Z. van Kesteren

Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector

This Letter describes the measurement of √ elliptic flow of charged particles in lead–lead collisions at √sNN = 2.76 TeV using the ATLAS detector at the Large Hadron Collider (LHC). The results are based on an integrated luminosity of approximately 7 μb^(−1). Elliptic flow is measured over a wide region in pseudorapidity, |η| < 2.5, and over a broad range in transverse momentum, 0.5 < p_T < 20 GeV. The elliptic flow parameter v_2 is obtained by correlating individual tracks with the event plane measured using energy deposited in the forward calorimeters. As a function of transverse momentum, v_2(p_T) reaches a maximum at p_T of about 3 GeV, then decreases and becomes weakly dependent on p_T above 7–8 GeV. Over the measured pseudorapidity region, v_2 is found to be only weakly dependent on η, with less variation than observed at lower beam energies. The results are discussed in the context of previous measurements at lower collision energies, as well as recent results from the LHC.

Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC

A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the LHC and correspond to an integrated luminosity of 315  nb⁻¹ collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% C.L. the q* mass interval 0.30<m(q*)<1.26  TeV, extending the reach of previous experiments.

Search for the Standard Model Higgs boson in the decay channel H→ZZ(*)→4ℓ with 4.8 fb-1 of pp collision data at √s=7 TeV with ATLAS

This Letter presents a search for the Standard Model Higgs boson in the decay channel H → ZZ → lll ′+l ′ −, where l, l ′ = e or μ, using proton-proton collisions at √ s = 7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb. The four-lepton invariant mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134−156 GeV, 182−233 GeV, 256−265 GeV and 268−415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125 GeV, 244 GeV and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.

Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in √s = 7 TeV pp collisions

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s=7  TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35  pb(-1). No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A(0)=0 GeV, tanβ=3, and μ>0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √ s = 7 TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35 pb. No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. For A0 = 0 GeV, tanβ = 3, μ > 0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.

Measurement of the production cross section for W-bosons in association with jets in pp collisions at root s=7 TeV with the ATLAS detector

This Letter reports on a first measurement of the inclusive W+jets cross section in proton-proton collisions at a centre-ofmass energy of 7 TeV at the LHC, with the ATLAS detector. Cross sections, in both the electron and muon decay modes of the W boson, are presented as a function of jet multiplicity and of the transverse momentum of the leading and nextto-leading jets in the event. Measurements are also presented of the ratio of cross sections σ(W+ ≥ n)/σ(W+ ≥ n− 1) for inclusive jet multiplicities n = 1−4. The results, based on an integrated luminosity of 1.3 pb, have been corrected for all known detector effects and are quoted in a limited and well-defined range of jet and lepton kinematics. The measured cross sections are compared to particle-level predictions based on perturbative QCD. Next-to-leading order calculations, studied here for n ≤ 2, are found in good agreement with the data. Leading-order multiparton event generators, normalized to the NNLO total cross section, describe the data well for all measured jet multiplicities.

Observation of a centrality-dependent dijet asymmetry in lead-lead collisions at √sNN=2.76 Tev with the ATLAS detector at the LHC

By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

Energization of interstellar media and cosmic ray production by jets from X‐ray binaries

Drawing on recent estimates of the power of jets from X-ray binary systems as a function of X-ray luminosity, combined with improved estimates of the relevant log(N )‐log(L X) luminosity functions, we calculate the total energy input to the interstellar medium (ISM) from these objects. The input of kinetic energy to the ISM via jets is dominated by those of the black hole systems, in contrast to the radiative input, which is dominated by accreting neutron stars. Summing the energy input from black hole jets LJ in the Milky Way, we find that it is likely to correspond to 1 per cent of LSNe, the time-averaged kinetic luminosity of supernovae, and 5 per cent of LCR, the cosmic ray luminosity. Given uncertainties in jet power estimates, significantly larger contributions are possible. Furthermore, in elliptical galaxies with comparable distributions of low mass X-ray binaries, but far fewer supernovae, the ratio L J/L SNe is likely to be larger by a factor of ∼5. We conclude that jets from X-ray binaries may be an important, distributed, source of kinetic energy for the ISM in the form of relativistic shocks, and as a result are likely to be a major source of cosmic rays. Ke yw ords: black hole physics ‐ binaries: close ‐ ISM: jets and outflows.

Properties of jets measured from tracks in proton-proton collisions at center-of-mass energy root s=7 TeV with the ATLAS detector

Aad, G.; et al., [Unknown]; Bentvelsen, S.C.M.; Bobbink, G.J.; Bos, K.; Boterenbrood, H.; Colijn, A.P.; Daum, C.; de Jong, P.J.; de Nooij, L.; Doxiadis, A.; Ferrari, P.; Garitaonandia, H.; Geerts, D.A.A.; Gosselink, M.; Hartjes, F.G.; Hessey, N.P.; Igonkina, O.; Kayl, M.S.; Klous, S.; Kluit, P.M.; Koffeman, E.N.; Koutsman, A.; Lee, H.C.; Lenz, T.; Linde, F.L.; Luijckx, G.; Massaro, G.G.G.; Mechnich, J.; Mussche, I.; Ottersbach, J.P.; Reichold, A.J.H.; Rijpstra, M.; Ruckstuhl, N.M.; Snuverink, J.; Ta, D.; Tsiakiris, M.; Turlay, E.; van der Graaf, H.; van der Kraaij, E.E.; van der Leeuw, R.H.L.; van der Poel, E.F.; van Eijk, B.; van Kesteren, Z.; van Vulpen, I.B.; Verkerke, W.; Vermeulen, J.C.; Vranjes Milosavljevic, M.; Vreeswijk, M.

Measurement of D*(+/-) meson production in jets from pp collisions at root s=7 TeV with the ATLAS detector

This paper reports a measurement of D meson production in jets from proton-proton collisions at a center-of-mass energy of √ s = 7 TeV at the CERN Large Hadron Collider. The measurement is based on a data sample recorded with the ATLAS detector with an integrated luminosity of 0.30 pb for jets with transverse momentum between 25 and 70 GeV in the pseudorapidity range |η| < 2.5. D mesons found in jets are fully reconstructed in the decay chain: D → Dπ, D → Kπ, and its charge conjugate. The production rate is found to beN(D)/N(jet) = 0.025±0.001 (stat.)± 0.004 (syst.) for D mesons that carry a fraction z of the jet momentum in the range 0.3 < z < 1. Monte Carlo predictions fail to describe the data at small values of z, and this is most marked at low jet transverse momentum.

Search for first generation scalar leptoquarks in pp collisions at s=7 TeV with the ATLAS detector

We report a search for first generation scalar leptoquarks us ing 1.03 fb−1 of proton-proton collisions data produced by the Large Hadron Collider at √ s = 7 TeV and recorded by the ATLAS experiment. Leptoquarks are s ought via their decay into an electron or neutrino and a quark, producing events with two oppositel y charged electrons and at least two jets, or events with an el ectron, missing transverse momentum and at least two jets. Control d ata samples are used to validate background predictions fro m M nte Carlo simulation. In the signal region, the observed event y ields are consistent with the background expectations. We e xclude at 95% confidence level the production of first generation scala r leptoquark with masses mLQ < 660 (607) GeV when assuming the branching fraction of a leptoquark to a charged lepton is equ al to 1.0 (0.5).We report a search for first generation scalar leptoquarks using 1.03 fb(-1) of proton-proton collisions data produced by the Large Hadron Collider at root s = 7 TeV and recorded by the ATLAS experiment. Leptoquarks are sought via their decay into an electron or neutrino and a quark, producing events with two oppositely charged electrons and at least two jets, or events with an electron, missing transverse momentum and at least two jets. Control data samples are used to validate background predictions from Monte Carlo simulation. In the signal region, the observed event yields are consistent with the background expectations. We exclude at 95% confidence level the production of first generation scalar leptoquark with masses m(LQ) < 660 (607) GeV when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5)