A QCD analysis is reported of ATLAS data on inclusive W{sup {+-}} and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit ...sensitivity to the light quark sea composition and magnitude at Bjorken x {approx} 0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00{sub -0.28}{sup +0.25} at absolute four-momentum transfer squared Q{sup 2} = 1.9 GeV{sup 2} and x = 0.023.
The X{sub b}(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at {radical}s = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to ...an integrated luminosity of 4.4 fb{sup -1}, these states are reconstructed through their radiative decays to Y(1S,2S) with Y {yields} {mu}{sup +}{mu}{sup -}. In addition to the mass peaks corresponding to the decay modes X{sub b}(1P,2P) {yields} Y(1S)y, a new structure centered at a mass of 10.530 {+-} 0.005(stat) {+-} 0.009(syst) GeV is also observed, in both the Y(1S)y and Y(2S)y decay modes. This structure is interpreted as the X{sub b}(3P) system.
We report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb-1 of pp collision data at s = 7 TeV recorded with the ATLAS detector at the LHC. Selected ...events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding s t = 83 +/- 4 (s t a t .) - 19 + 20 (s y s t .) pb , which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation | V t b | " | V t s | , | V t d | , the coupling strength at the W-t-b vertex is determined to be | V t b | = 1.13 - 0.13 + 0.14 . If it is assumed that | V t b | 0.75 is obtained at the 95% confidence level.
The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 1.04 fb-1 of proton-proton collision data recorded by the ATLAS ...detector during the first half of 2011 at s = 7 TeV . No excess beyond Standard Model expectations is observed, and upper limits on the fiducial cross sections for non-Standard Model production of these final states are set. Using models for string ball and black hole production and decay, exclusion contours are determined as a function of mass threshold and the fundamental gravity scale.
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.
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 ZZ production cross section in proton-proton collisions at root s = 7 TeV using data corresponding to an integrated luminosity of 1.02 fb(-1) recorded by the ATLAS experiment at ...the LHC is presented. Twelve events containing two Z boson candidates decaying to electrons and/or muons are observed, with an expected background of 0.3 +/- 0.3(stat)(-0.3)(+0.4)(syst) events. The cross section measured in a phase-space region with good detector acceptance and for dilepton masses within the range 66 to 116 GeV is sigma(fid)(ZZ -> l+l-l+l-) = 19.4(-5.2)(+6.3)(stat)(-0.7)(+0.9)(syst) +/- 0.7(lumi) fb. The resulting total cross section for on-shell ZZ production, sigma(tot)(ZZ) = 8.5(-2.3)(+2.7)(stat)(-0.3)(+0.4)(syst) +/- 0.3(lumi) pb, is consistent with the standard model expectation of 6.5(-0.2)(+0.3) pb calculated at the next-to-leading order in QCD. Limits on anomalous neutral triple gauge boson couplings are derived.
This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in pp collisions at {radical}s = 7 TeV at the Large ...Hadron Collider and correspond to a total integrated luminosity of 1.08 (1.21) fb{sup -1} in the e{sup +}e{sup -} ({mu}{sup +}{mu}{sup -}) channel. No statistically significant excess above the standard model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching fraction of Z' resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on the sequential standard model Z' boson is set. A Randall-Sundrum graviton with coupling k/{bar M}{sup -} = 0.1 is excluded at 95% C.L. for masses below 1.63 TeV.
A search for production of supersymmetric particles in final states containing jets, missing transverse momentum, and at least one hadronically decaying I lepton is presented. The data were recorded ...by the ATLAS experiment in s = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 2.05 fb a 1 of data. The results are interpreted in the context of gauge mediated supersymmetry breaking models with M mess = 250 TeV , N 5 = 3 , mu > 0 , and C grav = 1 . The production of supersymmetric particles is excluded at 95% C.L. up to a supersymmetry breaking scale I = 30 TeV , independent of tan beta, and up to I = 43 TeV for large tan beta.
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