Semileptonic τ decays: powerful probes of non-standard charged current weak interactions

When looking for heavy (O(few TeV)) New Physics, the most efficient way to bene?t from both high and low-energy measurements simultaneously is the use of the Standard Model Effective Field Theory (SMEFT). In this talk I highlight the importance of semileptonic τ decays in complementing, in this respect, the traditional low-energy precision observables and high-energy measurements. This is yet another reason for considering hadronic tau decays as golden channels at Belle-II beyond the unquestionable interest of the CP violation anomaly in τ → KS πντ decays, that I also discuss within the effective theory. A couple of new results for τ−→ K− ντ decays are also included.

Experimental review of τ lepton studies at the B factories

Recent results of a high-statistics τ lepton studies at B factories are reported. Reviewed are the measurements of Michel parameters in leptonic and radiative leptonic τ decays at Belle, as well as the measurement of the branching fractions of the radiative leptonic τ decays at BABAR. Searches for CP symmetry violation in hadronic decays with $ \mathop K\nolimits_S^0 $ are also briefly discussed.

No-Go Theorem for Nonstandard Explanations of the τ→K_{S}πν_{τ} CP Asymmetry

The CP asymmetry in τ→K_{S}πν_{τ}, as measured by the BABAR collaboration, differs from the standard model prediction by 2.8σ. Most nonstandard interactions do not allow for the required strong phase needed to produce a nonvanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least 2 orders of magnitude due to Watson's final-state-interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and D-D[over ¯] mixing. These observations together imply that it is extremely difficult to explain the current τ→K_{S}πν_{τ} measurement in terms of physics beyond the standard model originating in the ultraviolet.