Thesis BELLE2-MTHESIS-2020-007

First charmless B decay reconstruction in Belle II data

Sebastiano Raiz ; Diego Tonelli

University of Trieste Trieste (ITA)

Abstract: This is an experimental particle physics thesis aimed at the first reconstruction of charmless decays of B mesons in Belle II data. Belle II is a detector designed to reconstruct billions of decays of heavy mesons and τ leptons from 10 GeV electron-positron collisions in search for indirect indications of non-Standard-Model physics. Decays of b quarks in quarks other than c (charmless B decays) provide unique access to fundamental parameters of the weak interactions that are particularly sensitive to non-SM physics and play an important role in the Belle II scientific program. The work consists in three synergic parts. The common thread is the use of challenging physics to study, understand, and possibly optimize the most relevant experimental capabilities of the detector and reconstruction to reach optimal performance when the data set size will be competitive for world-leading results. I first focus on the reconstruction of B+ → K+π0 decays using the sample collected by Belle II in 2019 and corresponding to 8.7 fb−1 of integrated luminosity. The B+ → K+π0 channel probes several key aspects of the detector and reconstruction performance such as background suppression, reconstruction of energetic photons, and charged-hadron identification. I achieve the first reconstruction of B+ → K+π0 decays in Belle II data, with a signal yield of 27 ± 8 events, and expose a previously undetected energy- miscalibration in photon reconstruction, prompting the collaboration to adopt a novel approach to monitor the calorimeter energy-scale. A study of the charged-particle identification performance – a key capability in B physics – follows. I observe subtle instrumental dependences between particle-identification variables measured by different sub detectors, so far neglected in Belle II reconstruction, and assess their impact on long-term performance. Finally, I achieve the first measurement of charge-parity (CP) violating asymmetries in three-body decays B+ → K+K−K+ and B+ → K+π−π+ re- constructed in 2019–2020 Belle II data corresponding to 34.6 fb−1 of integrated lumi- nosity. The results, ACP (B+ → K+K−K+) = −0.049 ± 0.063(stat) ± 0.022(syst) and ACP (B+ → K+π−π+) = −0.063 ± 0.081(stat) ± 0.023(syst), demand a thorough data-based investigation of instrumental charge-asymmetries, which is a fundamental input for all future CP-violation measurements. All results are compatible with expectations from simulation and show performance on par with the best performance of the predecessor experiments. This work provides solid ground for upcoming analyses of B decays at Belle II and is expected to have significant impact on the quality of Belle II physics throughout the next few years, when the expected data set will enable competitive precisions with current world-best results.

Note: Presented on 18 09 2020
Note: MSc

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 Record created 2020-11-01, last modified 2020-11-01

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