Design and characterisation of the CsI Crystal Calorimeter, Cosmic Ray Tagger and SiPM front-end electronics for the Mu2e experiment at Fermilab

The Mu2e experiment at Fermi National Accelerator Laboratory will investigate Charged Lepton Flavour Violation by searching for coherent muon to electron conversion in the coulomb field of an Al nucleus. Although allowed due to neutrino oscillations, this conversion is suppressed by the Standard Model with an expected BR < 10^-54. Therefore, the observation of this process would be a clear evidence of New Physics beyond the SM. Over three years of run time, over 6x10^17 muons will be stopped on the Mu2e target, allowing a single event sensitivity of approximately 2.5×10^-17, probing 4 orders of magnitude beyond the current best experimental limit of 7×10^&#8722;13 @ 90% CL set by SINDRUM II. Mu2e will produce a high intensity pulsed negative muon beam, generated via the interaction of 8 GeV proton bunches on a W target. By means of a dedicated 28 metres long Nb-Ti superconducting solenoid system, low-momentum negative muons are selected and transported to the Detector Solenoid, where they are stopped on an Al target and undergo nuclear capture. The 104.96 MeV signature of the conversion electrons will be identified by a complementary measurement carried out by a high-resolution straw-tube tracker and the electromagnetic calorimeter. The calorimeter has a high granularity, high energy resolution (&#963;E/E < 10%) and fast timing (&#963;t < 500 ps). It is composed of 1348 undoped CsI crystals, each read by two custom UV-extended SiPMs, arranged in two annular disks. The EMC will need to maintain extremely high levels of stability and in the harsh Mu2e operating environment, for radiation exposure up to 15 krad/y and for neutron fluency up to 10^11 1MeV_neq/cm2. The calorimeter design has been validated through an electron beam test on a large-scale 51-crystals prototype at the LNF BTF facility. An extensive test campaign has been carried out in order to characterise and verify the performance of crystals, photodetectors and of the front-end electronics, including hardware stress tests and irradiation campaigns using both neutrons and gamma. A Cosmic Ray Tagger system has also been developed at LNF, to allow each disk to be individually calibrated using cosmic rays prior to installation. The CRT system will provide 3D MIP track reconstruction capabilities, in order to allow the equalisation and calibration of the energy response of all channels to a level below 1% against the expected 21 MeV MIP peak deposit. The CRT consists of two sub-modules, featuring a single layer of 8 parallel 1600 mm long bars made of EJ-200 plastic scintillator with a dual-sided Mu2e-SiPM readout. The scintillating elements and the relative readout system have been characterised at the LNF laboratories, showing a timing resolution better than 100 ps. A template fit algorithm was used for timing reconstruction and time-of-flight difference was used for the reconstruction of the interaction point, with a spatial resolution relative to the axial coordinate better than 6 mm. The final verification steps of the front-end system, including the complete characterisation of the optical and electronic performance of the hardware are reported, with focus on the most recent irradiation tests carried out with an intense 60-Co &#611;-source at ENEA Calliope facility. Furthermore, the work relative to the design and characterisation of the CRT system is included, starting from the first R&D stages, up to the characterisation steps of the instrument with cosmic rays.