Person:

Kagan, M

This dissertation presents a measurement of the (W^{\pm}Z) production cross section and limits on anomalous triple gauge couplings in proton-proton collisions at a center of mass energy of 7 TeV using data produced by LHC collisions and acquired by the ATLAS detector in 2011. The measurement and limits probe the electroweak sector of the Standard Model at high energies and allow for generic tests for new physics that could be present at high energy scales. This analysis is also useful for understanding the ATLAS detector response in the presence of multi-lepton signatures. The dataset used corresponds to an integrated luminosity of (1.02 fb^{−1}). The measurement relies on the leptonic decay modes of the W and Z, resulting in final states with electrons, muons, and missing energy. Events are selected by requiring three high momentum leptons, a large missing transverse energy, a Z candidate (reconstructed from two of the leptons) with a mass consistent with the Z pole mass, and a W candidate (reconstructed from the third lepton and the missing energy) with a large transverse mass. The backgrounds to the (W^{\pm}Z) process are estimated using Monte Carlo simulations and Data-Driven techniques. A total of 71 (W^{\pm}Z) candidate events are observed in data, with 50.3 signal and 12.1 background events expected. The production cross section is extracted from these events using a maximum likelihood method, and is found to be in good agreement with the Standard Model expectation. Limits at the 95% confidence interval on anomalous triple gauge couplings are extracted from the observed event yields using a frequentist limit setting approach.

A systematic method for optimizing multivariate discriminants is developed and applied to the important example of a light Higgs boson search at the Tevatron and the LHC. The Significance Improvement Characteristic (SIC), defined as the signal efficiency of a cut or multivariate discriminant divided by the square root of the background efficiency, is shown to be an extremely powerful visualization tool. SIC curves demonstrate numerical instabilities in the multivariate discriminants, show convergence as the number of variables is increased, and display the sensitivity to the optimal cut values. For our application, we concentrate on Higgs boson production in association with a W or Z boson with (H \rightarrow b\bar{b}) and compare to the irreducible standard model background, (Z/W + b\bar{b}). We explore thousands of experimentally motivated, physically motivated, and unmotivated single variable discriminants. Along with the standard kinematic variables, a number of new ones, such as twist, are described which should have applicability to many processes. We find that some single variables, such as the pull angle, are weak discriminants, but when combined with others they provide important marginal improvement. We also find that multiple Higgs boson-candidate mass measures, such as from mild and aggressively trimmed jets, when combined may provide additional discriminating power. Comparing the significance improvement from our variables to those used in recent CDF and D(\varnothing) searches, we find that a 10-20% improvement in significance against (Z/W + b\bar{b}) is possible. Our analysis also suggests that the (H + W/Z) channel with (H \rightarrow b\bar{b}) is also viable at the LHC, without requiring a hard cut on the (W/Z) transverse momentum.

In this paper, a measurement of τ polarization in W→τν decays is presented. It is measured from the energies of the decay products in hadronic τ decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb−1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the τ polarization is Pτ=−1.06±0.04 (stat)+0.05−0.07 (syst) , in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [−1,−0.91]. Measurements of τ polarization have not previously been made at hadron colliders.

The luminosity calibration for the ATLAS detector at the LHC during pp collisions at √s=7 TeV in 2010 and 2011 is presented. Evaluation of the luminosity scale is performed using several luminosity-sensitive detectors, and comparisons are made of the long-term stability and accuracy of this calibration applied to the pp collisions at √s=7 TeV. A luminosity uncertainty of Delta L/L = +/- 3.5% is obtained for the 47 pb-1 of data delivered to ATLAS in 2010, and an uncertainty of Delta L/L = +/- 1.8% is obtained for the 5.5 fb-1 delivered in 2011.

Abstract : The inclusive jet cross-section has been measured in proton-proton collisions at √s=2.76 TeV in a dataset corresponding to an integrated luminosity of 0.20pb-1 collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-kt algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum pT and jet rapidity y, covering a range of 20 <= pT < 430 GeV and |y| < 4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at √s=7 TeV, published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity xT = 2 pT / √s, in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at √s=2.76 TeV and √s=7 TeV are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.

A measurement of the top quark pair production cross section in the final state with a hadronically decaying tau lepton and jets is presented. The analysis is based on proton-proton collision data recorded by the ATLAS experiment at the LHC, with a centre-of-mass energy of 7 TeV. The data sample corresponds to an integrated luminosity of 1.67 inverse femtobarns. The cross section is measured to be 194 +/- 18 (stat.) +/- 46 (syst.) pb and is in agreement with other measurements and with the Standard Model prediction.

A search for top quark pair resonances in final states containing at least one electron or muon has been performed with the ATLAS experiment at the CERN Large Hadron Collider. The search uses a data sample corresponding to an integrated luminosity of 2.05 fb−1, which was recorded in 2011 at a proton-proton centre-of-mass energy of 7 TeV. No evidence for a resonance is found and limits are set on the production cross-section times branching ratio to tt¯ for narrow and wide resonances. For narrow Z′ bosons, the observed 95 % Bayesian credibility level limits range from 9.3 pb to 0.95 pb for masses in the range of m Z′=500 GeV to m Z′=1300 GeV. The corresponding excluded mass region for a leptophobic topcolour Z′ boson (Kaluza-Klein gluon excitation in the Randall-Sundrum model) is m Z′<880 GeV ( mgKK<1130 GeV ).

A measurement of the jet activity in tt events produced in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb^(−1) of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The tt events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, Powheg, Alpgen and Sherpa. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.

A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b-quarks, in 4.7 fb-1 of √s=7 TeV pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95% confidence level on fiducial cross-sections for the production of new particles are extracted. Results are interpreted in the context of the constrained minimal supersymmetric extension to the Standard Model and in supersymmetry-inspired models with diverse, high-multiplicity final states.

The inclusive jet cross-section has been measured in proton–proton collisions at s√=2.76 TeV in a dataset corresponding to an integrated luminosity of 0.20 pb−1 collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k t algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum p T and jet rapidity y, covering a range of 20≤p T<430 GeV and |y|<4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at √s=7 TeV , published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity xT=2pT/s√ , in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at √s=2.76 TeV and √s=7 TeV are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.