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 Title: Search for a charged Higgs Boson in proton-proton collisions at the LHC using the ATLAS detector Author(s): McCarn, Allison Director of Research: Neubauer, Mark S. Doctoral Committee Chair(s): Pitts, Kevin T. Doctoral Committee Member(s): Neubauer, Mark S.; Thaler, Jonathan J.; Willenbrock, Scott S. Department / Program: Physics Discipline: Physics Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Charged Higgs A Toroidal LHC Apparatus (ATLAS) Abstract: This thesis documents several searches for charged Higgs bosons ($H^{\pm}$) in the decay $H^{\pm} \rightarrow \tau_{had} \nu$, which are performed using data recorded in 2011 and 2012 by the ATLAS detector at the LHC. The $H^{\pm}$ bosons are predicted by some extensions of the Standard Model (SM) that include multiple Higgs doublet fields, such as the Minimal Supersymmetric Standard Model. None of these searches has found evidence for the existence of $H^{\pm}$, and exclusion limits are set on $B(t \rightarrow H^+ b) \times B(H^+ \rightarrow \tau \nu)$ and $\sigma_{H^+} \times B(H^+ \rightarrow \tau \nu)$. For the 2011 ATLAS dataset, searches are performed for light $\Hpm$, defined by $\mH < m_{top}$, on a data sample corresponding to an integrated luminosity of 4.6 $\invfb$ proton-proton collisions with $\sqrt{s}$ = 7 TeV. These searches are conducted using the decay channel $\ttbar \rightarrow Wb \Hpm b$, with $\Hpm \rightarrow \tau_{had} \nu$ and the $W$ boson decaying hadronically ($\tau$+jets channel) or leptonically ($\tau$+lepton). The combination of the channels in this search excludes at 95$\%$ confidence level (CL) a branching fraction of $B(t \rightarrow \Hp b) \times B(\Hp \rightarrow \tau \nu) = 1-5 \%$ for the mass range $\mH = 90-160$ GeV. An additional search, with the aim of observing $\Hpm$ through apparent violation of lepton universality in a ratio of yields, is also performed on this dataset. In this search, the $\ttbar \rightarrow \ell \nu b \tau \nu b$ event yield is compared to that of $\ttbar \rightarrow \ell \nu b \ell' \nu b$. The signal would contribute to these yields through $\Hpm$ replacing the $W$ boson in the SM $\ttbar$ decay and preferentially decaying to $\tau \nu$. Combining this search with the $\tau$+jets channel results in an improved 95$\%$ CL exclusion on the branching fraction of $B(t \rightarrow \Hp b) \times B(\Hp \rightarrow \tau \nu) = 0.8-3.4 \%$ for the mass range $\mH = 90-160$ GeV. For the 2012 ATLAS dataset, searches for an extended mass range of $\Hpm$ are performed on a data sample corresponding to an integrated luminosity of 19.5 $\invfb$ proton-proton collisions at $\sqrt{s}$ = 8~TeV. These $\Hpm$ searches are for final states involving a hadronic $\tau$ and jets, both in $\ttbar$ decays and top-associated production. For $\Hpm$ with $\mH < m_{top}$, 95$\%$ CL limits are set on $B(t \rightarrow \Hp b) \times B(\Hp \rightarrow \tau \nu)$ of $0.3-2.4 \%$ in a mass range of $\mH = 90-160$ GeV. For $\Hpm$ with $\mH > m_{top}$, 95$\%$ CL limits are set on $\sigma_{H^+} \times B(\Hp \rightarrow \tau \nu)$ of $0.03-0.8$ pb, for a mass range of $\mH = 180-600$ GeV. Issue Date: 2013-08-22 URI: http://hdl.handle.net/2142/45384 Rights Information: Copyright 2013 Allison McCarn Date Available in IDEALS: 2013-08-22 Date Deposited: 2013-08
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