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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Measurement of the polarisation of single top quarks and antiquarks produced in the t-channel at root s=13 TeV and bounds on the tWb dipole operator from the ATLAS experiment. J. High Energy Phys., 11(11), 040–62pp.
Abstract: A simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in t-channel single-top-quark production is presented. This analysis is based on data from proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb(-1), collected with the ATLAS detector at the LHC. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being b-tagged. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from the background contributions. The top-quark and top-antiquark polarisation vectors are measured from the distributions of the direction cosines of the charged-lepton momentum in the top-quark rest frame. The three components of the polarisation vector for the selected top-quark event sample are P-x' = 0.01 +/- 0.18, P-y' = -0.029 +/- 0.027, P-z' = 0.91 +/- 0.10 and for the top-antiquark event sample they are P-x' = -0.02 +/- 0.20, P-y' = -0.007 +/- 0.051, P-z' = -0.79 +/- 0.16. Normalised differential cross-sections corrected to a fiducial region at the stable-particle level are presented as a function of the charged-lepton angles for top-quark and top-antiquark events inclusively and separately. These measurements are in agreement with Standard Model predictions. The angular differential cross-sections are used to derive bounds on the complex Wilson coefficient of the dimension-six O-tW operator in the framework of an effective field theory. The obtained bounds are C-tW is an element of[-0.9, 1.4] and C-itW is an element of [-0.8, 0.2], both at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at √s=13TeV with the ATLAS experiment. Phys. Lett. B, 843, 137829–22pp.
Abstract: A measurement of the polarisation of Wbosons produced in top-quark decays is presented, using proton-proton collision data at a centre-of-mass energy of v s= 13TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139fb(-1). The measurement is performed selecting t tevents decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the cos theta* variable, where theta(*) is the angle between the momentum direction of the charged lepton from the Wboson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the Wboson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the cos theta* angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f(0) = 0.684 +/- 0.005 (stat.)+/- 0.014 (syst.), f(L)= 0.318 +/- 0.003 (stat.)+/- 0.008 (syst.) and f(R)=-0.002 +/- 0.002 (stat.)+/- 0.014 (syst.), in agreement with the Standard Model prediction.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Measurement of the production and lepton charge asymmetry of W bosons in Pb plus Pb collisions at root s(NN)=2.76 TeV with the ATLAS detector. Eur. Phys. J. C, 75(1), 23–30pp.
Abstract: A measurement of W boson production in lead-lead collisions at root s(NN) = 2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 nb(-1) and 0.15 nb(-1) in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons < N-part > and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of W boson production in multi-nucleon systems.
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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 the production cross section for a Higgs boson in association with a vector boson in the H -> WW* -> l nu l nu channel in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 798, 134949–24pp.
Abstract: A measurement of the Higgs boson production cross sections via associated WH and ZH production using H -> WW* -> l nu l nu decays, where l stands for either an electron or a muon, is presented. Results for combined WH and ZH production are also presented. The analysis uses events produced in proton-proton collisions collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The data correspond to an integrated luminosity of 36.1 fb(-1) recorded at a centre-of-mass energy of 13 TeV. The products of the H -> WW* branching fraction times the WH and ZH cross sections are measured to be 0.67(-0.27)(+0.31)(stat.)(-0.14)(+0.18)(syst.) pb and 0.54(-0.24)(+0.31)(stat.)(-0.07)(+0.15)(syst.) pb respectively, in agreement with the Standard Model predictions.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Measurement of the production cross section of jets in association with a Z boson in pp collisions at root s=7 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 032–51pp.
Abstract: Measurements of the production of jets of particles in association with a Z boson in pp collisions at root s = 7 TeV are presented, using data corresponding to an integrated luminosity of 4.6 fb(-1) collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum p(T) > 30 GeV and rapidity vertical bar y vertical bar < 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2021). Measurement of the production cross section of pairs of isolated photons in pp collisions at 13 TeV with the ATLAS detector. J. High Energy Phys., 11(11), 169–53pp.
Abstract: A measurement of prompt photon-pair production in proton-proton collisions at root s = 13 TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb(-1). Events with two photons in the wellinstrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of p(T,gamma 1(2)) > 40 (30) GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leadingorder calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.
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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 prompt j/psi mesons in association with a W (+/-) boson in pp collisions root s=7 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 172–37pp.
Abstract: The process pp -> W (+/-) J/psi provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb(-1) of root s= 7 TeV pp collisions at the LHC, the first observation is made of the production of W (+/-) + prompt J/psi events in hadronic collisions, using W (+/-) -> μnu(mu) and J/psi -> mu(+)mu(-) center dot A yield of W (+/-) + prompt J/psi events is observed, with a statistical significance of 5.1 sigma. The production rate as a ratio to the inclusive W (+/-) boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.
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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. (2018). Measurement of the production cross section of three isolated photons in pp collisions at root s=8 TeV using the ATLAS detector. Phys. Lett. B, 781, 55–76.
Abstract: A measurement of the production of three isolated photons in proton-proton collisions at a centre-of-mass energy root s = 8 TeV is reported. The results are based on an integrated luminosity of 20.2 fb(-1) collected with the ATLAS detector at the LHC. The differential cross sections are measured as functions of the transverse energy of each photon, the difference in azimuthal angle and in pseudorapidity between pairs of photons, the invariant mass of pairs of photons, and the invariant mass of the triphoton system. A measurement of the inclusive fiducial cross section is also reported. Next-to-leading-order perturbative QCD predictions are compared to the cross-section measurements. The predictions underestimate the measurement of the inclusive fiducial cross section and the differential measurements at low photon transverse energies and invariant masses. They provide adequate descriptions of the measurements at high values of the photon transverse energies, invariant mass of pairs of photons, and invariant mass of the triphoton system.
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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 production cross-section of a single top quark in association with a W boson at 8 TeV with the ATLAS experiment. J. High Energy Phys., 01(1), 064–48pp.
Abstract: The cross-section for the production of a single top quark in association with a W boson in proton-proton collisions at is measured. The dataset corresponds to an integrated luminosity of 20.3 fb(-1), collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central b-jet are selected. The W t signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of W t events is observed with a significance of 7.7 sigma. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The W t cross-section, inclusive of decay modes, is measured to be 23.0 +/- 1.3(stat.) (-aEuro parts per thousand 3.5) (+ 3.2) (syst.)+/- 1.1(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element |V (tb) | of 1.01 +/- 0.10 and a lower limit of 0.80 at the 95% confidence level. The cross-section for the production of a top quark and a W boson is also measured in a fiducial acceptance requiring two leptons with p (T) > 25 GeV and |eta| < 2.5, one jet with p (T) > 20 GeV and |eta| < 2.5, and E (T) (miss) > 20 GeV, including both W t and top-quark pair events as signal. The measured value of the fiducial cross-section is 0.85 +/- A 0.01(stat.) (-aEuro parts per thousand 0.07) (+ 0.07) (syst.)+/- 0.03(lumi.) 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. (2018). Measurement of the production cross-section of a single top quark in association with a Z boson in proton-proton collisions at 13 TeV with the ATLAS detector. Phys. Lett. B, 780, 557–577.
Abstract: The production of a top quark in association with a Z boson is investigated. The proton-proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of root s = 13 TeV are used, corresponding to an integrated luminosity of 36.1 fb(-1). Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, tt($)over-bar and Z + jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2 sigma in the data and the expected significance is 5.4 sigma. The measured cross-section for tZq production is 600 +/- 170(stat.)+/- 140(syst.)fb.
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