Testing the Efficiency of the Cosmic Ray Veto at the Mu2e Experiment*

The Mu2e experiment at Fermilab will search for the charged-lepton flavor-violating process of coherent muon-to-electron conversion in the field of an aluminum nucleus. A crucial source of background comes from cosmic-ray muons, requiring a Cosmic Ray Veto (CRV) system capable of achieving an efficiency greater than 99.99%. The CRV is composed of scintillator counters read out by silicon photomultipliers and front-end electronics, with layered geometry to allow coincidence tagging of through-going muons. We analyze data from several CRV modules, accompanied by a recently fabricated muon tagger system positioned above and below the modules. By applying photoelectron thresholds, fit quality requirements, and layer-based coincidence criteria, we measure the module's inefficiency and study the dependence on selection cuts. Preliminary results demonstrate consistency with the design efficiency goals. This work provides an essential validation step for the CRV and informs strategies for ensuring robust background rejection in the full Mu2e detector.