Spin-dependent μ→e conversion

• DOI: 10.1016/j.physletb.2017.05.053
• Publication Date: 2017-05-22T21:30:52Z
• Authors: Vincenzo Cirigliano; Sacha Davidson; Yoshitaka Kuno
• Keywords: Physics; QC1-999; 0103 physical sciences; 01 natural sciences; 7. Clean energy

The experimental sensitivity to μ→e conversion on nuclei is expected to improve by four orders of magnitude in coming years. We consider the impact of μ→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→e conversion in three ways: in nuclei with spin they mediate a spin-dependent transition; in all nuclei they contribute to the coherent (A2-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→e conversion.