The Mu2e experiment at Fermilab aims to search for the SM forbidden μ− → e− conversion in Al muonic atoms. The signal signature consists of 104.96 MeV electrons, identified by a straw-tube tracker and a crystal calorimeter, made of two annular disks. In order to calibrate the calorimeter disks with minimum ionizing particles (MIP) before the installation, we have realized a Cosmic Ray Tagger (CRT) at Laboratori Nazionali di Frascati (LNF) of INFN. The CRT consists of two planes of eight 2.5×1.5×160 cm3 plastic scintillator (EJ-200) bars, coupled to SiPMs on both edges, so as to estimate longitudinal hit positions from time differences. 3D MIP tracking is achieved by reconstructing hit positions in the two planes, placed above and below the disks, and allows to calibrate the energy response, to align the time offsets, and to study the detector performances dependence along the crystals axis.

{"references": ["N. Atanov, et al., The Mu2e e.m. calorimeter: Crystals and sipms production status, IEEE Transactions on Nuclear Science 67 (6) (2020) 978\u2013982. doi:10.1109/TNS.2020.2988422.", "N. Atanov, et al., The Mu2e calorimeter final technical design report, arXiv e-prints (2018) arXiv:1802.06341"]}