Search for Lepton Flavor Violation in Two Body Muon and Pion Decay at Rest

The ability of the Mu2e experiment to probe, or discover beyond the Standard Model physics in direct Charged Lepton Flavor Violation µ + and π + decay modes is estimated. These direct modes are searched for simultaneously with proposed Mu2e detector validation runs, and are complementary to the Mu2e main search goal, an indirect search for µ − → e − conversion at the sensitivity level of ∼ 10−17. The µ + validation run will operate at 50% nominal magnetic field and reduced proton beam intensity to less than 1/100th nominal, in order to observe the e+ spectrum from µ + decay, at and below the Michel edge Ee . 53 MeV. The π+ validation run, based on measuring the mono-energetic e+ emission in the decay π + → e +ν, at 76% of nominal magnetic field and reduced beam intensity less than 1/5 th nominal. Both of these runs can be used to fix the momentum scale for the Mu2e conversion search. In addition the muon validation dataset can be used to correct for systematic errors in the detector response by mapping the well known to O(α 3 ) corrected theoretical Michel spectrum, to the observed spectrum. One direct search is for two-body Charged Lepton Flavor Violation µ + → e +X decay, where X is a light new physics particle. This allows Mu2e to explore well motivated models including axion like particles with flavor violating couplings and massive Z 0 bosons with more sensitivity then present astrophysical and laboratory constraints. In two weeks of data-taking, Mu2e can achieve direct mode 90% confidence level branching ratio limits of 10−7 derived from 3×1013 µ+ stops, over the mass range 20 ≤ mX ≤ 50 MeV, improving the current experimental limit at 10−5 by two orders of magnitude. In the mass range mX ≤ 20 MeV, assuming systematic error corrections can be made by correcting the Monte Carlo mapping, the achievable search sensitivity is found to be 2.3×10−7 for mX =0, an order of magnitude improvement over the current best limit at 2.6×10−6 , when assuming V+A or isotropic coupling. The π+ validation run, allows searching for π+ → e +N decay, where N is a heavy neutral lepton such as a heavy sterile neutrino, in the mass region 20 ≤ mN ≤ 65 MeV. A branching ratio limit at 90% confidence level of 3 × 10−8 derived from 9 × 1011 π +stops can be achieved in two weeks, an improvement of the current search sensitivity limit by an order of magnitude