Just a little bit of flavour: A search for B to mu nu using semileptonic-tagging at Belle II

Sumitted to PubDB: 2025-09-18

Category: Phd Thesis

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Principal Authors Shanette De La Motte
Date 2024-11-28
Belle II Number BELLE2-PTHESIS-2025-019
Abstract While all fundamental mechanisms predicted by the Standard Model have been observed, there still remains universal phenomena that cannot be characterised using this current formulation of particle physics. A natural way of extending Standard Model theory to include unexplained phenomena like dark matter and gravitation is to search for theoretical anomalies in precision measurements of particle physics parameters, such as those seen in B-meson decays. One such decay is B+ ! +, a rare leptonic decay that has not yet been seen in high energy physics experiments, but is expected to take place due to its similarity to the observed B+ ! + decay. Additionally, measurements of the branching fraction of B+ ! + can also be used to constrain the key flavour physics input parameter Vub, which has shown consistent disagreement when measured in exclusive and inclusive B decays. A measurement of B+ ! + with sufficient statistical significance could possibly confirm Standard Model expectations and resolve Vub tensions. Likewise, it could also support said tensions, indicating the presence of new physics mechanisms which potentially link to the unexplained universal phenomena. This thesis presents a search for B+ ! + using 362 fb􀀀1 of electron-positron collisions at the (4S) resonance, recorded by the Belle II experiment between 2019-2021. To restrict the search to events most likely to have coalesced as BB pairs, a machine learning exclusive B-tagging algorithm called the Full Event Interpretation is used to identify and reconstruct semileptonic B mesons that may decay alongside B+ ! +. To further reject non-B+ ! + decays, selection criteria are adopted in a Monte Carlo-based optimisation procedure of the signal-to-background ratio p S S+B. Due to the bias-reduction standards adopted, a final B+ ! + branching fraction pertaining to the Belle II dataset is left for future work. However, after a thorough systematic uncertainty analysis in Monte Carlo and signal-depleted collision events, a 90% upper limit on the branching fraction of B(B+ ! +) < 3.29 10􀀀9 is established using predicted signal yields. This predicted result does not improve upon the current limit, a Belle search using inclusively-tagged B mesons. When instead comparing with the previous semileptonic-tagged limit, having been measured by the BaBar experiment in a much smaller dataset of B decays, this thesis has been able to produce a more constrained result.

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