This dissertation presents a measurement of the W+b-jets \((pp → W + b(\bar{b}) + X)\) production cross-section in proton–proton collisions at a center-of-mass energy of 7 TeV at the LHC. The results are based on data corresponding to an integrated luminosity of \(4.6 fb^{−1}\), collected with the ATLAS detector. The measurement probes the QCD sector of the Standard Model at high energy, in a region where b-quark mass and double parton scattering play an important role. In addition, the measurement is relevant for searches for physics beyond the Standard Model in final states with a W boson and b-jets. The measurement relies on the leptonic decay modes of the W, and on the iden- tification of b-jets. The backgrounds to the W+b-jets process are estimated using Monte Carlo simulation and data-driven techniques. Cross-sections, corrected for all known detector effects and quoted in a limited kinematic range, are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson. / Physics

This thesis presents searches for a charged Higgs boson (H±) in proton-proton collisions with center-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. Multiple search channels are used with the common characteristic of at least one charged lepton (electron or muon) that effectively reduces the multi-jet background and is used for efficient triggering. Charged Higgs bosons decaying to a tau lepton and a neutrino are searched for using final states with two charged leptons, or one charged lepton and a hadronically decaying tau. A significant background originates from quark- or gluon-initiated jets that may be misidentified as hadronic tau decays. Methods to estimate this background are presented, including a largely data-driven matrix method. Signal processes with a charged Higgs boson mass below or above that of the top quark are considered. With the dataset collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb-1, upper limits at 95% confidence level are placed on the branching fraction B(t→bH±)×B(H±→τν) in the range 1.1–0.3% for charged Higgs boson masses between 80 GeV and 160 GeV, and on the top-quark associated H± production cross section in the range 0.93 to 0.03 pb for charged Higgs boson masses between 180 GeV and 1 TeV.

LHC proton-proton collisions at a centre of mass energy of $\sqrt{s} = 7 TeV were observed in 2011. From a 1.68 fb$^{-1}$ sample of the data collected using the ATLAS detector, electron-positron pairs originating from the Drell-Yan process were selected using a cut based analysis. After the selection process, an estimate of the background was determined followed by the selection efficiency, detector resolution, reconstruction efficiency, and kinematic acceptance. Using these, the Drell-Yan differential cross section was calculated as a function of invariant mass between 26 and 66 GeV/c$^2$. This measurement has a precision between 12.4\% and 8.01\% from the lower invariant mass bins to the higher ones. The Drell-Yan cross section in proton-proton collisions depends on empirical quantities known as parton distribution functions which parametrize the structure of the proton. The measurement outlined in this thesis observes a region in parton distribution function phase space previously untouched by experiments. / Graduate

Quarks and leptons are assumed to be fundamental particles in the Standard Model of particle physics. The Large Hadron Collider provided 7 TeV proton-proton collisions in 2010. These collisions permit the search for quark substructure at a smaller length scale than was previously possible. This thesis is an investigation of the angular distribution of high dijet mass events in 36 pb^{−1} of data recorded by the ATLAS detector. Further contributions to technical aspects of the analysis are described in the appendices. This analysis excludes quark substructure at Λ < 5.3 TeV, corresponding to 3.7 × 10^{−5} fm, at 95% confidence level. / Graduate / 0798 / frank@uvic.ca

A search for diphoton events with large missing transverse energy is conducted using 3.1 pb^{-1} of integrated luminosity of proton-proton collisions at center of mass energy \sqrt{s}=7 TeV. The data were collected with the ATLAS detector at the CERN Large Hadron Collider during the period from March 30, 2010 until August 30, 2010. No excess of such events is observed above the Standard Model background prediction. This result is interpreted in the context of a gravity mediated One Universal Extra Dimension model with \Lambda R=20, N=6 and M_{D}=5 TeV, where \Lambda is the cutoff scale, N is the number of large extra dimensions and M_{D} is the Planck scale in the higher dimensional theory. The compactification radius of the Universal Extra Dimension, R, is excluded for values of 1/R < 728 GeV at 95\% CL, providing the most stringent limit on this model at the time of publication.

A search for diphoton events with large missing transverse energy is conducted using 3.1 pb^{-1} of integrated luminosity of proton-proton collisions at center of mass energy \sqrt{s}=7 TeV. The data were collected with the ATLAS detector at the CERN Large Hadron Collider during the period from March 30, 2010 until August 30, 2010. No excess of such events is observed above the Standard Model background prediction. This result is interpreted in the context of a gravity mediated One Universal Extra Dimension model with \Lambda R=20, N=6 and M_{D}=5 TeV, where \Lambda is the cutoff scale, N is the number of large extra dimensions and M_{D} is the Planck scale in the higher dimensional theory. The compactification radius of the Universal Extra Dimension, R, is excluded for values of 1/R < 728 GeV at 95\% CL, providing the most stringent limit on this model at the time of publication.

This thesis examines the possible creation of (4+n)-dimensional black holes at Large Hadron Collider (LHC) at CERN, consequent decays of such black holes via Hawking radiation and probable formation of Higgs boson among black hole decay products. Firstly, a theoretical background was presented including black hole physics, Hawking radiation, large extra dimensions, brane-bulk models, 4+n black holes and Higgs mechanism. Then, a simulation modeling black hole formation and decay including 130 GeV Higgs as a decay product at LHC interfaced with a detector simulation of Compact Muon Selenoid (CMS) was analysed focusing especially on the Higgs decay channels and properties of Hawking radiation. Both theoretical assumptions and simulation analysis point out that black hole production and the signatures of black hole decay products could carry crucial information on dimensionality and structure of spacetime Furthermore there is a significant possibility to observe 130 GeV Higgs boson especially in the Black Hole -&gt / H -&gt / jj and Black Hole -&gt / H &amp / #8211 / &gt / WW/ZZ -&gt / lnln decay channels.

Frankfurt (Main), University, Diss., 2008.

Techn. Hochsch., Diss., 2000--Aachen.

The highly celebrated discovery of a new particle with a mass of 125 GeV in proton-proton collisions by the ATLAS and CMS experiments at the CERN Large Hadron Collider in 2012 has been shown to be compatible with the Standard Model description of the Higgs boson. However, in order to fully verify the Standard Model nature of the Higgs boson, most of its properties still remain to be measured. Such measurements include differential cross section measurements, which are shown here for the H→WW decay channel and the coupling of the Higgs boson to bottom quarks, for which a study of future prospects is presented. Differential fiducial cross section measurements of the Higgs boson were performed in the H → WW*→lvlv channel at the ATLAS detector with 20 fb−1 of √s = 8 TeV collision data. For Higgs bosons produced by gluon-gluon fusion, the cross section is measured as a function of kinematic variables, including transverse momentum and rapidity of the Higgs boson, as well as the number of jets associated with the Higgs event. The obtained distributions are unfolded to a fiducial volume using a two-dimensional iterative Bayesian algorithm. The measured fiducial differential cross sections are compared to predictions from Monte Carlo generators. The total cross section measured in the fiducial volume defined by the charged lepton and neutrino kinematic properties is 36.0 ± 9.7 fb. Additionally the jet-veto efficiency in the fiducial volume is extracted from the differential cross sections. An analysis is presented of Higgs boson production and decay into bottom quarks in association with a vector boson at the ATLAS detector for the future high-luminosity LHC with proton-proton collisions at √s = 14 TeV. The vector bosons are reconstructed from Z→l+l− or W→lv final states, where l is an electron or muon. The analysis uses generator-level Monte Carlo samples to which efficiency and resolution smearing functions are applied. These reproduce the expected resolution of the upgraded ATLAS detector for the foreseen amount of pile-up due to multiple overlapping proton-proton collisions. The analysis of the ZH(→ l+l−b¯b) channel is presented and results are combined with the WH(→lvb¯b) channel from a corresponding study. For an integrated luminosity of 300 fb−1 using an average pile-up of 60, the expected significance is 3.9 σ with an expected error on the signal strength of 25%. Likewise, for 3000 fb−1 using an average pileup of 140 the expected significance is 8.8 σ , and the error on the signal strength is expected to be about 15%.