Matteo Filipig ; Mirco Dorigo ; Michele Mantovano

2024
University of Trieste Trieste

Abstract: This is an experimental particle physics thesis that aims to study the exclusive semileptonic decay B0 → D∗−l+νl at Belle II for a new measurement of its decay dynamics and |Vcb|. Belle II is a particle physics experiment that focuses on weak-interaction processes of heavy quarks to search for indirect indications of non-standard-model physics. The B0 → D∗−l+νl decay provides access to the strength of the bottom-to-charm weak coupling, |Vcb|, a fundamental parameter of the standard model, whose determination presents a long-standing discrepancy between two measurement methods, the so-called inclusive and exclusive approaches. The B0 → D∗−l+νl decay is the golden mode of the exclusive approach. In this approach, to determine |Vcb| from experimental data, a parameterization of the strong-interaction contribution to the decay amplitude is needed. Recent calculations of this contribution in the B0 → D∗−l+νl decay showed some inconsistencies, making the exclusive determination less precise. This work aims to explore a novel experimental approach which provides a measurement of model-independent observables of the decay dynamics, which can be interpreted assuming any theoretical model to parameterize the strong interaction contribution. The analysis is conducted using a full, realistic simulation of the Belle II sample collected between 2019 and 2022. A selection is developed to obtain a high-purity signal sample, whose composition is studied in detailed to model the data for a multi-dimensional χ2 fit that measures the model-independent observables. The fit is inspected in search for configurations which minimize fit parameter biases. A comparison between two configurations of fit variables is performed to assess which one returns the best parameter sensitivity, and the model-independent observables obtained are interpreted with a theoretical model to obtain |Vcb|. The results show that in the fit performed, for the best sensitivity on |Vcb|, a better signal-to-background discrimination is preferred over adding more information about the decay dynamics.

Note: Presented on 18 10 2024
Note: MSc