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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). Search for single production of vector-like quarks decaying into Wb in pp collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 76(8), 442–26pp.
Abstract: A search for singly produced vector-like Q quarks, where Q can be either a T quark with charge +2/3 or a Y quark with charge -4/3, is performed in proton-proton collisions recorded with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb(-1) and was produced with a centre-of-mass energy of root s = 8 TeV. This analysis targets Q -> Wb decays where the W boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant t (t) over bar background. The reconstructed Q-candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on the Q -> Wb cross-section times branching ratio. The results are also interpreted as limits on the QWb coupling and the mixing with the Standard Model sector for a singlet T quark or a Y quark from a doublet. T quarks with masses below 0.95 TeV are excluded at 95 % confidence level, assuming a unit coupling and a BR(T -> Wb) = 0.5, whereas the expected limit is 1.10 TeV.
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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). Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector. Phys. Lett. B, 789, 444–471.
Abstract: Correlations of two flow harmonics v(n) and v(m) via three- and four-particle cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between v(2) and v(3) and a positive correlation between v(2) and v(4) for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the < v(n)(2)> from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions.
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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). Jet reconstruction and performance using particle flow with the ATLAS Detector. Eur. Phys. J. C, 77(7), 466–47pp.
Abstract: This paper describes the implementation and performance of a particle flow algorithm applied to 20.2 fb(-1) of ATLAS data from 8 TeV proton-proton collisions in Run 1 of the LHC. The algorithm removes calorimeter energy deposits due to charged hadrons from consideration during jet reconstruction, instead using measurements of their momenta from the inner tracker. This improves the accuracy of the charged-hadron measurement, while retaining the calorimeter measurements of neutral-particle energies. The paper places emphasis on how this is achieved, while minimising double-counting of charged-hadron signals between the inner tracker and calorimeter. The performance of particle flow jets, formed from the ensemble of signals from the calorimeter and the inner tracker, is compared to that of jets reconstructed from calorimeter energy deposits alone, demonstrating improvements in resolution and pile-up stability.
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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). Observation of electroweak W(+/-)Z boson pair production in association with two jets in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 793, 469–492.
Abstract: An observation of electroweak W(+/-)Z production in association with two jets in proton-proton collisions is presented. The data collected by the ATLAS detector at the Large Hadron Collider 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, either electrons or muons, and two jets are selected. The electroweak production of W(+/-)Z bosons in association with two jets is measured with an observed significance of 5.3 standard deviations. A fiducial cross-section for electroweak production including interference effects and for a single leptonic decay mode is measured to be sigma(WZjj-EW) = 0.57(-0.13)(+0.14) (stat.) (+0.07)(-0.06) (syst.) fb. Total and differential fiducial cross-sections of the sum of W(+/-)Zjj electroweak and strong productions for several kinematic observables are 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). Measurement of the cross section for inclusive isolated-photon production in pp collisions at root s=13 TeV using the ATLAS detector. Phys. Lett. B, 770, 473–493.
Abstract: Inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13 TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2 fb(-1). The cross section is measured as a function of the photon transverse energy above 125 GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.
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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 electroweak W j j production and constraints on anomalous gauge couplings with the ATLAS detector. Eur. Phys. J. C, 77(7), 474–74pp.
Abstract: Measurements of the electroweak production of a W boson in association with two jets at high dijet invariant mass are performed using root s = 7 and 8 TeV proton-proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb(-1) of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of a W boson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.
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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 colour flow with the jet pull angle in t(t)over-bar events using the ATLAS detector at root s=8 TeV. Phys. Lett. B, 750, 475–493.
Abstract: The distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard-scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in t (t) over bar events with one W boson decaying leptonically and the other decaying to jets using 20.3 fb(-1) of data recorded with the ATLAS detector at a centre-of-mass energy of root s = 8 TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models.
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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). Study of the spin and parity of the Higgs boson in diboson decays with the ATLAS detector. Eur. Phys. J. C, 75(10), 476–36pp.
Abstract: Studies of the spin, parity and tensor couplings of the Higgs boson in the H -> ZZ* -> 4l, H -> WW* -> e nu μnu and H -> gamma gamma decay processes at the LHC are presented. The investigations are based on 25 fb(-1) of pp collision data collected by the ATLAS experiment at root s = 7 TeV and root s = 8 TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers J (P) = 0(+), is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9% confidence level. Using the H -> ZZ* -> 4l and H -> WW* -> e nu μnu. decays, the tensor structure of the interaction between the spin-0 boson and the SM vector bosons is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.
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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). Search for long-lived neutral particles in pp collisions at root s=13 TeV that decay into displaced hadronic jets in the ATLAS calorimeter. Eur. Phys. J. C, 79(6), 481–31pp.
Abstract: This paper describes a search for pairs of neutral, long-lived particles decaying in the ATLAS calorimeter. Long-lived particles occur in many extensions to the Standard Model and may elude searches for new promptly decaying particles. The analysis considers neutral, long-lived scalars with masses between 5 and 400 GeV, produced from decays of heavy bosons with masses between 125 and 1000 GeV, where the long-lived scalars decay into Standard Model fermions. The analysis uses either 10.8 fb-1 of data (depending on the trigger) recorded in 2016 at the LHC with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV. No significant excess is observed, and limits are reported on the production cross section times branching ratio as a function of the proper decay length of the long-lived particles.
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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 inclusive and fiducial t(t)over-bar production cross-sections in the lepton plus jets channel in pp collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 78(6), 487–31pp.
Abstract: The inclusive and fiducial t (t) over bar production cross sections are measured in the lepton+jets channel using 20.2 fb(-1) of proton proton collision data at a centre-of mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W+jets process is modelled using Z+jets events in a data-driven approach. The inclusive t (t) over bar cross-section is measured with a precision of 5.7% to be (sigma(inc) (t (t) over bar) = 248.3 +/- 0.7 (stat.) +/- 13.4 (syst.) +/- 4.7 (lumi.) pb, assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is sigma(fid) (t (t) over bar) = 48.8 +/- 0.1 (stat.) +/- 2.0 (syst.) +/- 0.9 (lumi.) pb with a precision of 4.5%.
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