Measurement of Semileptonic B Decays and Global Fits of Leptoquark Models

Abstract

Flavour physics offers some of the most promising hints of physics beyond the Standard Model (BSM). Precision measurements in the last decade have shown significant deviations from SM predictions, which have been found to favour particular BSM interpretations. This work contributes to two aspects of the ongoing quest for deeper understanding of flavour physics: precision measurements of rare processes at the Belle II experiment, and global fits with flavour constraints in order to make conclusions about specific BSM models. The Full Event Interpretation (FEI) is a novel tagging algorithm developed by the Belle II collaboration. Here we demonstrate the use of the semileptonic FEI via a normalisation mode approach, which circumvents the ongoing difficulties in calibrating the semileptonic FEI. We use this approach to extract |𝑉𝑢𝑏|, obtaining a result that is consistent with world averages and has an uncertainty of 7% that is statistically dominated. There are avenues for improving the semileptonic FEI tagging in the future, and the statistical uncertainty may be greatly reduced with larger datasets. Anomalous measurements may be placed in a broader context by performing global fits. This allows us to map the high-dimensional parameter spaces of BSM models and determine which regions are favoured by all available results. This thesis presents preliminary results of a global fit of leptoquark models with a selection of flavour constraints, and investigates the viability of observing the non-excluded models at the LHC. We focus in particular on identifying regions of the parameter space which are still viable in light of the recent 𝑅(𝐾 (∗)) measurement by LHCb, which removes a previous long-standing anomaly. This thesis also details software work done regarding Belle II’s skimming procedure. This work helps to prepare Belle II for future data collection, when an optimised data processing pipeline will be critically necessary.