Evan Rule, Berkeley 

Nuclear Effective Theory of μ→e Conversion

Abstract:
The Mu2e and COMET μ→e collaborations plan to advance branching ratio sensitivities by four orders of magnitude, further constraining new sources of charged lepton flavor violation (CLFV). We formulate a non-relativistic nucleon-level effective theory for this process, in order to clarify what can and cannot be learned about CLFV operator coefficients from elastic μ→e conversion. We employ a treatment of the lepton Coulomb physics that is very accurate, yet yields transparent results and preserves connections to standard-model processes like β decay and μ capture. The formulation provides a bridge between the nuclear physics needed in form factor evaluations and the particle physics needed to relate low-energy constraints from μ→e conversion to UV sources of CLFV. Using state-of-the-art shell model methods we evaluate the nuclear responses, deriving bounds on operator coefficients from existing and anticipated μ→e conversion limits. Finally, we discuss the relation of μ→e conversion to μ→e+γ and μ→3e, illustrating how MEG-II and Mu3e results will complement those of Mu2e and COMET.