18-22 October 2021
Matsue, Shimane Prefecture, Japan
Asia/Tokyo timezone

A new search for time-reversal symmetry-breaking in muon decays

19 Oct 2021, 19:10
Room 403 (Kunibiki Messe)

Room 403

Kunibiki Messe

Parallel Session Presentation Future facilities and experiments Joint BSM - Accelerator - Future Session EU timezone


Sohtaro Kanda (KEK)


One of the key open questions on the nature of the neutrino is whether it is a Dirac or a Majorana particle. If the neutrino is a Majorana particle, then the interference terms that break the time-reversal symmetry of muon decay appear as the transverse spin polarization of the decay electron [1]. Experiments have been performed at Paul Scherrer Institute to measure the spin polarization of positrons in positive muon decays, using Bhabha scattering and annihilation-in-flight [2]. No significant transverse polarization of decay positrons has been observed so far, and experiments with higher sensitivity can push the scope of searches for physics beyond the Standard Model. This topic is revisited by investigating the possibility of a new experiment using a high-intensity pulsed muon beam at J-PARC. As a result of the Monte-Carlo simulations, the prospect of improving the measurement precision by at least one order of magnitude was obtained. In this contribution, the outline of the new experiment and the design of the polarimeter will be discussed.

[1] M. Doi, T. Kotani, H. Nishiura, K. Okuda, and E. Takasugi, Prog. Theor. Phys. 67, 281 (1982).
[2] N. Danneberg et al., Phys. Rev. Lett. 94, 021802 (2005).

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