Browsing by Subject "Particle physics"
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Item Aspects of new physics beyond the standard model : electroweak naturalness, dark matter and holographic quantum gravity(2017-08) Swaminathan, Sivaramakrishnan; Kilic, Can; Distler, Jacques; Kaplunovsky, Vadim; Paban, Sonia; Shapiro, PaulThis dissertation is based on studies of three aspects of physics beyond the Standard Model. First, we summarize a study of high temperature symmetry restoration in extensions of the Standard Model where the naturalness problem is resolved due to the Higgs being a pseudo-Nambu-Goldstone boson. Second, we analyze a framework for building quantum field theoretic models where dark matter has its cosmic origins in an asymmetry, while the overall dark charge is conserved and the dark matter relic is actually symmetric at late times. Finally, we study the relation between holographic `bulk' geometry, and the structure of entanglement in the quantum field theory living on the boundary as characterized by the Multiscale Entanglement Renormalization Ansatz (MERA).Item Simulation Studies for the KLEVER Small-Angle Calorimeter(2021-05) Tsai, Emily; Moulson, Matthew; Andeen, Timothy R. JrThe Standard Model of particle physics attempts to describe all observable particles and forces, excluding gravity. There are also many fundamentally conserved quantities under the SM, such as charge, lepton flavour, and baryon number. The Standard Model can be used to calculate branching ratios, or the probability of one decay happening out of all possible decays for a particle. The branching ratio for the KL → π0νν ̄ decay is predicted to be BR(KL → π0νν ̄) = (3.4 ± 0.6) × 10−11 [1] and is extremely suppressed under the Standard Model, making this a prime candidate for potentially observing physics beyond the Standard Model. BR(KL → π0νν ̄) has never been experimentally measured. Discovering evidence of and measuring phenomena beyond the Standard Model could help to understand the potential flavour structure of new physics, and would help solve many open questions, such as the hierarchy problem, matter- antimatter asymmetry, and the origin and nature of dark matter. This paper focuses on simulating and testing the effectiveness of a lead fluoride-based small- angle calorimeter in catching photons escaping from the beam hole of KLEVER, an experiment aiming to measure BR(KL → π0νν ̄) to ∼20% accuracy. By catching these photons, the SAC will differentiate KL → π0π0 decays (BR = 8.64 × 10−4) from the decay of interest. First, fast- simulation of the entire KLEVER detector was used to obtain the needed inefficiencies for the SAC. Then, a detailed simulation of the SAC was constructed to confirm its opacity to photons and transparency to hadrons and test its reached inefficiencies. Finally, future work focuses on integrating detector readout and oriented crystals into the Geant4 simulation, testing the SAC with a more non-idealized KL beam, and testing different configurations of the SAC. As a rough approximation, the SAC currently reaches the needed inefficiencies, though the SAC efficiency will be decreased by a factor of ∼10 once the full readout chain is simulated, which may be compensated in part by increases from detector design improvements.Item Study of WZ + heavy flavor production and prospects for differential measurements of tt̄H-multilepton with the ATLAS detector(2021-10-05) Webb, Aaron; Onyisi, Peter; Andeen, Timothy; Kilic, Can; Scott, JamesSeveral theories Beyond the Standard Model predict a modification of the momentum spectrum of the Higgs Boson, without a significantly altered rate of Higgs produced in association with top quark pairs (tt̄H). This provides a physical observable that can be used to search for new physics with data collected at the LHC. This thesis presents techniques and preliminary results for a differential measurement of the Higgs transverse momentum in tt̄H events with multiple leptons in the final state, using data collected at an energy of √s = 13 TeV by the ATLAS detector at the LHC. Preliminary results are presented for 80 fb⁻¹ of data, with projected results shown for 140 fb⁻¹. A measurement of WZ + heavy flavor production, a significant background to tt̄H that is poorly understood, is also presented. This study targets the fully leptonic decay mode, WZ→ lνll, corresponding to events with three leptons and one or two jets in the final state, using 140 fb⁻¹ of √s = 13 TeV data. Cross-section measurements for WZ + b and WZ + c for events with one or two associated jets are shown.Item Towards a measurement of neutral pion production in neutral current interactions with the NOνA near detector(2022-07-08) Tapia Oregui, Beatriz; Lang, Karol, 1955-; Andeen, Timothy; Ritchie, Jack; Kumar, PawanThis dissertation reports the steps towards a cross-section measurement of the neutral-current producing at least one neutral pion above detection threshold in the NOvA near detector. Neutral pions decay into two photons 99% of the time, so the signature for this analysis is two electromagnetic showers slightly away from the vertex. This neutrino cross-section measurement is very important in the context of electron neutrino appearance, since one or both of the two gammas can mistakenly be reconstructed as an electron. The event selection requires two fully contained reconstructed showers with a minimum number of hits, whose reconstructed vertex is within the fiducial volume. To reduce the charged-current background with a muon in the final state, the analysis includes a cut on a muon identifier (trained on simulated single particles). Several reconstructed variables like the invariant mass, widths and distances between the vertex and the start of the showers are fed to a boosted decision tree (BDT), which also contains an electromagnetic shower identifier. The plan is to perform a fit to data for each bin within the phase-space grid, such that signal and background templates vary in normalization. A BDT event classifier output is the distribution chosen for the fit and templates. Signal and background templates predicted from simulation help extract the actual signal component for data in several π⁰ kinematic bins of reconstructed energy and angle with respect to the beam direction. This template fit should account for bin-to-bin correlations of systematic uncertainties via a covariance matrix. The final result is the amount of simulated events for the phase-space proposed binning, which accounts for a total of 8,970 events.