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Title: A measurement of the recently discovered Higgs Boson in the decay into two photons with associated jets, using the ATLAS detector at the LHC
Author: Cantrill, Robert J. B.
ISNI:       0000 0004 5356 3608
Awarding Body: University of London
Current Institution: University of London
Date of Award: 2014
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A new boson has been discovered and measurements lire under way using the 7TcV and 8TeV proton-proton collision data from the L1rge Hadron Collider to determine whether or not this is the Higgs boson as predicted by the Standard Model of Particle Physics (SM). Experimentally measuring the nature of this particle's couplings to other particles will help determine this. The Standard Model Higgs boson is expected to be produced by a variety of production mechanisms. The SM prediction is that the gluon-gluon fusion (ggF) and vector boson fusion (VRF) production mechanisms are the two production processes with the highest and second-highest, rates respectively. This thesis concentrates on the study of the Higgs boson via its decay into two photons, which was one of the key discovery channels. Part of this analysis is to mensure the ratio (9l) of these rates using J3fb I of ATLAS .fi = 8TeV proton-proton collision data and determine if ~ is consistent with the SM prediction. Using the diphoton decay channel, events were selected to fonn a category 01" data events which is enriched in VBF events with little gluon-gluon fusion contamination. The selection procedure was optimised using a boosted decision tree (BOT) multi variate classifier. The distinguishing feature of this analysis was that the ~DT was trained using hackground events from the data sample, so as to reduce the dependency 0 11 the modelling of the background processes. It was shown that using a BDT classifier, the VBF signal significance improves by 24.0% relative to the standard cut·based analysis and suffers from 12.0% less ggF signal contamination. Using this event classification ~ was measured as ~ = OvnF /(a 881' + O"vBd = 0.037 ± 0.067(stat) ± O.035(syst) where O"VBF and OggF are the respecti ve cross .~ec tions of the vector hoson fusion process and the gluon-gluon fusion process. The SM prediction is 9l ;.;;; 0.075 Although the uncertainty on the currcllI measurement is large. it is shown using pseudodata, that this choice of categorisation will help reduce the uncertainty on 9l when more data are available.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available