Measurement of the properties of B0→π0π0 decays at Belle II

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 Y(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 Y(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)=[xx]{+0.197}{-0.187} (stat) ± 0.145(syst) 10^-6, and the CP}-violating decay-rate-asymmetry, A_CP=[xx]{+0.296}{-0.299} (stat) ± 0.064 (syst). The central values are hidden as the analysis is still under internal collaboration review. The results have precisions competitive with the current world-best results, even though based on a smaller sample, and are expected to improve significantly several Standard-Model parameters and constraints associated with charmless B decays.