A High Efficiency Cosmic Ray Veto Detector for the Mu2e Experiment

The Mu2e Experiment, at the Fermi National Accelerator Laboratory, will search for the coherent, neutrinoless conversion of stopped muons into electrons, a charged lepton flavor-changing process highly suppressed and hence undetectable in the Standard Model. Many scenarios for physics beyond the Standard Model predict small but observable rates. The sensitivity of this experiment is a factor of 10$^{\mathrm{4}}$ improvement over the current limit. One source of background is cosmic rays that can produce one event per day that would look like a muon to electron conversion. A Cosmic Ray Veto that surrounds the Mu2e spectrometer will be used to identify and reject such cosmic-ray induced events. It must have an overall efficiency of 99.99{\%} over an area of some 330 m$^{\mathrm{2}}$. A description of the Cosmic Ray Veto, its anticipated performance, and status will be presented.