School of Physics - Theses

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Subject: Belle Γ— Subject: B Physics Γ—

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    Measurement of Direct CP Asymmetry and Branching Fraction in B0β†’D0πœ‹0 and B+β†’D0πœ‹+ at the Belle Experiment
    Bloomfield, Tristan Joel ( 2019)
    This thesis describes the measurement of direct CP asymmetry and branching fraction for the hadronic B decays B0 -> D0 pi0 an B+ -> D0 pi+. The study uses the full dataset of 711 fb^(-1) collected at the Y(4S) resonance by the Belle experiment at the KEKB accelerator in Tsukuba, Japan. Event reconstruction, background suppression and modelling are first studied using Monte Carlo simulations, before yield and direct CP asymmetry are extracted in a three-dimensional unbinned extended maximum likelihood fit. B+ -> D0 pi+ is measured first as the control mode to validate the methodology, before same techniques are used on B0 -> D0 pi0 . The measured branching fractions and direct CP asymmetries are: Br(B0 -> D0 pi0) = (2.69 +/- 0.06 +/- 0.09) x 10^(-4), A_CP(B0 -> D0 pi0) = (0.10 +/- 2.05 +/- 1.29) x 10^(-2), Br(B+ -> D0 pi+) = (4.53 +/- 0.02 +/- 0.14) x 10^(-3), A_CP(B+ -> D0 pi+) = (0.19 +/- 0.36 +/- 0.60) x 10^(-2), for B0 -> D0 pi0 and B+ -> D0 pi+ respectively, where the first uncertainty is statistical and the second is systematic. The represents the world’s first measurement of direct CP asymmetry for B0 -> D0 pi0. This measurement of branching fraction of B0 -> D0 pi0 and B+ -> D0 pi+, and direct CP asymmetry of B+ -> D0 pi+ are the most precise to date, and consistent with the current world average values.
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    B0β†’K0Ο€0 and direct CP violation at Belle
    Hawthorne-Gonzalvez, Anton ( 2017)
    Rare B-meson decays such as the B0 β†’ KsΟ€0 which proceed without a charm quark provide a probe for physics beyond the standard model. This decay proceeds mainly via the b β†’ s penguin transition, with the b β†’ u transition being colour suppressed, allowing CP-violating effects to be observable. The asymmetric e+eβˆ’ 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 Β± 10.566) Γ— 106 BB events, the statistical uncertainty in ACP using this method is reduced from the latest Belle result of 0.13 to 0.1035 Β± 0.0032. This method would also provide an up to date measurement on B(B0 β†’ K0Ο€0).