ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2016). Measurements of four-lepton production in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 753, 552–572.
Abstract: The four-lepton (4l, l = e, mu) production cross section is measured in the mass range from 80 to 1000GeV using 20.3 fb(-1)of data in pp collisions at root s = 8 TeV collected with the ATLAS detector at the LHC. The 4l events are produced in the decays of resonant Zand Higgs bosons and the nonresonant ZZcontinuum originating from q (q) over bar, gg, and qg initial states. A total of 476 signal candidate events are observed with a background expectation of 26.2 +/- 3.6 events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system. In the mass range above 180GeV, assuming the theoretical constraint on the q (q) over bar production cross section calculated with perturbative NNLO QCD and NLO electroweak corrections, the signal strength of the gluon-fusion component relative to its leading-order prediction is determined to be mu(gg) = 2.4 +/- 1.0(stat.) +/- 0.5(syst.) +/- 0.8(theory).
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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 bottom squark pair production in proton-proton collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 76(10), 547–25pp.
Abstract: The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark ((b) over tilde (1)) is reported. The search uses 3.2 fb(-1) of pp collisions at root s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from b-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the (b) over tilde (1) is the lightest squark and is assumed to decay exclusively via (b) over tilde (1) -> b (chi) over tilde (0)(1), where (chi) over tilde (0)(1) is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the. (chi) over tilde (0)(1) mass below 360 (100) GeV whilst differences in mass above 100 GeV between the (b) over tilde (1) and the (chi) over tilde (0)(1) are excluded up to a (b) over tilde (1) mass of 500 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. (2016). Search for new phenomena in different-flavour high-mass dilepton final states in pp collisions at root s=13Tev with the ATLAS detector. Eur. Phys. J. C, 76(10), 541–28pp.
Abstract: A search is performed for a heavy particle decaying into different flavour dilepton pairs (, or ), using 3.2 fb of proton-proton collision data at TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95 % credibility level are set on the mass of a boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric sneutrino with R-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for , and final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Measurements of top-quark pair differential cross-sections in the lepton plus jets channel in pp collisions at root s=8 TeV using the ATLAS detector. Eur. Phys. J. C, 76(10), 538–39pp.
Abstract: Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, t (t) over bar system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV. The observables have been chosen to emphasize the t (t) over bar production process and to be sensitive to effects of initial-and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb(-1), recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Measurement of W +/- Z production cross sections and gauge boson polarisation in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 79(6), 535–34pp.
Abstract: This paper presents measurements of W +/- Z production cross sections in pp collisions at a centre-of-mass energy of 13TeV. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 36.1fb-1. The W +/- Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is sigma W +/- Zfid.=63.7fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of 61.5-1.3+1.4fb. Cross sections for W+Z and W-Z production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.
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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). Fiducial, total and differential cross-section measurements of t-channel single top-quark production in pp collisions at 8 TeV using data collected by the ATLAS detector. Eur. Phys. J. C, 77(8), 531–46pp.
Abstract: Detailed measurements of t-channel single top-quark production are presented. They use 20.2 fb(-1) of data collected by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be stot(tq) = 56.7(-3.8)(+4.3) pb for top-quark production and sigma(tot)((t) over barq) = 32.9(-2.7)(+3.0) pb for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be R-t = 1.72 +/- 0.09. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.
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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). The performance of the jet trigger for the ATLAS detector during 2011 data taking. Eur. Phys. J. C, 76(10), 526–47pp.
Abstract: The performance of the jet trigger for the ATLAS detector at the LHC during the 2011 data taking period is described. During 2011 the LHC provided proton-proton collisions with a centre-of-mass energy of 7 TeV and heavy ion collisions with a 2.76 TeV per nucleon-nucleon collision energy. The ATLAS trigger is a three level system designed to reduce the rate of events from the 40 MHz nominal maximum bunch crossing rate to the approximate 400 Hz which can be written to offline storage. The ATLAS jet trigger is the primary means for the online selection of events containing jets. Events are accepted by the trigger if they contain one or more jets above some transverse energy threshold. During 2011 data taking the jet trigger was fully efficient for jets with transverse energy above 25 GeV for triggers seeded randomly at Level 1. For triggers which require a jet to be identified at each of the three trigger levels, full efficiency is reached for offline jets with transverse energy above 60 GeV. Jets reconstructed in the final trigger level and corresponding to offline jets with transverse energy greater than 60 GeV, are reconstructed with a resolution in transverse energy with respect to offline jets, of better than 4 % in the central region and better than 2.5 % in the forward direction.
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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 TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at root s=13 TeV. Phys. Lett. B, 760, 520–537.
Abstract: A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb (1) of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at root s = 13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions.
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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 supersymmetry in a final state containing two photons and missing transverse momentum in root s=13 TeV pp collisions at the LHC using the ATLAS detector. Eur. Phys. J. C, 76(9), 517–23pp.
Abstract: A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of 3.2 fb-1 of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be 0.27(-0.10)(+0.22) events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest super-symmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.
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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). Study of the rare decays of B-s(0) and B-0 into muon pairs from data collected during the LHC Run 1 with the ATLAS detector. Eur. Phys. J. C, 76(9), 513–31pp.
Abstract: A study of the decays B-s(0) -> mu(+)mu(-) and B-0 -> mu(+)mu(-) has been performed using data corresponding to an integrated luminosity of 25 fb(-1) of 7 and 8 TeV proton-proton collisions collected with the ATLAS detector during the LHC Run 1. For the B-0 dimuon decay, an upper limit on the branching fraction is set at B(B-0 -> mu(+)mu(-)) < 4.2 x 10(-10) at 95% confidence level. For B-s(0), the branching fraction B(B-s(0) -> mu(+)mu(-)) = (0.9(-0.8)(+1.1)) x 10(-9) is measured. The results are consistent with the Standard Model expectation with a p value of 4.8%, corresponding to 2.0 standard deviations.
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