The generation of vector boson mass via the Higgs mechanism in the Standard Model has been confirmed by the 2012 discovery of a candidate Higgs boson in the H→WW, H→ZZ, and H→γγ decay channels. In contrast, the Yukawa couplings hypothesized to provide the mass of fermions in the Standard Model have yet to be observed. The H→ττ decay channel currently provides the best opportunity for observing these couplings. This thesis describes two separate but related searches for Higgs boson decays in the H→ττ decay channel using proton-proton collisions recorded by the ATLAS detector. The first analysis is a general search for all Higgs boson production mechanisms leading to a H→ττ decay using 4.5 fb<sup>−1</sup> of 7 TeV and 20.3 fb<sup>−1</sup> of 8 TeV proton-proton collision data. A deviation from the background-only hypothesis is observed with a significance of 4.5σ for a hypothetical Higgs boson mass of m<sub>H</sub> = 125 GeV — a strong indication of a H→ττ signal. For the same mass point, the best fit value for the signal strength is found to be 1.43 <sup>+0.43</sup><sub style='position: relative; left: -2.4em;'>−0.37</sub> x the Standard Model expectation. The second analysis is a search for Higgs boson production in association with a vector boson using 20.3 fb<sup>−1</sup> of 8 TeV proton-proton collision data. Results in the Z<sub>ll</sub>H→<sub>τlτh</sub> channel indicate limits of 9.14 x the Standard Model expectation for VH signal production at m<sub>H</sub> = 125 GeV. In addition, two studies on enhancement of computing performance in the ATLAS trigger and data analysis pipeline are presented.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664779 |
| Date | January 2015 |
| Creators | Howard, Jacob Russell |
| Contributors | Hays, Christopher |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:14e6e708-0cd6-4022-a4b2-343c18c8e260 |
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