Towards New Physics Through Precision Measurements in B$^0_S$ and $\tau$ Decays

Abstract

Despite the correct and consistent description of the elementary particle interactions, there is evidence that the Standard Model (SM) is an incomplete theory. Solving the issues faced by the SM requires the introduction of new interactions and, if such interactions exist, they can be observed as deviations in the SM predictions or forbidden processes. Particularly of interest for this work are the B 0 s mesons and τ leptons produced in the DØ and Belle II experiments, respectively, where the luminosity provides the statistics for high precision SM measurements and searches for new physics.In this work, the lifetime measurement of the B 0 s meson in the decay channel B 0 s → J/ψ f 0 (980) is reported, which is a CP-odd final state. Using 10.4 fb −1 of data collected with the DØ detector in Run II of the Tevatron, the lifetime of the CP-odd component of the B 0 s meson is measured to be τ(B 0 s ) = 1.70 ± 0.14 (stat) ± 0.05 (syst) ps. This measurement is closely related to the B 0 s − B0 s mixing and searches for new CP violation mechanisms. Additionally, the τ lepton physics program of Belle II is reviewed and two sensitivity studies are shown: (i) τ − → ηπ − π 0 ν τ , which in the isospin symmetry limit SU(2) is a cross-check of consistency with the cross section σ(e + e − → ηπ + π − ) at low energies; (ii) τ − → ηπ − ν τ , which has never been observed and, given the suppression by G-parity, it allows the searches of scalar and tensor interactions. Estimations from 1 to 20 ab −1 are shown, corresponding to the integrated luminosity expected during the first years of operation of Belle II.