ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 94(3), 032006–31pp.
Abstract: Measurements of the top-antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high-p(T) leptons (electrons or muons), are presented using data corresponding to an integrated luminosity of 20.3 fb(-1) from pp collisions at a center-of-mass energy root s = 8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the tt system arc performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: A(c)(ll) based on the selected leptons and A(c)(tt) based on the reconstructed tt final state. The inclusive asymmetries are measured in the full phase space to be A(c)(ll)= 0.008 +/- 0.006 and A(c)(tt)= 0.021 +/- 0.016, which are in agreement with the Standard Model predictions of A(c)(ll)= 0.0064 +/- 0.0003 and A(c)(tt)= 0.0111 +/- 0.0004.
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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. (2016). Search for resonances in the mass distribution of jet pairs with one or two jets identified as b-jets in proton-proton collisions at root S=13 TeV with the ATLAS detector. Phys. Lett. B, 759, 229–246.
Abstract: Searches for high-mass resonances in the dijet invariant mass spectrum with one or two jets identified as b-jets are performed using an integrated luminosity of 3.2 fb(-1) of proton-proton collisions with a centre-of-mass energy of root S = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% credibility level, excited b* quarks with masses from 1.1 TeV to 2.1 TeV and leptophobic Z' bosons with masses from 1.1 TeV to 1.5 TeV. Contributions of a Gaussian signal shape with effective cross sections ranging from approximately 0.4 to 0.001 pb are also excluded in the mass range 1.5-5.0 TeV.
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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. (2016). Search for charged Higgs bosons produced in association with a top quark and decaying via H-+/- -> tau nu using pp collision data recorded at root s=13 TeV by the ATLAS detector. Phys. Lett. B, 759, 555–574.
Abstract: Charged Higgs bosons produced in association with a single top quark and decaying via H-+/- -> tau nu are searched for with the ATLAS experiment at the LHC, using proton-proton collision data at root s = 13 TeV corresponding to an integrated luminosity of 3.2 fb(-1). The final state is characterised by the presence of a hadronic tau decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, sigma (pp -> [b]tH(+/-)) x BR(H-+/- -> tau nu), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton-proton collision data recorded at root s = 8 TeV.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of W-+/- and Z-boson production cross sections in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 759, 601–621.
Abstract: Measurements of the W-+/- -> l(+/-) v and Z -> l(+)l(-) production cross sections (where l(+/-) = e(+/-), mu(+/-)) in proton-proton collisions at root s = 13 TeV are presented using data recorded by the ATLAS experiment at the Large Hadron Collider, corresponding to a total integrated luminosity of 81 pb(-1). The total inclusive W+-boson production cross sections times the single-lepton-flavour branching ratios are sigma(tot)(w+) = 11.83 +/- 0.02 (stat) +/- 0.32 (sys) +/- 0.25 (lumi) nb and sigma(tot)(W-) = 8.79 +/- 0.02 (stat) +/- 0.24 (sys) +/- 0.18 (lumi) nb for W+ and W-, respectively. The total inclusive Z-boson production cross section times leptonic branching ratio, within the invariant mass window 66 < m(tt) < 116 GeV, is sigma(tot)(Z) = 1.981 +/- 0.007 (stat) +/- 0.038 (sys) +/- 0.042 (lumi) nb. The W+, W-, and Z-boson production cross sections and cross-section ratios within a fiducial region defined by the detector acceptance are also measured. The cross-section ratios benefit from significant cancellation of experimental uncertainties, resulting in sigma(fid)(W+)/sigma(fid)(W-) = 1.295 +/- 0.003 (stat) +/- 0.010 (sys) and sigma(fid)(W +/-)/sigma(fid)(Z) = 10.31 +/- 0.04 (stat) +/- 0.20 (sys). Theoretical predictions, based on calculations accurate to next-to-next-to-leading order for quantum chromodynamics and to next-to-leading order for electroweak processes and which employ different parton distribution function sets, are compared to these measurements.
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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. (2016). Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in root s=8 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 11(11), 112–37pp.
Abstract: A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb(-1) of LHC proton-proton collision data at root s – 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be -0.8 +/- 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.
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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. (2016). Search for pair production of Higgs bosons in the bbbb final state using proton-proton collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 94(5), 052002–29pp.
Abstract: A search for Higgs-boson pair production in the bbbb final state is carried out with 3.2 fb(-1) of protonproton collision data collected at root s = 13 TeV with the ATLAS detector. The data are consistent with the estimated background and are used to set upper limits on the production cross section of Higgs-boson pairs times branching ratio to bbbb for both nonresonant and resonant production. In the case of resonant production of Kaluza-Klein gravitons within the Randall-Sundrum model, upper limits in the 24 to 91 fb range are obtained for masses between 600 and 3000 GeV, at the 95% confidence level. The production cross section times branching ratio for nonresonant Higgs-boson pairs is also constrained to be less than 1.22 pb, at the 95% confidence level.
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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. (2016). Measurement of exclusive gamma gamma -> W+W- production and search for exclusive Higgs boson production in pp collisions at root s=8 TeV using the ATLAS detector. Phys. Rev. D, 94(3), 032011–32pp.
Abstract: Searches for exclusively produced W boson pairs in the process pp(gamma gamma) -> pW(+) W- p and an exclusively produced Higgs boson in the process pp(gg) -> pHp have been performed using e(+/-) mu(-/+) final states. These measurements use 20.2 fb(-1) of pp collisions collected by the ATLAS experiment at a center-of-mass energy root s = 8 TeV at the LHC. Exclusive production of W+ W- consistent with the Standard Model prediction is found with 3.0 sigma significance. The exclusive W+ W- production cross section is determined to be sigma(gamma gamma -> W+ W- -> e(+/-) mu(-/+) X) = 6.9 +/- 2.2(stat) +/- 1.4(sys) fb, in agreement with the Standard Model prediction. Limits on anomalous quartic gauge couplings are set at 95% confidence level as -1.7 x 10(-6) < a(0)(W) / Lambda(2) < 1.7 x 10(-6) GeV-2 and -6.4 x 10(-6) < a(C)(W) / Lambda(2) < 6.3 x 10(-6) GeV-2. A 95% confidence-level upper limit on the total production cross section for an exclusive Higgs boson is set to 1.2 pb.
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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. (2016). Measurement of the b(b)over-bar dijet cross section in pp collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 76(12), 670–24pp.
Abstract: The dijet production cross section for jets containing a b-hadron (b-jets) has been measured in protonproton collisions with a centre-of-mass energy of root s = 7 TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of 4.2 fb(-1). The cross section is measured for events with two identified b-jets with a transverse momentum pT > 20 GeV and a minimum separation in the eta-phi plane of Delta R = 0.4. At least one of the jets in the event is required to have p(T) > 270 GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b-jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization.
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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. (2016). Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run-1 data. Eur. Phys. J. C, 76(12), 666–42pp.
Abstract: The algorithms used by the ATLAS Collaboration to reconstruct and identify prompt photons are described. Measurements of the photon identification efficiencies are reported, using 4.9 fb(-1) of pp collision data collected at the LHC at root s = 7 TeV and 20.3 fb-1 at root s = 8 TeV. The efficiencies are measured separately for converted and unconverted photons, in four different pseudorapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared to the predictions from a simulation of the detector response, after correcting the electromagnetic shower momenta in the simulation for the average differences observed with respect to data. Data-to-simulation efficiency ratios used as correction factors in physics measurements are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 10% in 7 TeV data and between 0.5% and 5.6% in 8 TeV data, depending on the photon transverse momentum and pseudorapidity.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of the charged-particle multiplicity inside jets from root s=8 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 76(6), 322–23pp.
Abstract: The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with p(T) > 500 MeV inside high-momentum jets in dijet events using 20.3 fb(-1) of data recorded with the ATLAS detector in pp collisions at root s = 8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.
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