Narrow Results

In this paper, we present the major results obtained in $\tau$ physics by the Belle and Belle II experiments. For the Belle II experiment, we also discuss prospects for improved measurements of the $\tau$ lepton properties, new results in Michel parameters determination, and searches for CP and lepton flavor violation in $\tau$ decays.

We present here the CP asymmetries in the decay rate and angular distributions of $\tau \to K_S\pi {\nu }_{\tau }$ decays in the Standard Model (SM) and beyond (BSM). The CP asymmetries in the SM are induced by the CP violation in $K^0-{\bar{K}}^0$ mixing. To investigate the BSM CP-violating (CPV) effects, a model-independent analysis is performed by using the low-energy effective field theory (LEFT) framework at $\mu =2$ GeV. If one further assumes the BSM physics to stem from above the electroweak scale, the LEFT shall then be matched onto the SM effective field theory (SMEFT), the operators of which contributing to $\tau \to K_S\pi {\nu }_{\tau }$ decays will also contribute to the neutron electric dipole moment (EDM) and $D^0-{\bar{D}}^0$ mixing. The stringent bounds from the latter suggest that no remarkable CPV effects can be observed in either the decay rate or the angular distributions. The prospects for future measurements of these observables are also mentioned.

We present a new method for direct measurement of all Michel parameters in the ${\tau }^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }{\nu }_{\tau }$ decay related to the polarization of the daughter muon: ${\xi }^{\prime }$, ${\xi }^{\prime \prime }$, ${\eta }^{\prime \prime }$, ${\alpha }^{\prime }/A$, and ${\beta }^{\prime }/A$. The method is based on the reconstruction of the muon decays-in-flight in the tracker of the $e^{+}{e}^{-}$ collider experiments and relies on the correlation between muon spin and its daughter electron momentum. The low probability of decay-in-flight in the tracker volume is compensated by the unprecedented statistics of modern flavor factories, which ultimately leads to the possibility of achieving high accuracy. We discuss the application of the suggested method in the experiments at $e^{+}{e}^{-}$ colliders and present a feasibility study for the future Super Charm-Tau Factory experiment. We also discuss the result of the first use of the proposed method in the Belle experiment, confirming its applicability.

We report recent results on τ lepton physics obtained with the Belle detector. They include a search for lepton-flavor-violating decays τ → lhh′, a high-precision measurement of the τ lepton lifetime and a study of the hadronic decays with the in the final state.

Since Run-1 of the LHC, CMS has taken the opportunity to improve further particle reconstruction. A number of improvements were made to the hadronic tau reconstruction and identification algorithms. In particular, the reconstruction of the tau decay products leaving deposits in the electromagnetic calorimeter was improved to better model signal of ${\pi }^0$ from $\tau$ decays. This modification improves energy response and removes the tau footprint from isolation area. In addition to this, improvements were made to discriminators that combine isolation and tau lifetime variables, and the rejection of electrons misidentified as hadronic taus was improved using multivariate techniques. The results of these improvements using 13 TeV data at LHC Run-2 are presented and validation of tau identification using a variety of techniques has been highlighted.

To test the Lorentz structure of the charged weak interaction, we study leptonic decays of tau lepton in the Belle experiment. We perform this test through the measurement of Michel parameters which appear in the differential decay widths of leptonic tau decays. In this paper, we discuss the preliminary results of the measurement of Michel parameters at Belle.