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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(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 t(t)over-bar production cross-section using e μevents with b-tagged jets in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 761, 136–157.
Abstract: This paper describes a measurement of the inclusive top quark pair production cross-section (sigma(t (t) over bar)) with a data sample of 3.2 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13 TeV, collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron-muon pair in the final state. Jets containing b-quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two b-tagged jets are counted and used to determine simultaneously sigma(t (t) over bar) and the efficiency to reconstruct and b-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be: s(t (t) over bar)= 818 +/- 8 (stat) +/- 27 (syst) +/- 19 (lumi) +/- 12 (beam) pb, where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented.
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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 total cross section from elastic scattering in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 761, 158–178.
Abstract: A measurement of the total ppcross section at the LHC at root s = 8TeV is presented. An integrated luminosity of 500 μb(-1) was accumulated in a special run with high-beta* beam optics to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the -t range from 0.014 GeV2 to 0.1 GeV2 to extrapolate t -> 0, the total cross section, sigma(tot)( pp -> X), is measured via the optical theorem to be sigma(tot)(pp -> X) = 96.07 +/- 0.18 (stat.) +/- 0.85 (exp.) +/- 0.31 (extr.) mb, where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation t -> 0. In addition, the slope of the exponential function describing the elastic cross section at small t is determined to be B = 19.74 +/- 0.05 (stat.) +/- 0.23 (syst.) GeV-2.
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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 top quark mass in the t(t)over-bar -> dilepton channel from root s=8 TeV ATLAS data. Phys. Lett. B, 761, 350–371.
Abstract: The top quark mass is measured in the t (t) over bar -> dilepton channel (lepton= e, mu) using ATLAS data recorded in the year 2012 at the LHC. The data were taken at a proton-proton centre-of-mass energy of root s = 8 TeV and correspond to an integrated luminosity of about 20.2 fb(-1). Exploiting the template method, and using the distribution of invariant masses of lepton-b-jetpairs, the top quark mass is measured to be m(top) = 172.99 +/- 0.41(stat)+/- 0.74(syst) GeV, with a total uncertainty of 0.84 GeV. Finally, acombination with previous ATLAS m(top) measurements from root s = 7 TeV data in the t (t) over bar -> dilepton and t (t) over bar -> lepton + jets channels results in m(top) = 172.84 +/- 0.34(stat)+/- 0.61(syst) GeV, with a total uncertainty of 0.70 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 high-mass new phenomena in the dilepton final state using proton-proton collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 761, 372–392.
Abstract: A search is conducted for both resonant and non-resonant high-mass new phenomena in dielectron and dimuon final states. The search uses 3.2 fb(-1) of proton-proton collision data, collected at root s = 13 TeV by the ATLAS experiment at the LHC in 2015. The dilepton invariant mass is used as the discriminating variable. No significant deviation from the Standard Model prediction is observed; therefore limits are set on the signal model parameters of interest at 95% credibility level. Upper limits are set on the cross-section times branching ratio for resonances decaying to dileptons, and the limits are converted into lower limits on the resonance mass, ranging between 2.74 TeV and 3.36 TeV, depending on the model. Lower limits on the llqq contact interaction scale are set between 16.7 TeV and 25.2 TeV, also depending on the 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. (2016). Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in root s=8 TeV proton-proton collisions. J. High Energy Phys., 09(9), 175–44pp.
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 root 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((chi) over tilde (0)(1)) of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger m((chi) over tilde (0)(1)) due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small.
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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. (2016). Measurement of W+W- production in association with one jet in proton-proton collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 763, 114–133.
Abstract: The production of W boson pairs in association with one jet in pp collisions at root s = 8 TeV is studied using data corresponding to an integrated luminosity of 20.3 fb(-1) collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The cross section is measured in a fiducial phase-space region defined by the presence of exactly one electron and one muon, missing transverse momentum and exactly one jet with a transverse momentum above 25 GeV and a pseudorapidity of vertical bar eta vertical bar < 4.5. The leptons are required to have opposite electric charge and to pass transverse momentum and pseudorapidity requirements. The fiducial cross section is found to be sigma(fid,1-jet)(WW) = 136 +/- 6 (stat) +/- 14 (syst) +/- 3 (lumi) fb. In combination with a previous measurement restricted to leptonic final states with no associated jets, the fiducial cross section of WW production with zero or one jet is measured to be sigma(fid,<1-jet)(WW) = 511 +/- 9 (stat) +/- 26 (syst) +/- 10 (lumi) fb. The ratio of fiducial cross sections in final states with one and zero jets is determined to be 0.36 +/- 0.05. Finally, a total cross section extrapolated from the fiducial measurement of WWproduction with zero or one associated jet is reported. The measurements are compared to theoretical predictions and found in good agreement.
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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 dark matter produced in association with a hadronically decaying vector boson in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 763, 251–268.
Abstract: A search is presented for dark matter produced in association with a hadronically decaying W or Z boson using 3.2 fb(-1) of ppcollisions at root s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet compatible with a Wor Z boson and with large missing transverse momentum are analysed. The data are consistent with the Standard Model predictions and are interpreted in terms of both an effective field theory and a simplified model containing dark matter.
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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 W-+/- Z boson pair-production cross section in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 762, 1–22.
Abstract: The production of W-+/- Z events in proton-proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb(-1). The W-+/- Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is sigma(fid.)(W +/- Z -> L'vll) = 63.2 +/- 3.2 (stat.) +/- 2.6 (sys.) +/- 1.5 (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is 53.4(-2.8)(+3.6) fb. The extrapolation of the measurement from the fiducial to the total phase space yields sigma(tot.)(W +/- Z) = 50.6 +/- 2.6 (stat.) +/- 2.0 (sys.) +/- 0.9 (th.) +/- 1.2 (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of 48.2(-1.0)(+1.1) pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent W+ Z and W- Z cross sections and their ratio.
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