Spin-dependent $\mu \to e$ conversion

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

6 pages, I figure, typo corrected in eqn 6

References in corpus (3)

• Big-Bang Nucleosynthesis
• Double Beta Decay, Majorana Neutrinos, and Neutrino Mass
• Renormalisation-group improved analysis of $\mu \to e$ processes in a systematic effective-field-theory approach

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