Muon Decay

The decay of the muon has been studied at PSI with several precision measurements: The longitudinal polarization P_{\mathrm{L}}(E)PL(E) with the muon decay parameters \xi'ξ′, \xi''ξ″, the Time-Reversal Invariance (TRI) conserving transverse polarization P_{\mathrm{T_{1}}}(E)PT1(E) with the muon decay parameters \etaη, \eta''η″, the TRI violating transverse polarization P_{\mathrm{T_{2}}}(E)PT2(E), with \alpha'/Aα′/A, \beta'/Aβ′/A and the muon decay asymmetry with P_{\mu}\xiPμξ. The detailed theoretical analysis of all measurements of normal and inverse muon decay has led for the first time to a lower limit |g^{V}_{LL}| > 0.960|gLLV|>0.960 (“V-AV−A”) and upper limits for nine other possible complex couplings, especially the scalar coupling |g^{S}_{LL}| < 0.550|gLLS|<0.550 which had not been excluded before.

Resonant enhancement in leptogenesis

Vanilla leptogenesis within the type I seesaw framework requires the mass scale of the right-handed neutrinos to be above [Formula: see text] GeV. This lower bound can be avoided if at least two of the sterile states are almost mass degenerate, which leads to an enhancement of the decay asymmetry. Leptogenesis models that can be tested in current and upcoming experiments often rely on this resonant enhancement, and a systematic and consistent description is therefore necessary for phenomenological applications. In this paper, we give an overview of different methods that have been used to study the saturation of the resonant enhancement when the mass difference becomes comparable to the characteristic width of the Majorana neutrinos. In this limit, coherent flavor transitions start to play a decisive role, and off-diagonal correlations in flavor space have to be taken into account. We compare various formalisms that have been used to describe the resonant regime and discuss under which circumstances the resonant enhancement can be captured by simplified expressions for the CP asymmetry. Finally, we briefly review some of the phenomenological aspects of resonant leptogenesis.

New Precision Measurement for Proton Zemach Radius with Laser Spectroscopy

In this proceeding, a new proposal aiming to improve the precision of the proton Zemach radius will be presented. A circularly polarized laser will be shed on a sample of muonic hydrogen in its ground state. By observing the maximum muon decay asymmetry during scanning laser wave length, the ground-state hyperfine splitting energy can be identified, which is directly related to Zemach radius.citedupays The precision of Zemach radius by this measurement is estimated to be three times better compared to PSI experiment. This result will contribute to the solution of proton size puzzle.

Background Study for a Search τ→ μγ at Super-c-τ Factory

A Monte Carlo study of possible background processes in a search for τ → μγ decay has been performed for conditions of the Super-с–τ factory (at the e + e  center-of-mass energies 3.686 GeV, 3.77 GeV, and 4.17 GeV). The background from e+ e  τ + τ − events has been analyzed. Selection criteria for background suppression have been proposed, and necessary requirements on the detector characteristics have been determined. The search was completed for energy resolution 1.5 % and 2.5 % in the calorimeter. All main tag modes of the tagging tau had taken into account (the total branching ratio all tag modes is about 90 %). The possibility of the longitudinal polarization for initial electron beam was considered for this problem. Measurement of the decay asymmetry became executable in case such polarization in spite of small events statistics. The total sensitivity for branching ratio of the decay τ → μγ at Super-с–τ factory, in case π/μ suppression 1 per 30, is ~ 5 × 10–10. The total number τ + τ − pairs at future Super-B factory about 3 time higher, but the sensitivity for decay τ → μγ at Super–с–τ factory in 5 time better. So the Super-с–τ factory can successfully compete with the Super-B factory in a search for τ→ μγ decay