School of Physics - Theses
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ItemMeasurement of the branching fraction and CP asymmetry of B0 to pi0 pi0 decays at Belle II( 2023-08)This thesis presents a measurement of the branching fraction and CP-violation asymmetry in B0 -> pi0pi0 decays. The analysis uses a sample that corresponds to 198e6 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 -> pi0pi0 events at rates enabling competitive measurements to previous results. The large uncertainties of the branching fraction and CP-violation asymmetry of B0 -> pi0pi0 decays are the greatest limitation in determining the least known angle of the unitarity triangle, phi2. To enhance the precision of the B0 -> pi0pi0 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 background 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 -> pi0pi0) = (1.38 +- 0.27 +- 0.22) x 10-6 ACP(B0 -> pi0pi0) = 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.
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ItemInvestigation of B+ mesons decay to K+K−π+ at the Belle experiment( 2017)Charmless decays of B mesons to three charged hadrons are suppressed in the Standard Model, and thus provide an opportunity to search for physics beyond the Standard Model. An unexpected excess and a large CP asymmetry in the low invariant mass spectrum of the K+K− system for the decay B+ → K+K−π+ were observed by BaBar and LHCb in recent years. We present the measurements of branching fraction and direct CP asymmetry of the charmless decay B+ → K+K−π+. This analysis is performed on a data sample of 772 × 10^6BB pairs produced at the Υ(4S) resonance by the KEKB asymmetric-energy e+e− collider and collected by the Belle detector. We perform a blind analysis, examining signal reconstruc- tion and background suppression with Monte Carlo simulated samples, and extract signal yield and direct CP asymmetry with a 2D extended maximum likelihood fit to the data. The measured branching fraction and direct CP asymmetry are B(B+ → K+K−π+) = (5.38 ± 0.40 ± 0.35) × 10^−6 and ACP = −0.170 ± 0.073 ± 0.017, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the current world average. We extract the branching fraction and direct CP asymmetry as a function of the K+K− invariant mass. The K+K− invariant mass distribution of the signal candidates shows an excess in the region below 1.5 GeV/c^2, which is consistent with the previous studies from BaBar and LHCb. Strong evidence of a large direct CP asymmetry of −0.90 ± 0.17 ± 0.03 with 4.8σ significance is found in the K+K− low-invariant-mass region.
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ItemB0→K0π0 and direct CP violation at Belle( 2017)Rare B-meson decays such as the B0 → Ksπ0 which proceed without a charm quark provide a probe for physics beyond the standard model. This decay proceeds mainly via the b → s penguin transition, with the b → u transition being colour suppressed, allowing CP-violating effects to be observable. The asymmetric e+e− KEKB collider and the Belle detector provide the large luminosity and data collection required to observe these rare B decays. Methods to reduce the large qq backgrounds are investigated. The use of optimised neural networks using TensorFlow shows a significant improvement compared to the commonly used NeuroBayes software. Techniques for reducing correlations between variables introduced by TensorFlow are also investigated, proving that the use of adversarial neural networks can provide an improved background suppression as compared to NeuroBayes, whilst minimising correlations introduced by the neural network. An improved method of measuring the direct CP violation is introduced. Using Monte Carlo data with sample sizes corresponding to the full Belle datatset of (771.581 ± 10.566) × 106 BB events, the statistical uncertainty in ACP using this method is reduced from the latest Belle result of 0.13 to 0.1035 ± 0.0032. This method would also provide an up to date measurement on B(B0 → K0π0).