Thesis BELLE2-PTHESIS-2023-016

Search for B+ --> K+ tau tau with Belle and Belle II experiments


Université paris saclay Orsay

Abstract: In recent years, some deviations from the Standard Model (SM) have been observed in semileptonic B-meson decays, such as departures from lepton flavour universality in the b --> sll and b --> c tau nu transitions quantified by the R(K(*)) and R(D(*)) ratios. This generated a considerable interest in the flavour physics community: the pattern of anomalies could be explained by New Physics (NP) scenarios predicting large enhancements in the third generation of leptons compared to the first two, as in the b --> s tau tau transition, especially when the NP coupling is proportional to the square of the lepton mass. The current limits for all processes mediated by b --> s tau tau are far from the SM predictions by four orders of magnitude, due to the experimental challenges of reconstructing tau-leptons that decay quickly and produce undetectable neutrinos. Consequently, the possibility of large NP-induced enhancements remains unchallenged. An upper limit for the B+ --> K+ tau tau decay mode has been set by the BaBar experiment, but it has never been searched for by any other experiment. In this work, the search for B+ --> K+ tau tau is performed in the context of the Belle and Belle II experiments. Due to the presence of up to four neutrinos (missing energy), the signal B candidate cannot be fully reconstructed. At a B factory, e+e- collisions produce Y(4S) which mostly decay into a pair of B-mesons, with a well-known initial state. By fully reconstructing one of the B mesons and using its four-momentum, we can infer the properties of the other B-meson, which may decay into K+ tau tau. This technique, called B-tagging, effectively mitigates large background contributions in searches involving missing energy. The full reconstruction of hadronic B-mesons is done using Belle II's exclusive tagging algorithm, employing a network of boosted decision trees trained on Monte Carlo (MC). A significant discrepancy in B-tagging performance between MC and data has been observed, not only being a significant source of systematic uncertainty but also indicating a suboptimal training. To address this, a new control procedure, using B --> D pi decays without explicitly reconstructing D mesons, is introduced to calibrate the hadronic B-tagging. This provides large statistics, while having a high purity (in contrast to the traditional B --> X_cl nu), also enabling the study of how B decay modelling affects B-tagging performance. By implementing the corresponding corrections to the decay model, the agreement is significantly improved. Furthermore, retraining the B-tagging algorithm with the updated MC simulations increases the background rejection. It is important to note that these efforts and further improvements to the decay model obtained by measuring poorly known hadronic B-decays will lead to better performance of hadronic B-tagging, benefiting all future searches involving missing energy. After reconstructing the tagged B and the charged tracks from the signal side, events that contain a B+ --> K+ tau tau are expected to leave no extra energy in the electromagnetic calorimeter (E_ECL). Contamination from beam background photons and neutral clusters arising from hadronic split-off processes degrades the resolution of the E_ECL. On the other hand, the background mostly composed of semileptonic decays also has low E_ECL. To mitigate the resolution issue and optimise the separation from the background composed mostly of semileptonic B decays, a study of E_ECL is conducted using the control sample of B+ --> J/psi K+ decays. Finally, in the absence of a signal, an upper limit for the branching ratio of the B+ --> K+ tau tau is derived using combined datasets of Belle and Belle II MC simulations.

Note: Presented on 03 10 2023
Note: PhD

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 Record created 2023-10-17, last modified 2023-11-25

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