Search for the lepton flavour violating decay B0 -> ell tau at Belle II

Abstract

Today, numerous avenues are explored to search for any evidence of new physics beyond the Standard Model, one of which is rare particle decays. Such decays are promising as their suppressed decay rates allow for any enhancements from new physics contributions to be more readily seen. One such rare decay of interest is B0 → ell tau (ell = e, μ) which is a lepton flavour violating decay making it forbidden within the conventional Standard Model framework. However, within an extended Standard Model that includes neutrino oscillations, which there is strong evidence for, the decay can occur but at a rate far beyond any current or future experimental sensitivity. Therefore, observation of B0 → ell tau would be strong and clear evidence of new physics beyond the Standard Model. This study presents the first search for B0 → ell tau using 365/fb of data collected by the Belle II experiment from 2019-2022. The Belle II experiment is situated around the interaction point of the SuperKEKB collider which is an asymmetric e+e− collider designed to mass produce B mesons. The primary aim of the study presented in this thesis is to develop an analysis technique for future hadronic tag-based B0 → ell tau searches at Belle II that yields an improved sensitivity over previous analysis strategies employed by other experiments. This is achieved by partitioning the tau decay into leptonic and hadronic modes, and designing separate signal extraction methods that are optimised for each as opposed to employing a global strategy for all tau decays. Focusing only on the leptonic tau decays for now, upper limits on the B0 → ell tau branching fraction are derived using a hybrid cut-and-count approach based on the Rolke-Lopez interval method. The results from just the leptonic tau decays have a sensitivity that is competitive with the current worldleading upper limits using only a fraction of the data