Spin-dependent μ → e conversion

The experimental sensitivity to mu -> e conversion on nuclei is expected to improve by four orders of magnitude in coming years. We consider the impact of mu -> e flavour-changing tensor and axial-vector four-fermion operators which couple to the spin of nucleons. Such operators, which have not previously been considered, contribute to mu -> e conversion in three ways: in nuclei with spin they mediate a spin-dependent transition; in all nuclei they contribute to the coherent (A(2)-enhanced) spin-independent conversion via finite recoil effects and via loop mixing with dipole, scalar, and vector operators. We estimate the spin-dependent rate in Aluminium (the target of the upcoming COMET and Mu2e experiments), show that the loop effects give the greatest sensitivity to tensor and axial-vector operators involving first-generation quarks, and discuss the complementarity of the spin-dependent and independent contributions to mu -> e conversion. (C) 2017 The Author(s). Published by Elsevier B.V.