THE MU2E CALORIMETER: R&D AND CALIBRATION STRATEGIES

Abstract

The Mu2e experiment at Fermi National Laboratory will search for Charged Lepton Flavor Violation (CLFV), looking for the conversion of a muon into an electron in the field of an aluminum nucleus. About 6 × 1017 muons, provided by a dedicated muon beam line, will be stopped in the aluminum target in three years of running. The experiment single event sensitivity will be 3 × 10−17 [1]. This process is forbidden in the Standard Model [2]. When considering diagrams with neutrinos oscillation, the process is allowed but the expected rate is negligible (BR ∼ 10−52). Therefore observation of this process would be a clear evidence of New Physics beyond the Standard Model. Several extensions of the Standard Model predict a rate in the range of 10−14 − 10−18 [3]. The current best experimental limit, set by the SINDRUM II experiment, is 7 × 10−13 at 90% C.L.. The Mu2e experiment plans to improve this limit by four orders of magnitude to test many of the possible Standard Model extensions. To reach this ambitious goal, the Mu2e experiment will use an intense pulsed muon beam and a detector system composed of a very precise straw tube tracker and a calorimeter made of pure CsI crystals. The work of my PhD thesis focused on the Mu2e calorimeter R&D phase and on the experimental tests needed to proove that the requirements are satisfied. The calorimetric information on time, energy and position are needed to provide Particle Identification capability and can also be used to improve the track reconstruction performance. After the long R&D phase, a final down select of the calorimeter components was done: undoped CsI as scintillating crystals and a custom array of UV extended SiPMs as photosensor. A large size calorimeter prototype (Module-0) was tested with an electron beam in the energy range 60-120 MeV. This test demonstrated that the proposed detector satisfies the Mu2e requirements both for timing and energy response and resolution (σT < 500 ps, and σE/E<10%, at 100 MeV). I contributed personally on the construction phase and on the data analysis of the test beam data with an emphasis on timing reconstruction and determination of the timing resolution. Module-0 is now used to test all technical calorimeter functionality in vacuum and at low temperature. I am personally in charge of the Module-0 data taking for measuring the detector performance in special conditions, like radiation exposure. After completing the Module-0 test beam, I worked on the development of the Quality Assurance stations for crystals and sensors and on the direct test of their quality. The calorimeter production phase started on March 2018 and the results found so far for the CsI and SiPM properties are excellent. We expect to conclude production in late spring 2019, in order to complete the detector assembly for middle 2020. During the commissioning and then the physics run, an accurate equalization and calibration of all calorimeter chan nels will be needed in order to obtain the expected performance. Two calibration methods exploiting the main ex perimental source of backgrounds have been developed and discussed in this thesis: (i) The cosmic ray muons based calibration will provide an energy equalization using the specific energy loss (∼21 MeV) in the calorimeter cells, with an estimated precision of ∼1.5%. Moreover cosmic muons tracks will be used to align the time offsets between the channels. This can be performed with an expected accuracy (estimated with the RMS) lower than 90 ps, which is an acceptable value with respect to the achievable time resolution [4]; (ii) Electrons coming from muons decay in orbit (DIO) in the Stopping Target can provide an additional calibration source. The energy calibration procedure is based on the comparison between the DIO electron cluster energies, E, and the electron track momenta measured in the tracker, P. At the nominal 1 T magnetic field, this method has to deal with a very strong radial dependence of the occupancy and therefore with a low statistics at higher calorimeter disks radii. To provide a more uniform coverage and high statistical samples, the magnetic field should be reduced from 1 T to 0.5 T, during dedicated calibration runs. A Monte Carlo study demonstrated that a calibration accuracy of about ∼0.3% is achievable at 0.5 T. At least two additional runs at reduced magnetic fields need to be done in order to extrapolate the E/P calibration up to 1 T. As the behaviour of E/P versus the magnetic field and electron momentum is well modelled, a high-statistics set of datasets can provide a calibration accuracy of ∼0.2%.