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  • Aaboud, M., et al. (författare)
  • Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector
  • 2019
  • Ingår i: Physics Letters B. - 0370-2693. ; 789, s. 444-471
  • Tidskriftsartikel (refereegranskat)abstract
    • Correlations of two flow harmonics v(n) and v(m) via three- and four-particle cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between v(2) and v(3) and a positive correlation between v(2) and v(4) for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the < v(n)(2)> from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions.
  • Aaboud, M., et al. (författare)
  • Cross-section measurements of the Higgs boson decaying into a pair of τ-leptons in proton-proton collisions at √s=13 TeV with the ATLAS detector
  • 2019
  • Ingår i: Physical Review D : covering particles, fields, gravitation, and cosmology. - 2470-0010 .- 2470-0029. ; 99:7
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H → τ τ decay channel. The analysis is performed using 36.1 fb-1 of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of √s = 13 TeV. All combinations of leptonic (τ → l v v‾ with l = e, μ) and hadronic (τ→ hadrons v) τ decays arc considered. The H → τ τ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H → τ τ decays. Using the data taken at √s = 13 TeV, the total cross section in the H → τ τ decay channel is measured to be 3.77+0.60-0.59(stat)+0.87-0.74 (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H → τ τ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be σVBFH → τ τ = 0.28 ± 0.09 (stat)+0.11-0.09 (syst) pb and σggFH → τ τ = 3.1 ± 1.0 (stat)+1.6-1.3 (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.
  • Aaboud, M., et al. (författare)
  • Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in s√=8 TeV proton-proton collisions
  • 2016
  • Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; :9
  • Tidskriftsartikel (refereegranskat)abstract
    • A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use 20 fb−1 of proton-proton collision data at s√=8 TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino-higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass m(χ~01) of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger m(χ~01) due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small.
  • Aaboud, M, et al. (författare)
  • Determination of the strong coupling constant alpha(s) from transverse energy-energy correlations in multijet events at root s=8 TeV using the ATLAS detector
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:12
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of transverse energy-energy correlations and their associated asymmetries in multi-jet events using the ATLAS detector at the LHC are presented. The data used correspond to vs = 8 TeV proton-proton collisions with an integrated luminosity of 20.2 fb(-1). The results are presented in bins of the scalar sum of the transverse momenta of the two leading jets, unfolded to the particle level and compared to the predictions from Monte Carlo simulations. A comparison with next-to-leading-order perturbative QCD is also performed, showing excellent agreement within the uncertainties. From this comparison, the value of the strong coupling constant is extracted for different energy regimes, thus testing the running of alpha(s)(mu) predicted in QCD up to scales over 1 TeV. A global fit to the transverse energy-energy correlation distributions yields alpha(s)(m(Z)) = 0.1162 +/- 0.0011 (exp.)(-0.0070)(+0.0084) (theo.), while a global fit to the asymmetry distributions yields a value of alpha(s)(m(Z)) = 0.1196 +/- 0.0013 (exp.)(-0.0045)(+0.0075) (theo.).
  • Aaboud, M., et al. (författare)
  • Dijet azimuthal correlations and conditional yields in pp and p+Pb collisions at √sNN=5.02TeV with the ATLAS detector
  • 2019
  • Ingår i: Physical Review C : Covering Nuclear Physics. - 2469-9985 .- 2469-9993. ; 100:3
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a measurement of forward-forward and forward-central dijet azimuthal angular correlations and conditional yields in proton-proton (pp) and proton-lead (p + Pb) collisions as a probe of the nuclear gluon density in regions where the fraction of the average momentum per nucleon carried by the parton entering the hard scattering is low. In these regions, gluon saturation can modify the rapidly increasing parton distribution function of the gluon. The analysis utilizes 25 pb(-1) of pp data and 360 mu b(-1) of p + Pb data, both at root S-NN = 5.02 TeV, collected in 2015 and 2016, respectively, with the ATLAS detector at the Large Hadron Collider. The measurement is performed in the center-of-mass frame of the nucleon-nucleon system in the rapidity range between -4.0 and 4.0 using the two highest transverse-momentum jets in each event, with the highest transverse-momentum jet restricted to the forward rapidity range. No significant broadening of azimuthal angular correlations is observed for forward-forward or forward-central dijets in p + Pb compared to pp collisions. For forward-forward jet pairs in the proton-going direction, the ratio of conditional yields in p + Pb collisions to those in pp collisions is suppressed by approximately 20%, with no significant dependence on the transverse momentum of the dijet system. No modification of conditional yields is observed for forward-central dijets.
  • Aaboud, M., et al. (författare)
  • Direct top-quark decay width measurement in the t(t)over-bar lepton plus jets channel at root s=8 TeV with the ATLAS experiment
  • 2018
  • Ingår i: European Physical Journal C. - 1434-6044 .- 1434-6052. ; 78
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a direct measurement of the decay width of the top quark using t (t) over bar events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb(-1). The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Gamma(t) = 1.76 +/- 0.33 (stat.) (+0.79)(-0.68) (syst.) GeV for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.
  • Aaboud, M., et al. (författare)
  • Electron and photon energy calibration with the ATLAS detector using 2015-2016 LHC proton-proton collision data
  • 2019
  • Ingår i: Journal of Instrumentation. - IOP Publishing. - 1748-0221. ; 14:3
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb -1 of LHC proton-proton collision data recorded at s=13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV . For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV . Validations of the energy calibration with J/ψ → e + e - decays and radiative Z boson decays are also presented. © 2019 CERN for the benefit of the ATLAS collaboration.
  • Aaboud, M, et al. (författare)
  • Electron efficiency measurements with the ATLAS detector using 2012 LHC proton-proton collision data
  • 2017
  • Ingår i: European Physical Journal C. - 1434-6044. ; 77:3
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper describes the algorithms for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC). These algorithms were used for all ATLAS results with electrons in the final state that are based on the 2012 pp collision data produced by the LHC at root s = 8 TeV. The efficiency of these algorithms, together with the charge misidentification rate, is measured in data and evaluated in simulated samples using electrons from Z -> ee, Z -> ee gamma and J/Psi -> ee decays. For these efficiency measurements, the full recorded data set, corresponding to an integrated luminosity of 20.3 fb(-1), is used. Based on a new reconstruction algorithm used in 2012, the electron reconstruction efficiency is 97% for electrons with E-T = 15 GeV and 99% at E-T = 50 GeV. Combining this with the efficiency of additional selection criteria to reject electrons from background processes or misidentified hadrons, the efficiency to reconstruct and identify electrons at the ATLAS experiment varies from 65 to 95%, depending on the transverse momentum of the electron and background rejection.
  • Aaboud, M., et al. (författare)
  • Electron reconstruction and identification in the ATLAS experiment using the 2015 and 2016 LHC proton-proton collision data at s=13 TeV
  • 2019
  • Ingår i: European Physical Journal C. - 1434-6044. ; 79:8
  • Tidskriftsartikel (refereegranskat)abstract
    • Algorithms used for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC) are presented in this paper; these algorithms are used in ATLAS physics analyses that involve electrons in the final state and which are based on the 2015 and 2016 proton-proton collision data produced by the LHC at root s = 13 The performance of the electron reconstruction, identification, isolation, and charge identification algorithms is evaluated in data and in simulated samples using electrons from Z -> ee and J/psi -> eedecays. Typical examples of combinations of electron reconstruction, identification, and isolation operating points used in ATLAS physics analyses are shown.
  • Aaboud, M., et al. (författare)
  • Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector
  • 2018
  • Ingår i: Physical Review D. - American Physical Society. - 2470-0010. ; 97:7
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for the associated production of the Higgs boson with a top quark pair (ttH) is reported. The search is performed in multilepton final states using a data set corresponding to an integrated luminosity of 36.1 fb-1 of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy s=13 TeV at the Large Hadron Collider. Higgs boson decays to WW∗, ττ, and ZZ∗ are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the ttH production cross section is σ(ttH)=790-210+230 fb, in agreement with the Standard Model prediction of 507-50+35 fb. The combination of this result with other ttH searches from the ATLAS experiment using the Higgs boson decay modes to bb, γγ and ZZ∗→4, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the ttH production mode. © 2018 CERN, for the ATLAS Collaboration. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.
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