Spin-dependent $μ\to e$ conversion

Abstract

The experimental sensitivity to $μ\to e$ conversion on nuclei is expected to improve by four orders of magnitude in coming years. We consider the impact of $μ\to e$ 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 $μ\to e$ 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 $μ\to e$ conversion

6 pages, I figure, typo corrected in eqn 6