School of Physics - Theses
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ItemBelle II Silicon Vertex Detector and a measurement of B → D**lν decays at Belle( 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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ItemMeasurement of the branching fractions of $\bar{B}^{0}\to D^{*+} h^{-}$ decays at the Belle experiment and development of a global particle vertex fitting algorithm for Belle~II( 2021)In this thesis high precision QCD-factorisation tests were performed. The branching ratios B(B → D π ) = (2.67±0.02±0.09)×10 and B(B → D K ) = (2.27±0.06±0.08)×10−4 were measured using (771.58±10.56)×106 B-meson pairs recorded by the Belle experiment. Both values are in tension with the theoretical expectation. The ratio of the branching ratio is measured in a way that allows for the cancellation of the systematic uncertainties arising from the D∗-meson reconstruction; the value of RK/π = B(B → D K )/B(B → D π ) = (8.41±0.24±0.13)×10 was found. Both B(B → D π ) and B(B → D K ) have shown deviations from the prediction, this suggests that the estimation of the Feynman diagrams contributing to the predictions may be inaccurate. The new measured branching ratios were used to perform a high precision QCD factorisation test by measuring ratios with respect to semi-leptonic branching ratios at fixed momentum transfers for different particle species. A deviation for the ratio Γ(B → D h )/dΓ(B → D l ν ̄)/dq of 16% from theoretical predictions was found, suggesting large non-factorisable contributions and/or new physics contributions. Furthermore, SU(3)-symmetry was tested by measuring ratios for pions and kaons of a21(K)/a21(π) = 1.05±0.05 as well as for different particle species. The found value is consistent with unity and therefore no evidence for SU(3)-symmetry breaking effects was found in this test to 5% precision. Thus, for RK/π one can rule out SU(3)- symmetry breaking effects as an explanation for the deviation. Finally, a new vertex fitting algorithm and its implementation for the Belle II software framework was reported. It can improve D∗-meson reconstruction, is com- putationally very efficient and is now the standard vertex fitting tool of the Belle II experiment.