Selecting μ → e conversion targets to distinguish lepton flavour-changing operators

The experimental sensitivity to mu -> e conversion on nuclei is set to improve by four orders of magnitude in coming years. However, various operator coefficients add coherently in the amplitude for mu -> e conversion, weighted by nucleus-dependent functions, and therefore in the event of a detection, identifying the relevant new physics scenarios could be difficult. Using a representation of the nuclear targets as vectors in coefficient space, whose components are the weighting functions, we quantify the expectation that different nuclear targets could give different constraints. We show that all but two combinations of the 10 Spin-Independent (SI) coefficients could be constrained by future measurements, but discriminating among the axial, tensor and pseudoscalar operators that contribute to the Spin-Dependent (SD) process would require dedicated nuclear calculations. We anticipate that mu -> e conversion could constrain 10 to 14 combinations of coefficients; mu -> e gamma and mu -> e (e) over bare constrain eight more, that leaves 60 to 64 flat directions in the basis of QED x QCD-invariant operators which describe mu -> e flavour change below m(w). (C) 2019 The Author(s). Published by Elsevier B.V.