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Title: Double Drell-Yan measurement, heavy Higgs searches in the four leptons final state and forward muons performance with the ATLAS detector
Author: Kyriazopoulos, Dimitrios
ISNI:       0000 0004 5991 5248
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2015
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In this thesis, physics studies are presented using data collected by the ATLAS detector at CERN on 2012 corresponding to L = 20.3 fb-1, at the LHC \sqrt{s} = 8 TeV. The studies performed refer to the identification of events in final states with two Z bosons decaying to four leptons (electrons, muons). Furthermore, the effect of QED radiative corrections in the Z -> ll process and the muon reconstruction efficiency in the forward region of the ATLAS detector have been studied. The first study is the cross-section of the Double Drell-Yan process Z+Z -> 4l. The cross-section is measured to be 0.75^{+0.51}_{-0.44}(stat)^{+0.18}_{-0.16}(syst.) fb. Due to the large uncertainties a 95% CL upper limit in the cross-section is set of 1.54 fb. This result is converted to a 95% CL lower limit on the phenomelogical factor σeff of 0.40 mb. The second study is the search for an additional heavy Higgs boson in the H -> ZZ -> 4l final state. The search is performed in the mass range between 140 - 1000 TeV. No significant excess in data with respect to the background expectations are observed, and upper limits are set on the production cross-sections for all the mass points. The cross-section x branching ratio limits are extracted independently for the gluon-gluon fusion (ggF) and vector-boson fusion (VBF) production mechanisms. Additionally, the effect of non-collinear QED radiative corrections is examined in the H->ZZ->4l. QED Final State Radiation (FSR) of leptons originating from Z bosons is reconstructed focusing on non-collinear radiation from the lepton. The photons are reconstructed with high efficiency and purity looking at Z -> μμ and Z -> ee events in the 2012 data. At the ATLAS Run-I 2012 H -> ZZ->4l mass measurement, 2 out of 30 Higgs candidates in the signal window (120-130 GeV) had their final invariant mass corrected for non-collinear FSR effects. Lastly, the reconstruction performance of muons in the forward pseudorapidity region of the detector is examined in the 2012 data Z -> μμ decays. The study focuses on the region 2.5 < |η| < 2.7$. The reconstruction efficiency is at the level of >90%. Efficiency scale factors are extracted which are used to correct the simulation of the muon behaviour at this part of the detector, in order to match data observations.
Supervisor: Anastopoulos, Christos Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available