# Spin-dependent $\mu \to e$ conversion

## TL;DR

This paper explores how spin-dependent and independent operators influence $\, ext{mu to e}$ conversion, highlighting new contributions from tensor and axial-vector interactions and their implications for upcoming experiments.

## Contribution

It introduces the consideration of spin-dependent operators in $\, ext{mu to e}$ conversion, analyzing their effects and sensitivity in upcoming experimental setups.

## Key findings

- Spin-dependent operators contribute to $\, ext{mu to e}$ conversion in spinful nuclei.
- Loop effects enhance sensitivity to tensor and axial-vector operators.
- Spin-dependent and independent contributions are complementary in probing new physics.

## Abstract

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