Nuclear Effective Theory of Muon-to-Electron Conversion

Limits on the charged lepton flavor violating (CLFV) process of μ→e conversion are expected to improve by four orders of magnitude due to the next generation of experiments, Mu2e at Fermilab and COMET at J-PARC. While the kinematics of the decay of a trapped muon are ideal for detecting a signal of CLFV, the intervening nuclear physics presents a significant roadblock to the interpretation of experimental results. We introduce an effective theory of μ→e conversion formulated at the nuclear scale, which factorizes the nuclear physics from the CLFV leptonic physics, sequestering the latter quantity into unknown low-energy constants (LECs) that are probed directly by experiments. Utilizing state-of-the-art shell-model calculations of nuclear response functions, we estimate the limits that can be obtained on these LECs if the next-generation experiments achieve their design sensitivity.