000004116 001__ 4116
000004116 005__ 20240220230245.0
000004116 037__ $$aBELLE2-PTHESIS-2024-003
000004116 041__ $$aeng
000004116 100__ $$aFrancis Pham
000004116 245__ $$aMeasurement of the branching fraction and CP asymmetry of B0 → π0 π0 decays at Belle II
000004116 260__ $$aMelbourne$$bThe University of Melbourne$$c2023
000004116 300__ $$a259
000004116 500__ $$aPresented on 15 11 2023
000004116 502__ $$aPhD$$bMelbourne, The University of Melbourne$$c2023
000004116 520__ $$ahis thesis presents a measurement of the branching fraction and CP -violation asymmetry in B0 → π0π0 decays. The analysis uses a sample that corresponds to 198 × 106 BB pairs, collected by the Belle II experiment at the SuperKEKB accelerator in Tsukuba, Japan. Among collider experiments, only Belle II can efficiently record B0 → π0π0 events at rates enabling competitive measurements to previous results. The large uncertainties of the branching fraction and CP -violation asymmetry of B0 → π0π0 decays are the greatest limitation in determining the least known angle of the unitarity triangle, ϕ2. To enhance the precision of the B0 → π0π0 measurement, this analysis employs improved machine learning algorithms to suppress misreconstructed photons and continuum background. Simulated samples are used to optimise event selection criteria, compare observed data distributions with expectations, study back- ground sources, and model distributions. The branching fraction and direct CP asymmetry are extracted from a three-dimensional unbinned extended maximum likelihood fit simultaneously to events divided into seven data sets. The measured branching fractions and direct CP asymmetries are: B(B0 → π0π0) = (1.38 ± 0.27 ± 0.22) × 10−6 ACP (B0 → π0π0) = 0.14 ± 0.46 ± 0.07 where the first uncertainty is statistical and the second uncertainty is systematic. These values are in agreement with previous results. The statistical and systematic uncertainty of the B measured in this work is similar in size to those obtained by Belle despite using a dataset almost a quarter in size. This demonstrates Belle II’s potential for high-precision measurements of charmless hadronic B decays measurements, enabling the parameter space of new physics to be further constrained
000004116 700__ $$aMartin Sevior$$edir.
000004116 8560_ $$ffpham@student.unimelb.edu.au
000004116 8564_ $$uhttps://docs.belle2.org/record/4116/files/BELLE2-PTHESIS-2024-003.pdf
000004116 980__ $$aTHESIS