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Publications and source records attributed to Alexander, G..
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A search for the Higgs boson has been performed in the H→WW*→l⁺νl⁻ν¯ channel (l=e/μ) with an integrated luminosity of 2.05 fb⁻¹ of pp collisions at √s=7 TeV collected with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 110 GeV<mH<300 GeV. The observations exclude the presence of a standard model Higgs boson with a mass 145<mH<206 GeV at 95% confidence level.
Pseudorapidity gap distributions in proton-proton collisions at s√=7 ~TeVs=7 ~TeV are studied using a minimum bias data sample with an integrated luminosity of 7.1 μb -1 . Cross sections are measured differentially in terms of Δη F , the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at η= ±4.9, in which no final state particles are produced above a transverse momentum threshold p$cut\atop{T}$ . The measurements span the region 0<Δη F < 8 for 200~MeV< p$cut\atop{T}$ <800~MeV200~MeV F , the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp→Xp), enhanced by double dissociation (pp→XY) where the invariant mass of the lighter of the two dissociation systems satisfies M Y ≲7 GeV. The resulting cross section is dσ/dΔη F ≈1 mb for Δη F ≳3. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. Finally, the cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.
Here, detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J/ψ particles. Data collected in 2010 at √s = 7 TeV are used, corresponding to an integrated luminosity of almost 40 pb -1 . The inter-alignment of the inner detector and the electromagnetic calorimeter, the determination of the electron energy scale and resolution, and the performance in terms of response uniformity and linearity are discussed. The electron identification, reconstruction and trigger efficiencies, as well as the charge misidentification probability, are also presented.
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A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at {radical}s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb{sup -1} of data collected with the ATLAS detector at the LHC. The W + b-jet cross section is defined for jets reconstructed with the anti-kt clustering algorithm with transverse momentum above 25 GeV and rapidity within {+-}2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 {+-} 1.9(stat) {+-} 2.6(syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.
This Letter presents measurements of the differential cross-sections for inclusive electron and muon production in proton-proton collisions at a centre-of-mass energy of {radical}s = 7 TeV, using data collected by the ATLAS detector at the LHC. The muon cross-section is measured as a function of pT in the range 4 < pT < 100 GeV and within pseudorapidity |{eta}| < 2.5. In addition the electron and muon cross-sections are measured in the range 7 < pT < 26 GeV and within |{eta}| < 2.0, excluding 1.37 < |{eta}| < 1.52. Integrated luminosities of 1.3 pb{sup -1} and 1.4 pb{sup -1} are used for the electron and muon measurements, respectively. After subtraction of the W/Z/{gamma}{sm_bullet} contribution, the differential cross-sections are found to be in good agreement with theoretical predictions for heavy-flavour production obtained from Fixed Order NLO calculations with NLL high-pT resummation, and to be sensitive to the effects of NLL resummation.
A measurement of the production cross section of top quark pairs (t{bar t}) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the Large Hadron Collider is reported. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. Using a data sample corresponding to an integrated luminosity of 35 pb{sup -1}, a t{bar t} production cross section {sigma}{sub t{bar t}} = 177 {+-} 20(stat.) {+-} 14(syst.) {+-} 7(lum.) pb is measured for an assumed top quark mass of mt = 172.5 GeV. A second measurement requiring at least one jet identified as coming from a b quark yields a comparable result, demonstrating that the dilepton final states are consistent with being accompanied by b-quark jets. These measurements are in good agreement with Standard Model predictions.
We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb{sup -1} collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.
Invariant mass distributions of jet pairs (dijets) produced in LHC proton–proton collisions at a centre-of-mass energy √s = 7 TeV have been studied using a data set corresponding to an integrated luminosity of 1.0 fb⁻¹ recorded in 2011 by ATLAS. Dijet masses up to ~4 TeV are observed in the data, and no evidence of resonance production over background is found. Limits are set at 95% C.L. for several new physics hypotheses: excited quarks are excluded for masses below 299 TeV, axigluons are excluded for masses below 3.32 TeV, and colour octet scalar resonances are excluded for masses below 1.92 TeV.
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A search for new phenomena in tt¯ events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1.04 fb⁻¹ of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.
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