School of Physics - Theses
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ItemSemi-leptonic decays at the Belle and Belle II experiments( 2022)This thesis explores semi-leptonic decays of the type $B \to V L$ along two avenues; their contribution to high-precision tests and metrology of the Standard Model, and their potential for probing physics beyond the Standard Model. The magnitude of the Cabbibo-Kobayashi-Maskawa matrix element corresponding to $b\to c$ quark transitions, $|V_{cb}|$, has a long-standing discrepancy between its measurements from exclusive and inclusive decays, which presents a puzzling gap in our knowledge of the Standard Model. In this thesis, recent Lattice QCD calculations of form factors at nonzero hadronic recoil are incorporated into novel analysis methods to examine the phenomenology and analysis techniques behind the exclusive $|V_{cb}|$ measurement in $B \to D^{*}\ell^+ \nu_\ell$ decays. This results in a measurement of $|V_{cb}|=(38.49 \pm 0.54 \pm 0.92 \pm 0.91) \times 10^{-3}$, the least model-dependent result to date. The results of this analysis of the $B \to D^{*}\ell^+ \nu_\ell$ branching fraction and decay rate are used in a test of Standard Model QCD predictions of hadronic factorisation, with the first measurement of the parameter $|a_1|$ within a single experiment, based on a ratio that cancels most experimental systematic uncertainties, resulting in $|a_1| = 0.884 \pm 0.004 \pm 0.003 \pm 0.016$. This corresponds to an $8.9\sigma$ discrepancy from the Standard Model, suggesting the existence of large non-factorisable contributions in hadronic $B$-decays, or potential contributions from new physics amplitudes. A probe for new physics in photon-dipole interactions, described by the $\mathcal{O}_7$ term in an operator product expansion, is then performed to constrain non-Standard Model right-handed currents in flavour-changing neutral current semi-leptonic decays. New lepton identification techniques in the Belle experiment are developed for use in the first study of $B \to K^* e^+ e^-$ decays at the very low dilepton invariant mass region, $q^2 < 1.12 \gevccsq$, to constrain the Wilson coefficient $\mathcal{C}_7$, and its right-handed counterpart $\mathcal{C}_7^{\prime}$, expected to be zero in the Standard Model.
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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.
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ItemMeasurement of Direct CP Asymmetry and Branching Fraction in B0→D0𝜋0 and B+→D0𝜋+ at the Belle Experiment( 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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ItemMeasurement of R(D) and R(D*) with a semileptonic tag at the Belle experiment( 2019)In recent years, several measurement of the ratios R(D) = B(B ̄ → Dτ−ν ̄τ )/B(B ̄ → Dl−ν ̄l )andR(D∗)=B(B ̄→D∗τ−ν ̄τ )/B(B ̄→D∗l−ν ̄l ),whereldenotesanelectron or a muon, have obtained results in disagreement with the theoretical predictions of the Standard Model (SM) of particle physics. When combining all experimental results together, the statistical significance of the discrepancy with the SM accounts to 4σ. If confirmed, this discrepancy would directly require an extension of the SM with new physics theories, to accommodate the experimental results. This thesis reports a new independent measurement of the ratios R(D) and R(D∗). The results are based on a data sample containing 772 × 106 BB ̄ pairs recorded at the Υ (4S ) resonance with the Belle detector at the KEKB e+ e− collider in Japan. The analysis uses a semileptonic reconstruction of the tag-side B-meson and leptonic τ decays. The measured values are R(D) = 0.307±0.037 (stat.) ± 0.016 (syst.) and R(D∗) = 0.283 ± 0.018 (stat.) ± 0.014 (syst.), which are in agreement with the SM predictions within 0.2σ and 1.1σ respectively. The R(D)-R(D∗) combined result is in agreement with the SM predictions within 1.2σ. The discrepancy of the experimental world average with the SM expectations de- creases from 4 to 3σ when including these latest results, which represent to most precise measurement of R(D) and R(D∗) ever performed.
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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).
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ItemMeasurement of the branching fraction of B0 → π0 π0 decays using the final Belle dataset( 2015)This thesis presents a measurement of the branching fraction of B0 → π0 π0 using data collected by the Belle experiment based at the KEKB accelerator in Tsukuba, Japan. This study is performed using the final and complete Belle dataset of 711 fb^−1, comprising about 771 × 10^6 BB pairs collected at the Υ(4S) resonance. The measurement of B0 → π0 π0 is a vital component of the study into the B → ππ system, through which a measurement of the CP violation parameter φ2 (also called α) can be made. This analysis makes use of an artificial neural network to improve continuum suppression as well as making use of timing information pertaining to photons detected in the Belle electromagnetic calorimeter to help distinguish between signal and background originating from QED backgrounds. Photon reconstruction was enhanced through the recovery of photons that converted within the inner detector. The timing information for 20% of the Belle dataset was recovered specifically for the use of this analysis. This is the first time a branching fraction for B → π0 π0 has been presented using an artificial neural network and such QED suppression techniques. The branching fraction of B → π0 π0 is measured to be Br(B → π0 π0 ) = (1.22 +0.28 −0.27,+0.10 −0.12 ) × 10^−6.