ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Precision measurement and interpretation of inclusive W+, W- and Z/gamma* production cross sections with the ATLAS detector. Eur. Phys. J. C, 77(6), 367–62pp.
Abstract: High-precision measurements by the ATLAS Collaboration are presented of inclusive W+ -> l(+) nu, W- -> l(-) (nu) over bar and Z/gamma* -> ll (l = e, mu) Drell-Yan production cross sections at the LHC. The data were collected in proton-proton collisions at root s = 7 TeV with an integrated luminosity of 4.6 fb(-1). Differential W+ and W- cross sections are measured in a lepton pseudorapidity range vertical bar eta(l)vertical bar < 2.5. Differential Z/gamma* cross sections are measured as a function of the absolute dilepton rapidity, for vertical bar y(ll)vertical bar < 3.6, for three intervals of dilepton mass, m(ll), extending from 46 to 150 GeV. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive e(+/-) p scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element vertical bar V-cs vertical bar is also provided.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Measurement of transverse energy-energy correlations in multi-jet events in pp collisions at root s=7 TeV using the ATLAS detector and determination of the strong coupling constant alpha(s)(m(Z)). Phys. Lett. B, 750, 427–447.
Abstract: High transverse momentum jets produced in pp collisions at a centre of mass energy of 7 TeV are used to measure the transverse energy-energy correlation function and its associated azimuthal asymmetry. The data were recorded with the ATLAS detector at the LHC in the year 2011 and correspond to an integrated luminosity of 158 pb(-1). The selection criteria demand the average transverse momentum of the two leading jets in an event to be larger than 250 GeV. The data at detector level are well described by Monte Carlo event generators. They are unfolded to the particle level and compared with theoretical calculations at next-to-leading-order accuracy. The agreement between data and theory is good and provides a precision test of perturbative Quantum Chromodynamics at large momentum transfers. From this comparison, the strong coupling constant given at the Z boson mass is determined to be alpha(s)(m(Z)) = 0.1173 +/- 0.0010 (exp.) (+0.0065)(-0.0026) (theo.).
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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). Measurements of gluon-gluon fusion and vector-boson fusion Higgs boson production cross-sections in the H -> WW* -> e nu μnu decay channel in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 789, 508–529.
Abstract: Higgs boson production cross-sections in proton-proton collisions are measured in the H -> WW*-> e nu μnu decay channel. The proton-proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1). The product of the H -> WW* branching fraction times the gluon-gluon fusion and vector-boson fusion cross-sections are measured to be 11.4(-1.1)(+1.2)(stat.)(-1.7)(+1.8)(syst.) pb and 0.50(-0.22)(+0.24)(stat.) +/- 0.17(syst.) pb, respectively, in agreement with Standard Model predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Search for flavor-changing neutral currents in top quark decays t -> Hc and t -> Hu in multilepton final states in proton-proton collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 98(3), 032002–25pp.
Abstract: Flavor-changing neutral currents are not present in the Standard Model at tree level and are suppressed in loop processes by the unitarity of the Cabibbo-Kobayashi-Maskawa matrix; the corresponding rates for top quark decay processes are experimentally unobservable. Extensions of the Standard Model can generate new flavor-changing neutral current processes, leading to signals which, if observed, would be unambiguous evidence of new interactions. A data set conesponding to an integrated luminosity of 36.1 fb(-1) of pp collisions at a center-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider is used to search for top quarks decaying to up or charm quarks with the emission of a Higgs boson, with subsequent Higgs boson decay to final states with at least one electron or muon. No signal is observed and limits on the branching fractions B(t -> Hc) < 0.16% and B(t -> Hu) < 0.19% at 95% confidence level are obtained (with expected limits of 0.15% in both cases).
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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 fiducial cross-sections for t(t)over-bar production with one or two additional b-jets in pp collisions at root s=8 TeV using the ATLAS detector. Eur. Phys. J. C, 76(1), 11–37pp.
Abstract: Fiducial cross-sections for t (t) over bar production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb(-1) of proton-proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for t (t) over bar events with at least one additional b-jet is measured to be 950 +/- 70 (stat.)(-190)(+240) (syst.) fb in the lepton-plus-jets channel and 50 +/- 10 (stat.)(-10)(+15) (syst.) fb in the e μchannel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 +/- 3.5 (stat.) +/- 5.7 (syst.) fb in the dilepton channel (e mu, μmu, and ee) using a method based on tight selection criteria, and 13.5 +/- 3.3 (stat.) +/- 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 +/- 0.33 (stat.) +/- 0.28 (syst.)% for the ratio of t (t) over bar production with two additional b-jets to t (t) over bar production with any two additional jets. All measurements are in good agreement with recent theory predictions.
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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 event-shape observables in Z -> l(+)l(-) events in pp collisions at root s=7 TeV with the ATLAS detector at the LHC. Eur. Phys. J. C, 76(7), 375–40pp.
Abstract: Event-shape observables measured using charged particles in inclusive Z-boson events are presented, using the electron and muon decay modes of the Z bosons. The measurements are based on an integrated luminosity of 1.1 fb(-1) of proton-proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy root s = 7 TeV. Charged-particle distributions, excluding the lepton-antilepton pair from the Z-boson decay, are measured in different ranges of transverse momentum of the Z boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and F-parameter, which are in particular sensitive to properties of the underlying event at small values of the Z-boson transverse momentum. The measured observables are compared with predictions from PYTHIA 8, Sherpa, and HERWIG 7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high Z-boson transverse momenta than at low Z-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2020). Performance of electron and photon triggers in ATLAS during LHC Run 2. Eur. Phys. J. C, 80(1), 47–41pp.
Abstract: Electron and photon triggers covering transverse energies from 5 GeV to several TeV are essential for the ATLAS experiment to record signals for a wide variety of physics: from StandardModel processes to searches for new phenomena in both proton-proton and heavy-ion collisions. To cope with a fourfold increase of peak LHC luminosity from 2015 to 2018 (Run 2), to 2.1 x 10(34) cm(-2) s(-1), and a similar increase in the number of interactions per beam-crossing to about 60, trigger algorithms and selections were optimised to control the rates while retaining a high efficiency for physics analyses. For proton-proton collisions, the single-electron trigger efficiency relative to a single-electron offline selection is at least 75% for an offline electron of 31 GeV, and rises to 96% at 60 GeV; the trigger efficiency of a 25GeVleg of the primary diphoton trigger relative to a tight offline photon selection is more than 96% for an offline photon of 30 GeV. For heavy-ion collisions, the primary electron and photon trigger efficiencies relative to the corresponding standard offline selections are at least 84% and 95%, respectively, at 5 GeV above the corresponding trigger threshold.
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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(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Measurement of the transverse momentum and phi(eta)*. distributions of Drell-Yan lepton pairs in proton-proton collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 76(5), 291–61pp.
Abstract: Distributions of transverse momentum p(T)(ll) and the related angular variable phi(eta)*. of Drell-Yan lepton pairs are measured in 20.3 fb(-1) of proton-proton collisions at root s = 8 TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton-proton collisions at root s = 7 TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of phi(eta)*. < 1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of phi(eta)*. this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of p(T)(ll) while the fixed-order prediction of Dynnlo falls below the data at high values of p(T)(ll). ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the phi(eta)*. and p(T)(ll) distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.
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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. (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).
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