Radiative muon capture mysteries and the search for μ- N(A,Z) --> e+ N'(A,Z-2)

Experimentally observed conservation of flavor for charged leptons makes them very different from other fermions - the quarks and neutrinos - for which mixing among generations is large and has been established long ago. In the Standard Model (SM), charged lepton flavor violating (CLFV) processes are suppressed by small neutrino masses down to the level well below experimental reach. For example, the expected Br(μ- N --> e- N) is approximately 1e-52. Many models beyond the SM (BSM) predict much higher rates of CLFV and lepton number violating (LNV) processes.

The conversion μ- N(A,Z) --> e+ N'(A,Z-2) is both a CLFV and LNV process which depends on off-diagonal lepton flavor matrix elements, not directly accessible by double beta decay searches. The strongest limits on this process come from the TRIUMF and SINDRUM II experiments. The dominant background to this search comes from the process of radiative muon capture (RMC), often described in a simplified form using the closure approximation (CA). CA has failed to describe the positron spectrum in all recently published searches by the TRIUMF and SINDRUM II experiments, where either ad hoc alterations to the model were added to better describe the experimental data, the poor description was ignored, or a limit was not published. The published data are in significant tension with the current models of RMC, making it impossible to distinguish between new physics and SM physics.

The Mu2e experiment aims to improve the sensitivity of searches for μ --> e conversion in the field of a nucleus by a factor of 10,000. In order to utilize the four orders of magnitude gain in sensitivity and the discovery potential of Mu2e, a better theoretical description of RMC is needed. We will discuss the tension between the data and predictions in the previous experimental searches, and the potential impact these discrepancies could have on the Mu2e search for μ- N(A,Z) --> e+ N'(A,Z-2) conversion.