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

Sumitted to PubDB: 2025-05-06

Category: Phd Thesis

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Principal Authors Priyanka Cheema
Date 2025-03-14
Belle II Number BELLE2-PTHESIS-2025-014
Abstract Despite the successes of the Standard Model, its limitations provide strong indications that new physics beyond the Standard Model must exist. Today, numerous avenues are explored to search for any evidence of new physics, 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 $B^0\rightarrow \ell^\pm\tau^\mp$ ($\ell = e, \mu$) 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 $B^0\rightarrow \ell^\pm\tau^\mp$ would be strong and clear evidence of new physics beyond the Standard Model. This study presents the first search for $B^0\rightarrow \ell^\pm\tau^\mp$ using 365 fb$^{-1}$ 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 $B^0\rightarrow \ell^\pm\tau^\mp$ 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 $B^0\rightarrow \ell^\pm\tau^\mp$ 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 world-leading upper limits using only a fraction of the data.

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