Francis Pham ; Martin Sevior

2023
The University of Melbourne Melbourne

Abstract: his thesis presents a measurement of the branching fraction and CP -violation asymmetry in B0 → π0π0 decays. The analysis uses a sample that corresponds to 198 × 106 BB pairs, collected by the Belle II experiment at the SuperKEKB accelerator in Tsukuba, Japan. Among collider experiments, only Belle II can efficiently record B0 → π0π0 events at rates enabling competitive measurements to previous results. The large uncertainties of the branching fraction and CP -violation asymmetry of B0 → π0π0 decays are the greatest limitation in determining the least known angle of the unitarity triangle, ϕ2. To enhance the precision of the B0 → π0π0 measurement, this analysis employs improved machine learning algorithms to suppress misreconstructed photons and continuum background. Simulated samples are used to optimise event selection criteria, compare observed data distributions with expectations, study back- ground sources, and model distributions. The branching fraction and direct CP asymmetry are extracted from a three-dimensional unbinned extended maximum likelihood fit simultaneously to events divided into seven data sets. The measured branching fractions and direct CP asymmetries are: B(B0 → π0π0) = (1.38 ± 0.27 ± 0.22) × 10−6 ACP (B0 → π0π0) = 0.14 ± 0.46 ± 0.07 where the first uncertainty is statistical and the second uncertainty is systematic. These values are in agreement with previous results. The statistical and systematic uncertainty of the B measured in this work is similar in size to those obtained by Belle despite using a dataset almost a quarter in size. This demonstrates Belle II’s potential for high-precision measurements of charmless hadronic B decays measurements, enabling the parameter space of new physics to be further constrained

Note: Presented on 15 11 2023
Note: PhD