The expected search sensitivity for μ^- → e^- at Mu2e in Run I

The standard model (SM) predicts lepton flavor to be a conserved quantity of nature. With the discovery of neutrino oscillations which violate lepton flavor, the SM can be extended to include neutrino mass terms that explicitly violate lepton flavor conservation. These neutrino mass terms lead to charged lepton flavor violation (CLFV) due to loop-level interactions, but the CLFV rates are significantly suppressed by the small neutrino masses to well below an experimentally accessible level (< ∼10^-50). New physics models can predict much higher rates of CLFV which are experimentally observable. The observation of a CLFV process would be unambiguous evidence of beyond the SM interactions.

The Mu2e experiment at FNAL will search for the CLFV process of neutrinoless muon to electron conversion in the field of a nucleus, μ^- → e^-, using an aluminum target. The current best experimental limit on this process comes from the SINDRUM-II experiment using a gold target: R(μ^- Au → e^- Au) < 7 x 10^-13 (90% CL), where R(μ^- → e^-) is the μ^- → e^- rate divided by the muon capture rate. The Mu2e experiment aims to improve the experimental sensitivity to μ^- → e^- by three orders of magnitude in its first data-taking run, and an overall four orders of magnitude in total.

I will present the main backgrounds and their expected impacts and uncertainties, focusing on Run I of the Mu2e experiment. I will present the current Mu2e expected discovery and exclusion sensitivity during Run I, comparing counting and spectrum fit approaches. I will also discuss the impact of systematic uncertainties on the experimental upper limit in the absence of a signal.