School of Physics - Theses

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    Belle II Silicon Vertex Detector and a measurement of B → D**lν decays at Belle
    Webb, James Maitland ( 2021)
    The Belle II Silicon Vertex Detector (SVD) is a silicon strip detector designed to possess a high irradiation tolerance and short shaping time, making the detector suitable for operation at the high luminosity SuperKEKB collider. In this thesis, the construction of the inner most layer of the detector ''Layer-3'' and subsequent electrical characterisation of the devices are described. Each of the 11 Layer-3 ladders produced were of a high electrical quality, with a strip failure rate of less than 0.2%, demonstrating each of the ladders to be a suitable candidate for installation into the Belle II detector. In the early stages of the detector commissioning phase, numerous high occupancy regions were discovered on the origami sensors. This problem was identified to be caused by crosstalk between control lines on the pitch adapters and the electrodes of the sensor beneath. An algorithm was developed to identify events in which these clusters were present, such that further studies into the affect of the crosstalk clusters could be performed. In particular, the impact on the track finding performance was studied in the search of an offline software approach to mitigating the crosstalk clusters. It was found that the signal-to-noise Ratio (SNR) of the crosstalk clusters were distinct from clusters deposited by signal tracks and an SNR cut based approach demonstrated an improvement to the tracking computation time of order 10%, and a slight improvement to the track parameter resolution. The hit occupancy in the SVD is expected to continually increase as the instantaneous luminosity of SuperKEKB increases over the course of the experiment. As a means of reducing the exponentially growing number of 2D hit candidates which are supplied to the track finding software, detector information was utilised to filter background events. Through exploiting cluster charge, cluster time, and cluster size correlations between each side of the strip detector, a quality index was assigned to each of the reconstructed 2D hits. The quality index of the 2D hits was included in the track candidate multivariate classifier (MVC), having the second highest impact of all the included variables. Through inclusion as a feature of the MVC, the purity of the global track quality ranking was improved. Additionally, a measurement of the semi-inclusive $B\rightarrow D^{**}\ell\nu$ rates, (where $\ell$ denotes either an electron or a muon) were obtained from the entire 711 $fb^{-1}$ Belle data-set. $B\rightarrow D^{**}\ell\nu$ decays are of particular interest due to the uncertainty in the branching fractions calculated by previous measurements. A more precise measurement is of importance for the difference between the inclusive charmed semileptonic decay rate and the sum of the exclusive charmed semileptonic decays (the ``gap problem'') and for improving the precision of future measurements of $\mathcal{R}(D)$ and $\mathcal{R}(D^{*})$, where new physics may be observed. The tag-side $B$ meson is fully reconstructed in a hadronic decay mode with the latest \ac{BDT} tagging algorithm. The measured branching fractions are $\mathcal{B}(B^{+}\rightarrow D^{-}\pi^{+}\ell^{+}\nu) = (0.396 \pm 0.014 \pm 0.020)\% $, $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu) = (0.509 \pm 0.019 \pm 0.030)\%$, $ \mathcal{B}(B^{0}\rightarrow \bar{D}^{0}\pi^{-}\ell^{+}\nu) = (0.364 \pm 0.020 \pm 0.020)\%$, $\mathcal{B}(B^{0}\rightarrow \bar{D}^{*0}\pi^{-}\ell^{+}\nu) = (0.589 \pm 0.030 \pm 0.040)$. Each of which are in agreement with current world averages, apart from $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu)$, which falls below the world average by 1.8$\sigma$. Each of these measurements offer a higher precision than previous results.
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    Measurement of Direct CP Asymmetry and Branching Fraction in B0→D0𝜋0 and B+→D0𝜋+ 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.