has been studied by means of a 100 MeV electron beam, confirming that expected calorimeter performances well satisfy the Mu2e requirements.