School of Physics - Theses
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ItemProbing Higgs Boson physics in decays to tau leptons with the ATLAS experiment( 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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ItemMeasurements of the ATLAS tau trigger reconstruction and identification efficiencies using 2016 data from \(pp\) collisions at\(\sqrt s \) = 13 T\(e\)V( 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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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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ItemPhenomenology of particle dark matter( 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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ItemPhysics beyond the standard model( 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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ItemSearch for the Higgs Boson produced in association with a Vector Boson at ATLAS( 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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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.
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ItemInteracting dark matter: decay and bremsstrahlung processes( 2013)Though there is substantial indirect astrophysical evidence for the existence of dark matter (DM), it has yet to be directly detected. Consequently, little is known about its internal structure. It is possible that there is a small but finite non-gravitational interaction between dark matter and the Standard Model (SM) which may have observable consequences. The purpose of this thesis is the exploration of some of these interactions and consequences. In particular we consider the possibility that dark matter is unstable on long timescales, as motivated by discrepancies between simulation and observation of structure on sub-galactic scales. We also consider the consequences of electroweak radiative corrections to annihilation processes involving dark matter, as such corrections are necessarily present in many well motivated models. We consider this possibility in the contexts of dark matter annihilation in galactic halos, and production in colliders. Chapter 1 provides an introduction to dark matter, including some of its astrophysical and particle aspects. As a motivation for the following sections, we begin by briefly outlining some of the observational evidence for dark matter. We go on to discuss structure formation, and the cold dark matter distribution on galactic scales. Next we discuss the possibility of non-gravitational interactions involving dark matter, including decay, annihilation, scattering off nuclei, and production. Finally we discuss the determination of the relic abundance in the early Universe, including a discussion of models involving coannihilation. Late decaying dark matter has been proposed as a solution to the small scale structure problems inherent to cold dark matter cosmology. In these models the parent dark matter particle is unstable, and decays into a daughter with near degenerate mass, plus a relativistic final state. In Chapter 2 we review the observational constraints on decaying dark matter, and construct explicit particle physics models to realize this scenario. To achieve this, we introduce a pair of fermionic dark matter candidates and a new scalar field, which obey either a Z4, or a U(1) symmetry. Through the spontaneous breaking of these symmetries, and coupling of the new fields to standard model particles, we demonstrate that the desired decay process may be obtained. We also discuss the dark matter production processes in these models. In Chapter 3 we investigate electroweak radiative corrections to dark matter annihilation into leptons, in which a W or Z boson is also radiated. In many dark matter models the annihilation rate into fermions is helicity suppressed. We demonstrate that bremsstrahlung processes can remove this helicity suppression, causing the branching ratios Br($\ell \nu W $), Br($\ell^+\ell^-Z$), and Br($\bar\nu \nu Z$) to dominate over Br($\ell^+\ell^-$) and Br($\bar\nu \nu$). We find this effect to be most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Finally, in Chapter 4, we investigate a mono-Z process as a potential dark matter search strategy at the Large Hadron Collider (LHC). In this channel a single Z boson recoils against missing transverse momentum attributed to dark matter particles, $\chi$, which escape the detector. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. We look for muonic decays of the Z, showing the Standard Model backgrounds to this process to be easily removable with modest selection cuts. We compare signal with Standard Model backgrounds and demonstrate that there exist regions of parameter space where the signal may be clearly visible above background in future LHC data.
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ItemSearching for the Light Supersymmetric Top Quark with the ATLAS experiment( 2011)The nature of dark matter and the source of the matter-antimatter asymmetry in the universe are two of the most important questions in particle physics and cosmology. The current Standard Model of particle physics, while being a very successful description of the observed fundamental particles and their interactions, cannot fully account for either of these phenomena. Theoretical extensions of the Standard Model, however, possibly can. One such extension is the Minimal Supersymmetric Standard Model (MSSM). This thesis begins by exploring the MSSM parameter space in which the matter- antimatter asymmetry of the universe is dynamically generated through electroweak baryogenesis. In this scenario, one of the supersymmetric partners to the top quark, the light stop quark, must be lighter than the top quark. It is found that this parameter space region is highly constrained by experimental limits on the electric dipole moment of the electron and the branching ratio of a bottom quark into a strange quark and a photon. If the additional requirement of matching the observed dark matter abundance by the relic density of the lightest supersymmetry particle is necessitated, the allowed MSSM parameter space is further constrained. The focus of the thesis then moves to the investigation of the collider phenomenology of supersymmetric electroweak baryogenesis, in particular, the evaluation of the discovery potential of light stop quark pair production at the LHC using the ATLAS experiment. This study assumes a light stop decay topology involving the lightest chargino and neutralino where the visible final state products mimic those from top quark pair production. Feasibility studies are performed for proton-proton collisions at centre of mass energies of 10 TeV and an integrated luminosity of 1/fb, concentrating on the dileptonic and semileptonic decay channels where there are two or one charged leptons in the final state. It is found that signal points with stop masses less then 120 GeV and stop-neutralino mass differences greater than 60 GeV have the greatest discovery potential in the dileptonic decay channel, while the semileptonic decay channel is swamped by backgrounds and requires detailed understanding of the detector and backgrounds in order to extract a signal. Finally, a preliminary study is conducted on 41.4/pb of data collected at collisions with centre of mass energies of 7 TeV in the dielectron decay channel, focusing on the understanding of selection variables and backgrounds.