ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Electron efficiency measurements with the ATLAS detector using 2012 LHC proton-proton collision data. Eur. Phys. J. C, 77(3), 195–45pp.
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.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton-proton collisions at the LHC. Eur. Phys. J. C, 77(5), 332–35pp.
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 μm is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20 μm 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.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Performance of the ATLAS trigger system in 2015. Eur. Phys. J. C, 77(5), 317–53pp.
Abstract: During 2015 the ATLAS experiment recorded 3.8 fb(-1) of proton-proton collision data at a centre-of-mass energy of 13 TeV. The ATLAS trigger system is a crucial component of the experiment, responsible for selecting events of interest at a recording rate of approximately 1 kHz from up to 40 MHz of collisions. This paper presents a short overview of the changes to the trigger and data acquisition systems during the first long shutdown of the LHC and shows the performance of the trigger system and its components based on the 2015 proton-proton collision data.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). High-E-T isolated-photon plus jets production in pp collisions at root s=8 TeV with the ATLAS detector. Nucl. Phys. B, 918, 257–316.
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.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 77(3), 144–40pp.
Abstract: Two searches for new phenomena in final states containing a same-flavour opposite-sign lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton-proton collision data, collected during 2015 and 2016 at a centre-of-mass energy root s = 13 TeV by the ATLAS detector at the large hadron collider, which correspond to an integrated luminosity of 14.7 fb(-1). Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g.. (chi) over tilde (0)(2) -> l(+)l(-1)(chi) over tilde (0)(1)), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 TeV (980 GeV).
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of (WW +/-)-W-+/- vector-boson scattering and limits on anomalous quartic gauge couplings with the ATLAS detector. Phys. Rev. D, 96(1), 012007–34pp.
Abstract: This paper presents the extended results of measurements of (WW +/-)-W-+/- jj production and limits on anomalous quartic gauge couplings using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two leptons (e or mu) with the same electric charge and at least two jets are analyzed. Production cross sections are determined in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. An additional fiducial region, particularly sensitive to anomalous quartic gauge coupling parameters alpha 4 and alpha 5, is introduced, which allows more stringent limits on these parameters compared to the previous ATLAS measurement.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of jet activity produced in top-quark events with an electron, a muon and two b-tagged jets in the final state in pp collisions root s=13TeV with the ATLAS detector. Eur. Phys. J. C, 77(4), 220–38pp.
Abstract: Measurements of jet activity in top-quark pair events produced in proton-proton collisions are presented, using 3.2 fb of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, . The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the threshold for additional jets, and is also presented for different invariant mass regions of the system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurements of psi(2S) and X(3872) -> J/psi pi (+) pi (-) production in pp collisions at root s=8 Tev with the ATLAS detector. J. High Energy Phys., 01(1), 117–43pp.
Abstract: Differential cross sections are presented for the prompt and non-prompt production of the hidden-charm states X(3872) and psi(2S), in the decay mode J/psi pi (+) pi (-), measured using 11.4 fb(-1) of pp collisions at root s = 8 Tev by the ATLAS detector at the LHC. The ratio of cross-sections X(3872)/psi(2S) is also given, separately for prompt and non-prompt components, as well as the non-prompt fractions of X(3872) and psi(2S). Assuming independent single effective lifetimes for non-prompt X(3872) and psi(2S) production gives separating short- and long-lived contributions, assuming that the short-lived component is due to B (c) decays, gives R (B) = (3.57 +/- 0.33(stat) +/- 0.11(sys)) x 10(-2), with the fraction of non-prompt X(3872) produced via B (c) decays for p (T)(X(3872)) > 10 GeV being (25 +/- 13(stat) +/- 2(sys) +/- 5(spin))%. The distributions of the dipion invariant mass in the X(3872) and psi(2S) decays are also measured and compared to theoretical predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). 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. J. High Energy Phys., 02(2), 071–49pp.
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 = 8 TeV 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.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of the ZZ production cross section in proton-proton collisions at root s=8 TeV using the ZZ -> l(-) l(+) l '(-) l '(+) and ZZ -> l(-) l(+) nu(nu)over-bar decay channels with the ATLAS detector. J. High Energy Phys., 01(1), 099–53pp.
Abstract: A measurement of the ZZ production cross section in the l(-)l(+)l'(-)l'(+) and l(-)l(+) nu(nu) over bar channels (l = e, mu) in proton-proton collisions at root s = 8TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb(-1) collected by the ATLAS experiment in 2012 is presented. The fi ducial cross sections for ZZ -> l(-)l(+)l'(-)l'(+) and ZZ -> l(-)l(+) nu(nu) over bar are measured in selected phase-space regions. The total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV is measured from the combination of the two channels to be 7.3 +/- 0.4 (stat) +/- 0.3 (syst) (-0.2)(-0.1) (lumi) pb, which is consistent with the Standard Model prediction of 6.6(-0.6)(+0.7) pb. The di ff erential cross sections in bins of various kinematic variables are presented. 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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