The prompt and non-prompt production cross-sections for the chi_c1 and chi_c2 charmonium states are measured in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector at the LHC using 4.5 fb^-1 of integrated luminosity. The chi_c states are reconstructed through the radiative decay chi_c -> J/psi gamma (with J/psi -> mu+mu-) where photons are reconstructed from gamma -> e+e- conversions. The production rate of the chi_c2 state relative to the chi_c1 state is measured for prompt and non-prompt chi_c as a function of J/psi transverse momentum. The prompt chi_c cross-sections are combined with existing measurements of prompt J/psi production to derive the fraction of prompt J/psi produced in feed-down from chi_c decays. The fractions of chi_c1 and chi_c2 produced in b-hadron decays are also measured.
A search for scalar particles decaying via narrow resonances into two photons in the mass range $65-600$ GeV is performed using 20.3 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV $pp$ collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95 % confidence level on the production cross-section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.
The differential cross section for the process $Z/\gamma^*\rightarrow ll$ ($l=e,\mu$) as a function of dilepton invariant mass is measured in pp collisions at $\sqrt{s}=$ 7 TeV at the LHC using the ATLAS detector. The measurement is performed in the $e$ and $\mu$ channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb$^{-1}$ collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb$^{-1}$ of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leading-order QCD calculations, unless the latter are matched to a parton shower calculation.
A measurement is presented of the phi to K+K- production cross section at sqrt(s) = 7 TeV using pp collision data corresponding to an integrated luminosity of 383 mub-1, collected with the ATLAS experiment at the LHC. Selection of phi(1020) mesons is based on the identification of charged kaons by their energy loss in the pixel detector. The differential cross section is measured as a function of the transverse momentum, pTphi, and rapidity, |yphi|, of the phi(1020) meson in the fiducial region 500 < pTphi< 1200 MeV, |yphi| < 0.8, kaon pTK> 230 MeV and kaon momentum pK< 800 MeV.The integrated phi(1020)-meson production cross section in this fiducial range is measured to be s(phi K+K-) = 570 pm 8 (stat) pm 66 (syst) pm 20 (lumi) mub.
The production cross-section of B+ mesons is measured as a function of transverse momentum pT and rapidity y in proton--proton collisions at center-of-mass energy sqrt(s) = 7 TeV, using 2.4 fb-1 of data recorded with the ATLAS detector at the Large Hadron Collider. The differential production cross-sections, determined in the range 9<pT<120 GeV and y<2.25, are compared to next-to-leading-order theoretical predictions.
A measurement of jet shapes in top-quark pair events using 1.8 fb$^{-1}$ of $\sqrt{s}$ = 7 TeV pp collision data recorded by the ATLAS detector is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays $W \to q\bar{q}'$ in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.
The measurement of charged-particle event shape variables is presented in inclusive inelastic pp collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor and transverse sphericity, each defined using the final-state charged particles' momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged particle transverse momentum, charged particle multiplicity and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.
A search for squarks and gluinos in final states containing jets, missing transverse momentum and no high-pT electrons or muons is presented. The data represent the complete sample recorded in 2011 by the ATLAS experiment in 7 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 4.7 fb^-1. No excess above the Standard Model background expectation is observed. Gluino masses below 860 GeV and squark masses below 1320 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino, for squark or gluino masses below 2 TeV, respectively. Squarks and gluinos with equal masses below 1410 GeV are excluded. In MSUGRA/CMSSM models with tan beta = 10, A_0 = 0 and mu > 0, squarks and gluinos of equal mass are excluded for masses below 1360 GeV. Constraints are also placed on the parameter space of SUSY models with compressed spectra. These limits considerably extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.
Distributions sensitive to the underlying event are studied in events containing one or more charged-particle jets produced in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect 800 inverse microbarns of data taken during 2010. Jets are reconstructed using the antikt algorithm with radius parameter R varying between 0.2 and 1.0. Distributions of the charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the average charged-particle pT are measured as functions of pT^JET in regions transverse to and opposite the leading jet for 4 GeV < pT^JET < 100 GeV. In addition, the R-dependence of the mean values of these observables is studied. In the transverse region, both the multiplicity and the scalar sum of the transverse momentum at fixed pT^JET vary significantly with R, while the average charged-particle transverse momentum has a minimal dependence on R. Predictions from several Monte Carlo tunes have been compared to the data; the predictions from Pythia 6, based on tunes that have been determined using LHC data, show reasonable agreement with the data, including the dependence on R. Comparisons with other generators indicate that additional tuning of soft-QCD parameters is necessary for these generators. The measurements presented here provide a testing ground for further development of the Monte Carlo models.
A measurement of the ZZ production cross section in proton-proton collisions at sqrt(s) = 7 TeV using data recorded by the ATLAS experiment at the Large Hadron Collider is presented. In a data sample corresponding to an integrated luminosity of 4.6 fb-1 collected in 2011, events are selected that are consistent either with two Z bosons decaying to electrons or muons or with one Z boson decaying to electrons or muons and a second Z boson decaying to neutrinos. The ZZ*->llll and ZZ->llnunu cross sections are measured in restricted phase-space regions. These results are then used to derive the total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV, sigmaZZtot = 6.7 +-0.7 +0.4-0.3 +-0.3 pb, which is consistent with the Standard Model prediction of 5.89+0.22-0.18 pb calculated at next-to-leading order in QCD. The normalized differential cross sections in bins of various kinematic variables are presented. Finally, the differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.
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