Spin-dependent μ→e\mu \to e conversion

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 $\mu \to e$ conversionComment: 6 pages, I figure, typo corrected in eqn

Future Experimental Improvement for the Search of LNV Process in eμe\mu Sector

Exploring the leptonic sector in frontier experiments is more of importance nowadays, since the conservation of lepton flavor and total lepton number are not guaranteed anymore in the Standard Model after the discovery of neutrino oscillations. $\mu^- + N(A,Z) \rightarrow e^+ + N(A,Z-2)$ conversion in a muonic atom is one of the most promising channels to investigate the lepton number violation process, and the measurement of this process is planned in future $\mu^--e^-$ conversion experiments with a muonic atom in a muon-stopping target. This paper discusses how to maximize the experimental sensitivity of the $\mu^--e^+$ conversion by introducing the new requirement of the mass relation of $M(A,Z-2)<M(A,Z-1)$, where $M(A,Z)$ is the mass of the muon-stopping target nucleus, to get rid of the background from radiative muon capture. The sensitivity of the $\mu^--e^+$ conversion is anticipated to have four orders of magnitude of improvement in forthcoming experiments using a proper target nucleus, which satisfies the mass relation. The most promising isotopes found are $^{40}$Ca and $^{32}$S.Comment: 8 pages, 4 figures; Figures, some numbers and a reference in text are modifie

GPU-Accelerated Event Reconstruction for the COMET Phase-I Experiment

This paper discusses a parallelized event reconstruction of the COMET Phase-I experiment. The experiment aims to discover charged lepton flavor violation by observing 104.97 MeV electrons from neutrinoless muon-to-electron conversion in muonic atoms. The event reconstruction of electrons with multiple helix turns is a challenging problem because hit-to-turn classification requires a high computation cost. The introduced algorithm finds an optimal seed of position and momentum for each turn partition by investigating the residual sum of squares based on distance-of-closest-approach (DCA) between hits and a track extrapolated from the seed. Hits with DCA less than a cutoff value are classified for the turn represented by the seed. The classification performance was optimized by tuning the cutoff value and refining the set of classified hits. The workload was parallelized over the seeds and the hits by defining two GPU kernels, which record track parameters extrapolated from the seeds and finds the DCAs of hits, respectively. A reasonable efficiency and momentum resolution was obtained for a wide momentum region which covers both signal and background electrons. The event reconstruction results from the CPU and GPU were identical to each other. The benchmarked GPUs had an order of magnitude of speedup over a CPU with 16 cores while the exact speed gains varied depending on their architectures