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Sökning: LAR1:lu > Kungliga Tekniska Högskolan > Refereegranskat

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1.
  • Aaboud, M., et al. (författare)
  • A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp-collision data with the ATLAS detector
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:1
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb(-1) of proton-proton collision data at root s = 7 TeV from 2010 and 0.1 nb(-1) of data at root s = 8 TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of GEANT4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2-5% for jets with transverse momenta above 2 TeV, where this method provides the jet energy scale uncertainty for ATLAS.
2.
  • Aaboud, M, et al. (författare)
  • High-E-T isolated-photon plus jets production in pp collisions at root s=8 TeV with the ATLAS detector
  • 2017
  • Ingår i: Nuclear Physics B. - 0550-3213. ; 918, s. 257-316
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamics of isolated-photon plus one-, two- and three-jet production in pp collisions at a centre-of-mass energy of 8 TeV are studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 20.2 fb(-1). Measurements of isolated-photon plus jets cross sections are presented as functions of the photon and jet transverse momenta. The cross sections as functions of the azimuthal angle between the photon and the jets, the azimuthal angle between the jets, the photon-jet invariant mass and the scattering angle in the photon-jet centre-of-mass system are presented. The pattern of QCD radiation around the photon and the leading jet is investigated by measuring jet production in an annular region centred on each object; enhancements are observed around the leading jet with respect to the photon in the directions towards the beams. The experimental measurements are compared to several different theoretical calculations, and overall a good description of the data is found.
3.
  • Aaboud, M, et al. (författare)
  • Measurement of the prompt J/psi pair production cross-section in pp collisions at root s=8 TeV with the ATLAS detector
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The production of two prompt J/psi mesons, each with transverse momenta p(T) > 8.5 GeV and rapidity vertical bar y vertical bar < 2.1, is studied using a sample of proton-proton collisions at root s = 8 TeV, corresponding to an integrated luminosity of 11.4 fb(-1) collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised J/psi production, is measured as a function of the transverse momentum of the lower-p(T) J/psi meson, di-J/psi p(T) and mass, the difference in rapidity between the two J/psi mesons, and the azimuthal angle between the two J/psi mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/psi mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be sigma(eff) = 6.3 +/- 1.6(stat) +/- 1.0(syst) mb.
4.
  • Aaboud, M, et al. (författare)
  • Measurement of the W boson polarisation in t(t)over-barevents from pp collisions at root s=8 TeV in the lepton plus jets channel with ATLAS
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:4
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a measurement of the polarisation of W bosons from t (t) over bar t decays, reconstructed in events with one high-p(T) lepton and at least four jets. Data from pp collisions at the LHC were collected at root s = 8 TeV and correspond to an integrated luminosity of 20.2 fb(-1). The angle theta* between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos theta* is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left-and right-handed polarisation states are F-0 = 0.709 +/- 0.019, F-L = 0.299 +/- 0.015 and F-R = - 0.008 +/- 0.014, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the W tb vertex are set.
5.
  • Aaboud, M., et al. (författare)
  • Measurements of charge and CP asymmetries in b-hadron decays using top-quark events collected by the ATLAS detector in pp collisions at root s=8 TeV
  • 2017
  • Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708. ; :2
  • Tidskriftsartikel (refereegranskat)abstract
    • Same-and opposite-sign charge asymmetries are measured in lepton+ jets t (t) over bar events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb(-1) from proton-proton collisions at a centre-of-mass energy of root s = 8TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b -hadron and are measured in a fi ducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.
6.
  • Aaboud, M, et al. (författare)
  • Measurements of top-quark pair differential cross-sections in the e mu channel in pp collisions at root s=13 TeV using the ATLAS detector
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:5
  • Tidskriftsartikel (refereegranskat)abstract
    • This article presents measurements of t (t) over bar differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb(-1) of proton-proton data at a centre-of-mass energy of root s = 13 TeV recorded by the ATLAS experiment at the LHC in 2015. Differential crosssections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the t (t) over bar system. The t (t) over bar events are selected by requiring one electron and onemuonof opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box + Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the t (t) over bar system.
7.
  • Aaboud, M., et al. (författare)
  • Measurements of top-quark pair to Z-boson cross-section ratios at root s=13, 8, 7 TeV with the ATLAS detector
  • 2017
  • Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708. ; :2, s. 1-54
  • Tidskriftsartikel (refereegranskat)abstract
    • Ratios of top-quark pair to Z-boson cross sections measured from proton-proton-collisions at the LHC centre-of-mass energies of root S = 13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given root S for the two processes and at different root S , for each process, as well as double ratios of the two processes at different root S , are evaluated. The ratios are constructed using previously published ATLAS measurements of the t (t) over bar and Z-boson production cross sections, corrected to a common phase space where required, and a new analysis of Z -> l(+)l(-) where l = e, mu at root S = 13 TeV performed with data collected in 2015 with an integrated luminosity of 3.2 fb(-1). Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the Z -> e (+) e (-) and the Z -> mu (+) mu (-) channels for each value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-x values near 0.1 and the light-quark sea for x < 0.02.
8.
  • Aaboud, M, et al. (författare)
  • Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at root s=8 TeV with the ATLAS detector
  • 2017
  • Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708. ; :4
  • Tidskriftsartikel (refereegranskat)abstract
    • To probe the Wtb vertex structure, top-quark and W-boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb(-1), recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W-boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling gR are also set from model-independent measurements.
9.
  • Aaboud, M, et al. (författare)
  • Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton-proton collisions at the LHC
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:5
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents the method and performance of primary vertex reconstruction in proton-proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of root s = 8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30 mu m is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20 mu m and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.
10.
  • Aaboud, M., et al. (författare)
  • Search for heavy resonances decaying to a Z boson and a photon in pp collisions at root s=13 TeV with the ATLAS detector
  • 2017
  • Ingår i: Physics Letters B. - 0370-2693. ; 764, s. 11-30
  • Tidskriftsartikel (refereegranskat)abstract
    • This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a Z boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb(-1) of pp collisions collected at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The Z bosons are identified through their decays either to charged, light, lepton pairs (e(+) e(-), mu(+) mu(-)) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to Z gamma of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.
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