ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector. Eur. Phys. J. C, 77(2), 70–31pp.
Abstract: Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H -> e tau, H -> μtau, and Z -> μtau. The searches are based on the data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb(-1) at a centre-of-mass energy of root s = 8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H -> e tau) < 1.04%, Br(H -> μtau) < 1.43%, and Br(Z -> μtau) < 1.69 x 10(-5).
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 05(5), 071–52pp.
Abstract: Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.
|
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.
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2012). Time-dependent angular analysis of the decay B-s(0) -> J/psi phi and extraction of Delta Gamma(s) and the CP-violating weak phase phi(s) by ATLAS. J. High Energy Phys., 12(12), 072–34pp.
Abstract: A measurement of B-s(0) -> J/psi phi decay parameters, including the CP-violating weak phase phi(s) and the decay width difference Delta Gamma(s) is reported, using 4.9 fb(-1) of integrated luminosity collected in 2011 by the ATLAS detector from LHC pp collisions at a centre-of-mass energy root s = 7 TeV. The mean decay width Gamma(s) and the transversity amplitudes vertical bar A(0)(0)vertical bar(2) and vertical bar A(parallel to)(0)vertical bar(2) are also measured. The values reported for these parameters are: phi(s) = 0.22 +/- 0.41 (stat.) +/- 0.10 (syst.) rad Delta Gamma(s) = 0.053 +/- 0.021 (stat.) +/- 0.010 (syst.) ps(-1) Gamma(s) = 0.677 +/- 0.007 (stat.) +/- 0.004 (syst.) ps(-1) vertical bar A(0)(0)vertical bar(2) = 0.528 +/- 0.006 (stat.) +/- 0.009 (syst.) vertical bar A(parallel to)(0)vertical bar(2) = 0.220 +/- 0.008 (stat.) +/- 0.007 (syst.) where the values quoted for phi(s) and Delta Gamma(s) correspond to the solution compatible with the external measurements to which the strong phase delta(perpendicular to) is constrained and where is Delta Gamma(s) constrained to be positive. The fraction of S-wave KK or f(0) contamination through the decays B-s(0) -> J/psi K+K- (f(0)) is measured as well and is found to be consistent with zero. Results for phi(s) and Delta Gamma(s) are also presented as 68%, 90% and 95% likelihood contours, which show agreement with Standard Model expectations.
|
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 jet activity in top quark events using the e μfinal state with two b-tagged jets in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 09(9), 074–62pp.
Abstract: Measurements of the jet activity in t (t) over bar events produced in proton-proton collisions at root s = 8 TeV are presented, using 20.3 fb(-1) of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton e μdecay channel with two identified b-jets. The numbers of additional jets for various jet transverse momentum ( p(T)) thresholds, and the normalised differential cross-sections as a function of p(T) for the five highest-p(T) additional jets, were measured in the jet pseudorapidity range broken vertical bar eta broken vertical bar < 4.5. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum p(T) of a single jet or the scalar sum of p(T) of all additional jets. These fractions were also measured in different intervals of the invariant mass of the e μb<(b)over bar> b system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order t (t) over bar t event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector. J. High Energy Phys., 04(4), 075–51pp.
Abstract: A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at root s = 7 TeV with the ATLAS experiment at the Large flatiron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb(-1). No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles.
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for charged Higgs bosons through the violation of lepton universality in t(t)over-bar events using pp collision data at root s=7 TeV with the ATLAS experiment. J. High Energy Phys., 03(3), 076–36pp.
Abstract: In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson H+, t -> bH(+), in addition to the Standard Model decay t -> bW. Since W bosons decay to the three lepton generations equally, while H+ may predominantly decay into tau nu, charged Higgs bosons can be searched for using the violation of lepton universality in top quark decays. The analysis in this paper is based on 4.6 fb(-1) of proton-proton collision data at root s = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons (e or mu) and/or a hadronically decaying tau (tau(had)) are used. Event yield ratios between e+ tau(had) and e + mu, as well as between μ+ tau(had) and μ+ e, final states are measured in the data and compared to predictions from simulations. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviation from the Standard Model predictions is observed. With the assumption that the branching fraction B(H+ -> tau nu) is 100%, upper limits in the range 3.2%-4.4% can be placed on the branching fraction B(t -> bH(+)) for charged Higgs boson masses m(H+) in the range 90-140 GeV. After combination with results from a search for charged Higgs bosons in t (t) over bar decays using the tau(had) + jets final state, upper limits on B(t -> bH(+)) can be set in the range 0.8%-3.4%, for m(H+) in the range 90-160 GeV.
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Performance of jet substructure techniques for large-R jets in proton-proton collisions at root s=7 TeV using the ATLAS detector. J. High Energy Phys., 09(9), 076–83pp.
Abstract: This paper presents the application of a variety of techniques to study jet substructure. The performance of various modified jet algorithms, or jet grooming techniques, for several jet types and event topologies is investigated for jets with transverse momentum larger than 300 GeV. Properties of jets subjected to the mass-drop filtering, trimming, and pruning algorithms are found to have a reduced sensitivity to multiple proton-proton interactions, are more stable at high luminosity and improve the physics potential of searches for heavy boosted objects. Studies of the expected discrimination power of jet mass and jet substructure observables in searches for new physics are also presented. Event samples enriched in boosted W and Z bosons and top-quark pairs are used to study both the individual jet invariant mass scales and the efficacy of algorithms to tag boosted hadronic objects. The analyses presented use the full 2011 ATLAS dataset, corresponding to an integrated luminosity of 4.7 +/- 0.1 fb(-1) from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of root s = 7 TeV.
|
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 prompt J/psi pair production cross-section in pp collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 77(2), 76–34pp.
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.
|
ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Measurement of the production cross-section of psi(2S) -> J/psi(-> mu(+)mu(-))pi(+) pi(-) in pp collisions at root s=7 TeV at ATLAS. J. High Energy Phys., 09(9), 079–49pp.
Abstract: The prompt and non-prompt production cross-sections for psi(2S) mesons are measured using 2.1 fb(-1) of pp collision data at a centre-of-mass energy of 7TeV recorded by the ATLAS experiment at the LHC. The measurement exploits the psi(2S) --> J/psi(--> mu(+)mu(-)) pi(+)pi(-) decay mode, and probes psi(2S) mesons with transverse momenta in the range 10 <= p(T) < 100 GeV and rapidity |y| < 2.0. The results are compared to other measurements of psi(2S) production at the LHC and to various theoretical models for prompt and non-prompt quarkonium production.
|
