Performance of Mu2e Cosmic Ray Veto Detector Modules

Charged Lepton Flavor Violation (CLFV) processes provide model-independent probes of new physics. The Mu2e experiment at Fermilab will search for the CLFV neutrino-less muon-to-electron conversion in the presence of aluminum nuclei, through the detection of a single monoenergetic electron of 105 MeV energy. The Mu2e experiment aims for a single-event sensitivity of 3×10^-17, setting an upper limit for the conversion-to-capture ratio of 8×10^-17 at 90% CL. This would improve the current experimental limit by four orders of magnitude. A major experiment background comes from cosmic rays, which can produce electrons indistinguishable from the signature. To reach the desired sensitivity, a cosmic ray veto (CRV) system will be installed to veto the cosmic rays. The plastic-scintillator-based CRV is designed to have a veto efficiency of 99.99%. We report on the CRV's performance using fabricated CRV detector components, confirming that it can reach the designed performance.