Sebastiano Raiz ; Diego Tonelli

2024
University of Trieste Trieste

Abstract: This experimental particle-physics thesis reports measurements of the branching fraction and charge-parity (CP) violating decay-rate asymmetry of B0 → π0π0 decays reconstructed in electron-positron collisions with the Belle II detector. These quantities constrain parameters of potential processes not described by the Standard Model of particle physics; offer data-driven information to refine phenomenological models of hadronic bottom-meson amplitudes; and provide important inputs for the determination of α, a fundamental parameter of quark dynamics. The analysis uses the full data set of electron-positron collisions at the Υ(4S) resonance produced as of June 2022 by the energy-asymmetric SuperKEKB collider and collected by the Belle II experiment as of this writing. Belle II is an hermetic solenoidal magnetic spectrometer surrounded by particle-identification detectors, calorimeter, and muon detectors, designed to reconstruct the decay products of 10 GeV electron-positron collisions. The data set contains 387 million pairs of bottom-antibottom mesons. The analysis is developed using simulated and control-data samples and applied to the signal sample in data only when all procedures are established. Data are enriched in B0 → π0π0 events by minimizing the expected statistical variance of the quantities of interest as a function of selections on two multivariate statistical-learning selections that suppress the most prominent sources of background. The flavor of the neutral B meson, needed to measure the asymmetry, is determined by using information associated with the other, nonsignal B-meson produced in the Υ(4S) decay. A multidimensional fit of sample composition statistically identifies the signal and determines its CP-violating asymmetry. The fit results, combined with acceptance and efficiency corrections determined from simulation and validated in control data, allow for determining the B0 → π0π0 branching fraction, B(B0 → π0π0) = (1.26+0.20−0.19 (stat) ± 0.12 (syst)) × 10−6, and the CP-violating decay-rate-asymmetry, ACP(B0 → π0π0) = 0.06 ± 0.30 (stat) ± 0.05 (syst). The results are compatible with previous results. Even though based on a smaller sample, they have precision superior to, or comparable with, the world-best results, and are expected to improve significantly several Standard-Model parameters and constraints associated with charmless B decays.

Note: Presented on 17 05 2024
Note: PhD