Browsing by Author "Mwewa, Chilufya"

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    Observation of the electroweak production of two same-sign W bosons in proton-proton collisions with the ATLAS detector
    (2020) Mwewa, Chilufya; Yacoob, Sahal; Hamilton Andrew
    The production of same sign WW bosons (W±W±) is an extremely rare process predicted within the Standard Model (SM). It results from electroweak mediated processes such as Vector Boson Scattering (VBS), a process linked with electroweak symmetry breaking. Therefore, an observation of the W±W± VBS process does not only confirm the predictions of the SM at such a small cross-section, but it also provides an opportunity to test the electroweak sector and the Higgs mechanism. Evidence for this process was found, at a significance of 4.5 and 2.0 by the ATLAS and CMS experiments respectively, during Run I of the Large Hadron Collider (LHC). Following this potential for discovery, this search was repeated in Run II of the LHC. The CMS experiment reported an observation of this process at a significance of 5.5 in September 2017 while ATLAS reported it at a significance of 6.9 in July 2018. In ATLAS, the search was conducted by looking for events with two same sign leptons (e±e±, e± µ± and µ±µ± ), large missing transverse momentum (Emiss T ) and two forward jets using 36.1 fb1 of data collected at a proton-proton center of mass energy of 13 TeV in 2015 and 2016. This thesis presents an overview of the observation of this rare process by the ATLAS experiment while focussing on the study of backgrounds resulting from photon conversions as well as studies for the enhancement of the the VBS signal which were the student's primary contribution to the full analysis. In addition, the student's contribution to the ATLAS upgrade project is also highlighted.
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    Statistical interpretation of exotics monojet data in search of an invisibly decaying Higgs Boson
    (2014) Mwewa, Chilufya; Hamilton, Andrew
    Following the recent discovery of a Standard Model Higgs-like particle at the Large Hadron Collider, this study searches for the evidence of invisible decays of this particle. Assuming that this is the Standard Model Higgs boson, its decay to invisible particles is not expected to be measurable in the current data. However, it could have a large contribution from its decay to stable non-Standard Model particles such as the hypothetical dark matter particles. This study corresponds to 4.7 fb!1 of 7 TeV proton-proton collisions and 20.3 fb!1 of 8 TeV proton-proton collisions. At the time of thesis submission, the 8 TeV results were not unblinded by the ATLAS Collaboration, so toy-data are presented here to demonstrate the procedure. The performance of the statistical framework to be used in the combination of the 7 TeV data with the real 8 TeV data is assessed and is found to perform very well. The results are interpreted to set 95 confidence level limits on the branching ratio to invisible particles of the newly discovered Higgs-like particle at a mass of 125 GeV. Limits are also set on the production cross section ⇥ branching ratio of additional Higgs-like particles that decay invisibly in the mass range: 115 GeV to 300 GeV. In the combination of the 7 TeV data and 8 TeV toy-data, an expected (observed) upper limit of0.89 (0.59) is set on the branching ratio to invisible particles of a 125 GeV Higgs boson. In the mass range 115 to 300 GeV, no excess beyond the Standard Model expectation is observed.