Browsing by Author "Hamilton, Andrew"

Now showing 1 - 9 of 9
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    Data-driven QCD background estimation to W production in association with jets using the ATLAS detector
    (2016) Radovanovic, Lidija; Hamilton, Andrew
    A preliminary analysis was conducted using data from the proton-proton collisions at p s = 8 TeV recorded by the ATLAS experiment, corresponding to an integrated luminosity of approximately 18.9 ƒƅ⁻¹. It has been performed to extract the fraction of QCD events in the chosen phase space for the W boson production in association with jets. A data-driven technique was developed for measuring the QCD background. The fraction of QCD events in the signal region was estimated in exclusive jet multiplicities by performing an extended likelihood fit on the Emiss T distribution in data. The fraction of QCD events ranges 2:9 ± 0:01% for the zero jet bin and increases to 10:74 ± 0:11% for the two jet bin.The fraction of QCD events decreases with increasing jet multiplicity. Using the results from the fit, some important kinematic distributions have been made. The overall agreement between the data and Monte Carlo simulation is vastly improved by including the data-driven QCD background estimate. This agreement could be further improved by using a data-driven tt estimate where the tt MC tends to overestimate the amount of events in the higher jet multiplicities.
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    Exclusive J/Ψ Vector-Meson production in high-energy nuclear collisions: a cross-section determinaton in the Colour Glass Condensate effective field theory and a feasibility study using the STARlight Monte Carlo event generator
    (2014) Ramnath, Andrecia; Weigert, Heribert; Hamilton, Andrew
    The cross-section calculation for exclusive J /Ψ vector-meson production in ultra-peripheral heavy ion collisions is approached in two ways. First, the setup for a theoretical calculation is done in the context of the Colour Glass Condensate effective field theory. Rapidity-averaged n-point correlators are used to describe the strong interaction part of this process. The JIMWLK equation can be used to predict the energy evolution of a correlator. In order to facilitate practical calculations, an approximation scheme must be employed. The Gaussian Truncation is one such method, which approximates correlators in terms of new 2-point functions. This work takes the first step beyond this truncation scheme by considering higher-order n-point functions in the approximation. An expression for the cross-section is written, which takes parametrised 2- and 4-point correlators as input. This expression can be used as the basis for a full cross-section calculation. The second part of the thesis is a feasibility study using Monte Carlo simulations done by the STARlight event generator. A prediction is made for how many exclusive J /Ψ vector-mesons are expected to be detected by ATLAS in a data set corresponding to 160 μb−1 total integrated luminosity. It is found that the muon reconstruction efficiencies for low pT muons is too poor in ATLAS to do this analysis effectively. On the order of 150 candidate events are expected from all the Pb-Pb collision data collected in 2011. The feasibility study acts as a preliminary investigation for a full cross-section measurement using ATLAS collision data. Once this is completed, it can be compared with the theoretical prediction for the cross-section.
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    Measurement of exclusive Di-Muon production in the ATLAS experiment
    (2015) Schenck, Ferdinand Wilhelm; Hamilton, Andrew
    In this thesis a method for identifying exclusively produced di-muon pairs in proton-protoncollisions at the ATLAS experiment is described.
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    Parallel computing, benchmarking and ATLAS software on ARM
    (2015) Smith, Joshua Wyatt; Hamilton, Andrew
    This thesis explores the use of the ARM architecture in high energy physics computing. ARM processors are predominantly found in smartphones and mobile tablets. Results from benchmarks which were performed on the armv7l architecture are presented. These provide qualitative data as well as confirmation that specialized high energy physics software does run on ARM. This thesis presents the first ever port of the ATLAS software stack to the ARM architecture, as well as the issues that ensued. A new framework, ANA, is introduced which facilitates the compilation of the ATLAS software stack on ARM.
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    Single top channel in association with H→bb̄ production at ATLAS : a feasible study
    (2015) Antel, Claire; Hamilton, Andrew; Gray, Heather M
    A degeneracy in the minima of the profile likelihood function of the Higgs boson coupling to fermions and bosons, where boson couplings are assumed to be positive, is reported by both ATLAS and CMS. Although one minimum lies in the region of positive fermionic coupling scale factors, consistent with the Standard Model, the second minimum lies in the negative region-a region which would indicate new physics. This degeneracy can potentially be resolved by studying a fermionic-bosonic interference that takes place in the single top quark production in association with a Higgs particle, in which the cross-section is significantly enhanced for anomalous coupling values. A truth level feasibility study of the tH process with H→bb̄ is conducted at 8 TeV and 21 fb-¹ of integrated luminosity. the equivalent of the 2012 run at the LHC. A sensitivity of l.7σ is found for the enhanced case of a coupling scale factor of cF = -1. This is insufficient for a detection. A projection to 14 TeV centre-of-mass energies indicates that at truth-level a 3σ significance with 15 fb-¹ of integrated luminosity can be reached for the enhanced case. However, assuming as low as 5% systematic uncertainty lowers this to below 2σ across the whole luminosity range, indicating that the upcoming run at LHC will also be insufficient to resolve the degeneracy.
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    Studies of the W±W± scattering process in pp collisions at the once and future ATLAS detector
    (2017) Van Tonder, Raynette; Yacoob, Sahal; Hamilton, Andrew
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    Suppression of the fake lepton background in same-sign W-boson scattering with the ATLAS experiment
    (2017) McConnell, Lucas Henry; Hamilton, Andrew; Yacoob, Sahal
    Same-sign W-boson scattering is a rare Standard Model process that is useful for probing the nature of electroweak symmetry breaking and the Higgs mechanism. Analysis is currently underway to measure the cross-section to a significance of 5σ or higher using √s = 13 TeV data from the ATLAS detector's Run 2. The two scattered W-bosons decay leptonically leaving a distinctive experimental signature of two same-sign leptons, two forward jets, and missing transverse energy carried away by two neutrinos. Non-prompt leptons are defined as leptons coming from the decay of hadrons. Such leptons, together with jets misreconstructed as leptons, contribute to the background processes in same-sign W-boson scattering; making up the so-called fake lepton background. In this thesis the fake lepton background is suppressed using two strategies: 1) implementing an optimised veto on events found to contain a b-jet; and 2) optimising the isolation requirements set on signal lepton candidates using the cumulative significance quantity. The approach using the cumulative significance is then extended to optimise additional analysis cuts on the lepton invariant mass mₗₗ, jet invariant mass mⱼⱼ , and the jet separation rapidity Δyⱼⱼ.