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This thesis presents work towards the first measurement of the mass of the W boson (m_W) at ATLAS at the Large Hadron Collider using √s = 7 TeV collision data taken in 2011. The electron energy calibration is presented, including a detailed study of the linearity of the ATLAS electromagnetic calorimeter. Separately, the measurement of the W boson recoil is calibrated using Z boson events. Recoil corrections for the underlying event, pileup, recoil magnitude resolution, recoil angular resolution and recoil response mismodelling are determined, with statistical uncertainties on these corrections corresponding to an estimated uncertainty on m_W of 3.9 MeV. The corrections for calorimeter non-linearity and recoil modelling improve the description of the data, though systematic biases remain. To achieve a precision commensurate with the statistics of the data, these biases will need to be understood.

This thesis presents the measurement and results of the Z/γ* → μ⁺μ⁻ Drell-Yan triple-differential cross-section, using 20.24 fb⁻¹ of ATLAS data recorded in 2012 at a centre-of-mass energy of √s = 8TeV. The triple-differential cross-section is measured as a function of dimuon invariant mass, m_μμ, dimuon rapidity, γ_μμ , and lepton decay angle in the Collins-Soper frame, cosθ*. These dimensions provide sensitivity to the parton composition of the proton through the parton density functions, PDFs, and the weak effective mixing angle, sin²θeff./W, via the forward-backward asymmetry, A_FB. The measurement is performed on and around the Z-boson's invariant mass peak, 46 < m_μμ < 200 GeV, in a kinematic ducial volume of muon transverse momentum, pT > 20 GeV, and muon pseudo-rapidity |η|≤2.4. The results are unfolded from the detector level to the Born, bare and dressed levels, where a precision of < 0:6% is reported in the central bins. The data is combined with an electron channel measurement resulting in a combined result with reduced total uncertainty. The combined result is shown to constrain PDF uncertainties and achieve the most constrained sin²θeff./W uncertainty yet reported at the LHC.

This thesis presents the results for two searches for new physics performed with the ATLAS experiment. The first, a search for the rare B-meson decay Bs → μμ and measurement of its branching ratio, uses 25 fb⁻¹ of √s = 7 and 8 TeV data recorded during 2011 and 2012. After observing a small number of these decays, a branching ratio of B(Bs → μμ) = (0.9⁺¹·¹₋₀.₈) x 10⁻⁹ is measured, assuming non-negative event yields. This is compatible with the Standard Model at the 2σ level. The second, a search for direct pair production of the supersymmetric top quark partner, is performed using 36.07 fb⁻¹ of √s = 13 TeV data recorded during 2015 and 2016. Final states with a high jet multiplicity, no leptons and large missing transverse momentum are selected to target these decays, with several signal regions designed to cover a wide range of particle masses. No excess is observed, with all signal regions being compatible with the Standard Model within 2σ. Limits are set on the stop mass, excluding up to mt̃1 = 940 GeV for values of mx̃⁰₁ below 160 GeV, assuming a 100% branching fraction to t̃1 → tX̃⁰₁ decays. In addition two reinterpretations of this data are presented, for a gluino-mediated stop production scenario and a direct dark matter production scenario. No excess is observed for either model, and limits are set on the mass of the relevant particles. Finally a viability study into using machine learning techniques to improve on existing SUSY search methods has been performed, with the initial results proving promising.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (p. 49-50). / In this thesis I explore the identification of [tau] leptons from simulated reconstructed data that will be collected by the Compact Muon Solenoid (CMS) detector on the Large Hadron Collider (LHC) at CERN. The two components of particle identification, efficiencies of [tau] identification from generator level information, along with fake rates of the current default algorithm have been determined and analyzed for a photon plus jets background sample and QCD background sample. I propose a new [tau] lepton identification algorithm that employs a signal cone parametrized with respect to the 7 transverse energy, and an isolation cone parametrized with respect to charged particle density surrounding the [tau] jet. Using the default algorithm an efficiency of 27.7% is achieved along with a photon plus jets fake rate of 1.96%. Using the proposed algorithm and matching the efficiency of the default algorithm, an efficiency of 26.9% and a fake rate of 0.44% is achieved. Approximately matching fake rates, an efficiency of 37.4% is achieved with a fake rate of 2.36%. / by by Philip James Ilten. / S.B.

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