School of Physics - Theses

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    Estimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment
    Mahmood, Ibtihal ( 2022)
    Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.
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    Measurement 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
    Krohn, Jo-Frederik ( 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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    Probing Higgs Boson physics in decays to tau leptons with the ATLAS experiment
    Le, Brian ( 2019)
    The discovery of the Higgs boson in 2012 has opened up a new sector of particle physics to measurements of yet undiscovered couplings. Measurements of the numerous couplings of the Higgs boson - primarily through studies of bosonic decays - have consistently confirmed the Standard Model of Particle Physics. The measurement of fermionic Yukawa couplings, which is a free parameter of the Standard Model, is a critical test of the origin of mass in the Standard Model and serves as a probe for a wide array of new physics models. This thesis presents a measurement, search and feasibility study of couplings of the Higgs boson to the third generation lepton, the tau . A measurement is presented of the H -> tau tau cross-section which is performed using 36.1 inverse fb of sqrt(s) = 13 TeV collision data collected at the Large Hadron Collider by the ATLAS detector. A 6.4 sigma excess over the background-only hypothesis was observed in combination with sqrt(s) = 7, 8 TeV which constitutes a discovery of the final remaining coupling to third generation fermions. The total cross-section was measured as 3.70 +- 0.58(stat) +0.89-0.75(syst) pb, consistent with the expectation from the Standard Model. Several new physics models predict phenomena which can probed in supressed Higgs boson decays: one such phenomena is lepton flavour violation. A search for lepton flavour violating Higgs boson decays to e,mu and mu,e final states, using a new multivariate approach, is also detailed. This search used the same dataset as for the H -> tau tau cross-section. No excess is observed and upper limits on the branching ratio for H -> mu tau and H -> e tau of 0.47% and 0.28% are set, respectively. The limit on H -> e tau is now the new global limit. Finally a feasibility study for measuring the state of the Higgs boson using decays to two tau leptons is demonstrated. At tree-level, the H -> tau tau decays are sensitive to CP admixture couplings to a Higgs boson. One of the promising decay channels, where the tau lepton decays to three charged pions, is shown to be viable thanks to development of a robust neural network approach.
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    Measurements of the ATLAS tau trigger reconstruction and identification efficiencies using 2016 data from \(pp\) collisions at\(\sqrt s \) = 13 T\(e\)V
    Mason, Lara ( 2017)
    This thesis presents the performance of the tau trigger algorithm used by the ATLAS experiment to select hadronically decaying tau leptons in the LHC Run 2. Using the 33.3 \(f{b^{ - 1}}\) of \(pp\) collisions data recorded in 2016 at\(\sqrt s \) = 13 T\(e\)V, the performance of this algorithm is studied using a `tag-and-probe' based analysis in order to select Z boson decays to tau leptons, where one tau decays hadronically and the other leptonically. The reconstruction and identification efficiencies of the tau trigger algorithm are measured, and good performance is observed. The efficiency of the tau trigger in data is compared with that in simulation, and is parametrised as a function of the tau decay topology, its kinematics, and the average number of interactions per bunch crossing. The selection efficiency at each step of the high level trigger is measured, using dedicated intermediary triggers, and good agreement between data and simulation is observed. Using the comparison between reconstruction and identification efficiencies in data and simulation, correction factors for simulated events are measured, which are utilised by the entire ATLAS collaboration.
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    Investigation of B+ mesons decay to K+K−π+ at the Belle experiment
    Hsu, Chia-Ling ( 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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    B0→K0π0 and direct CP violation at Belle
    Hawthorne-Gonzalvez, Anton ( 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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    Phenomenology of particle dark matter
    Leane, Rebecca Kate ( 2017)
    The fundamental nature of dark matter (DM) remains unknown. In this thesis, we explore new ways to probe properties of particle DM across different phenomenological settings. In the first part of this thesis, we overview evidence, candidates and searches for DM. In the second part of this thesis, we focus on model building and signals for DM searches at the Large Hadron Collider (LHC). Specifically, in Chapter 2, the use of effective field theories (EFTs) for DM at the LHC is explored. We show that many widely used EFTs are not gauge invariant, and how, in the context of the mono-W signal, their use can lead to unphysical signals at the LHC. To avoid such issues, the next iteration of a minimal DM framework, called simplified models, are considered. We discuss use of such models at the LHC in Chapter 3, and show that in the context of a renormalizable gauge-invariant theory, any isospin violating effects in mono-W signals cannot be large. In Chapter 4, we discuss an alternative search strategy to mono-X searches at the LHC — in the case that DM does not couple directly to hadrons, the mono-X signature does not exist, and instead a leptophilic DM signature can be probed. We focus on the prospects for leptophilic DM with a spin-1 mediator at the LHC, and discuss constraints from other experiments. In the third part of this thesis, we turn to astrophysical signals of DM. In Chapter 5, we show that a consequence of enforcing gauge invariance in simplified DM models provides a new dominant s-wave DM annihilation process for indirect detection searches, and set limits on the annihilation cross section from Pass 8 observations of the Fermi Gamma-ray Space Telescope. In Chapter 6, we demonstrate the impact of mass generation for simplified models, finding that the relic density and indirect detection constraints, along with the DM interaction types, are strongly dictated by the mass generation mechanism chosen. In Chapter 7, we show that the multi-mediator approach advocated in the previous two chapters can also lead to a new dominant signal, in the form of dark initial state radiation. Finally in Chapter 8, we look to the Sun to find that if DM annihilates to long-lived mediators, the gamma rays and neutrinos produced can be strongly probed by gamma-ray telescopes and observatories Fermi-LAT, HAWC, and LHAASO, as well as neutrino telescopes IceCube and KM3Net. Interestingly, these telescopes can provide the strongest probe of the DM spin dependent scattering cross section, outperforming standard high-energy solar neutrino searches and direct detection experiments by several orders of magnitude.
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    Physics beyond the standard model
    Clarke, Jackson David ( 2016)
    In this Thesis we present a collection of original bodies of work pertaining to a number of theoretical/phenomenological questions of the Standard Model, as studied from a "bottom-up" perspective. In Chapter 2: Higgs Sector, we consider the implications of extending the Standard Model Higgs sector by a very light (100 MeV < $m_s$ < $m_h/2$) real singlet scalar field. We identify the regions of parameter space which experiments at the Large Hadron Collider are uniquely sensitive to. There is a strong focus on low background displaced decay signatures. In Chapter 3: Naturalness, we show how a Higgs mass sensitivity measure can be rigorously derived from Bayesian probability theory. We use this measure to constrain the masses of various fermionic and scalar gauge multiplets, obtaining naturalness bounds of O(1-100) TeV. In Chapter 4: Neutrino Mass, we write down the minimal UV completions for all the Standard Model dimension 7 operators which might be responsible for the radiative generation of Majorana neutrino masses. A detailed collider study of a one-loop realisation is performed. In Chapter 5: Baryon Asymmetry of the Universe, we present a proof that the three-flavour Type I seesaw model cannot provide an explanation for neutrino masses and the baryon asymmetry of the Universe via hierarchical leptogenesis without introducing a Higgs naturalness problem. We then describe a minimal extension (the "$\nu$2HDM") which can avoid this conclusion. In Chapter 6: Strong CP Problem, we describe a very minimal model (the "$\nu$DFSZ") which can explain neutrino masses, the baryon asymmetry of the Universe, the strong CP problem, and dark matter, without introducing a naturalness problem for the Higgs. This model serves as an existence proof that weakly coupled high scale physics can naturally explain phenomenological shortcomings of the Standard Model. Lastly, in Chapter 7: Dark Matter, we consider the implications of a class of self-interacting "plasma dark matter" models for direct detection experiments. A number of qualitatively unique signatures (when compared to single component collisionless dark matter) are identified. We emphasise the prediction for a signal which modulates with sidereal day.
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    Search for the Higgs Boson produced in association with a Vector Boson at ATLAS
    Jennens, David ( 2015)
    On the 4th of July, 2012, the ATLAS and CMS experiments at CERN announced the discovery of a particle consistent with the Standard Model Higgs boson through its decay to bosons. Subsequent measurements have established that the particle’s spin, parity and coupling strengths are in line with Standard Model expectation, making this the strongest evidence of the Standard Model electroweak symmetry breaking mechanism. This thesis concerns the search for Higgs boson decays to two tau leptons, one of the leading Standard Model Higgs decay modes, with particular focus on events in which the Higgs is produced in association with a weak vector boson. Evidence for the existence of such events constitutes a crucial test of the Higgs’ coupling to leptons. The search is conducted using the 20.3 fb−1 of data collected by the ATLAS detector in 2012 from proton-proton collisions in the Large Hadron Collider at a centre-of-mass √ energy of s = 8 TeV. The search is optimised for events in which the associated vector boson decays to an electron or muon and one or both tau leptons from the Higgs decay subsequently decay to hadrons. The dominant background comes from events in which a jet is misidentified as the hadronic decay products of a tau decay, which is estimated from data samples in a signal-free control region using the rate of misidentification. The trigger and identification efficiencies are measured using dedicated calibration data samples with a tag-and-probe technique. No significant excess above the expected background yield is observed in any signal region. The signal strength of best fit, for a Higgs boson mass of 125 GeV, is 2.3 ± 1.6. Instead, limits are placed on the rate at which the event rate might exceed the Standard Model expectation. The event rate is found to be no greater than 5.6 times that expected by the Standard Model at a 95% confidence interval. This observed limit is less than the expected limit of 3.5 times the SM event rate, under the no signal hypothesis, or 3.7 times the SM event rate with signal injected, but is consistent within uncertainties.
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    Measurement of the branching fraction of B0 → π0 π0 decays using the final Belle dataset
    JULIUS, T'MIR ( 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.