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). Measurement of Azimuthal Anisotropy of Muons from Charm and Bottom Hadrons in pp Collisions at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 124(8), 082301–20pp.
Abstract: The elliptic flow of muons from the decay of charm and bottom hadrons is measured in pp collisions at root s = 13 TeV using a data sample with an integrated luminosity of 150 pb(-1) recorded by the ATLAS detector at the LHC. The muons from heavy-flavor decay are separated from light-hadron decay muons using momentum imbalance between the tracking and muon spectrometers. The heavy-flavor decay muons are further separated into those from charm decay and those from bottom decay using the distance-of-closest-approach to the collision vertex. The measurement is performed for muons in the transverse momentum range 4-7 GeV and pseudorapidity range vertical bar eta vertical bar < 2.4. A significant nonzero elliptic anisotropy coefficient nu(2) is observed for muons from charm decays, while the nu(2) value for muons from bottom decays is consistent with zero within uncertainties.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2020). Measurement of the Lund Jet Plane Using Charged Particles in 13 TeV Proton-Proton Collisions with the ATLAS Detector. Phys. Rev. Lett., 124(22), 222002–21pp.
Abstract: The prevalence of hadronic jets at the LHC requires that a deep understanding of jet formation and structure is achieved in order to reach the highest levels of experimental and theoretical precision. There have been many measurements of jet substructure at the LHC and previous colliders, but the targeted observables mix physical effects from various origins. Based on a recent proposal to factorize physical effects, this Letter presents a double-differential cross-section measurement of the Lund jet plane using 139 fb(-1) of root s = 13 TeV proton-proton collision data collected with the ATLAS detector using jets with transverse momentum above 675 GeV. The measurement uses charged particles to achieve a fine angular resolution and is corrected for acceptance and detector effects. Several parton shower Monte Carlo models are compared with the data. No single model is found to be in agreement with the measured data across the entire plane.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2020). Search for Heavy Higgs Bosons Decaying into Two Tau Leptons with the ATLAS Detector Using pp Collisions at root s=13 TeV. Phys. Rev. Lett., 125(5), 051801–22pp.
Abstract: A search for heavy neutral Higgs bosons is performed using the LHC Run 2 data, corresponding to an integrated luminosity of 139 fb(-1) of proton-proton collisions at root s = 13 TeV recorded with the ATLAS detector. The search for heavy resonances is performed over the mass range 0.2-2.5 TeV for the tau(+)tau(-) decay with at least one tau-lepton decaying into final states with hadrons. The data are in good agreement with the background prediction of the standard model. In the M-h(125) scenario of the minimal supersymmetric standard model, values of tan beta > 8 and tan beta > 21 are excluded at the 95% confidence level for neutral Higgs boson masses of 1.0 and 1.5 TeV, respectively, where tan beta is the ratio of the vacuum expectation values of the two Higgs doublets.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2020). Search for Heavy Resonances Decaying into a Photon and a Hadronically Decaying Higgs Boson in pp Collisions at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 125(25), 251802–20pp.
Abstract: This Letter presents a search for the production of new heavy resonances decaying into a Higgs boson and a photon using proton-proton collision data at root s = 13 TeV collected by the ATLAS detector at the LHC. The data correspond to an integrated luminosity of 139 fb(-1). The analysis is performed by reconstructing hadronically decaying Higgs boson (H -> b (b) over bar) candidates as single large-radius jets. A novel algorithm using information about the jet constituents in the center-of-mass frame of the jet is implemented to identify the two b quarks in the single jet. No significant excess of events is observed above the expected background. Upper limits are set on the production cross-section times branching fraction for narrow spin-1 resonances decaying into a Higgs boson and a photon in the resonance mass range from 0.7 to 4 TeV, cross-section times branching fractions are excluded between 11.6 fb and 0.11 fb at a 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2021). Longitudinal Flow Decorrelations in Xe plus Xe Collisions at root s(NN )=5.44 TeV with the ATLAS Detector. Phys. Rev. Lett., 126(12), 122301–20pp.
Abstract: The first measurement of longitudinal decorrelations of harmonic flow amplitudes v(n) for n = 2-4 in Xe + Xe collisions at root s(NN) = 5.44 TeV is obtained using 3 μb(-1) of data with the ATLAS detector at the LHC. The decorrelation signal for v(3) and v(4) is found to be nearly independent of collision centrality and transverse momentum (p(T)) requirements on fmal-state particles, but for v(2) a strong centrality and p(T) dependence is seen. When compared with the results from Pb + Pb collisions at. root s(NN) = 5.02 TcV, the longitudinal decorrelation signal in midcentral Xe + Xe collisions is found to be larger for v(2), but smaller for v(3). Current hydrodynamic models reproduce the ratios of the v(n) measured in Xe + Xe collisions to those in Pb + Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe + Xe and Pb + Pb. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions.
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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 the charge asymmetry in highly boosted top-quark pair production in root s=8 TeV pp collision data collected by the ATLAS experiment. Phys. Lett. B, 756, 52–71.
Abstract: In the pp -> t (t) over bar process the angular distributions of top and anti-top quarks are expected to present a subtle difference, which could be enhanced by processes not included in the Standard Model. This Letter presents a measurement of the charge asymmetry in events where the top-quark pair is produced with a large invariant mass. The analysis is performed on 20.3 fb(-1) of pp collision data at root s=8 TeV collected by the ATLAS experiment at the LHC, using reconstruction techniques specifically designed for the decay topology of highly boosted top quarks. The charge asymmetry in a fiducial region with large invariant mass of the top-quark pair (m(t (t) over bar) > 0.75 TeV) and an absolute rapidity difference of the top and anti-top quark candidates within -2 vertical bar y(t)vertical bar – vertical bar y(<(t) over bar)vertical bar < 2is measured to be 4.2 +/- 3.2%, in agreement with the Standard Model prediction at next-to-leading order. A differential measurement in three <(t)over bar> mass bins is also presented.
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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 new phenomena in final states with large jet multiplicities and missing transverse momentum with ATLAS using root s=13 TeV proton-proton collisions. Phys. Lett. B, 757, 334–355.
Abstract: Results are reported of a search for new phenomena, such as supersymmetric particle production, that could be observed in high-energy proton-proton collisions. Events with large numbers of jets, together with missing transverse momentum from unobserved particles, are selected. The data analysed were recorded by the ATLAS experiment during 2015 using the 13 TeV centre-of-mass proton-proton collisions at the Large Hadron Collider, and correspond to an integrated luminosity of 3.2 fb(-1). The search selected events with various jet multiplicities from >= 7 to >= 10 jets, and with various b-jet multiplicity requirements to enhance sensitivity. No excess above Standard Model expectations is observed. The results are interpreted within two supersymmetry models, where gluino masses up to 1400 GeV are excluded at 95% confidence level, significantly extending previous limits.
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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). Charged-particle distributions in root s=13 TeV pp interactions measured with the ATLAS detector at the LHC. Phys. Lett. B, 758, 67–88.
Abstract: Charged-particle distributions are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using a data sample of nearly 9 million events, corresponding to an integrated luminosity of 170 μb(-1), recorded by the ATLAS detector during a special Large Hadron Collider fill. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on the charged-particle multiplicity are presented. The measurements are performed with charged particles with transverse momentum greater than 500 MeV and absolute pseudorapidity less than 2.5, in events with at least one charged particle satisfying these kinematic requirements. Additional measurements in a reduced phase space with absolute pseudorapidity less than 0.8 are also presented, in order to compare with other experiments. The results are corrected for detector effects, presented as particle-level distributions and are compared to the predictions of various Monte Carlo event generators.
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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 a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in pp collisions at root s=8 TeV. Phys. Lett. B, 758, 249–268.
Abstract: A search is performed for the process pp -> G* -> B-H(b) over bar/(B) over bar (H)b -> Hb (b) over bar -> b (b) over barb (b) over bar, predicted in composite Higgs scenarios, where G* is a heavy colour octet vector resonance and B-H a vector-like quark of charge -1/3. The data were obtained from pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.5 fb(-1), recorded by the ATLAS detector at the LHC. The largest background, multijet production, is estimated using a data-driven method. No significant excess of events with respect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from a specific benchmark model are made, resulting in lower mass limits in the two-dimensional mass plane of m(G*) vs. m(BH).
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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 resonances in the mass distribution of jet pairs with one or two jets identified as b-jets in proton-proton collisions at root S=13 TeV with the ATLAS detector. Phys. Lett. B, 759, 229–246.
Abstract: Searches for high-mass resonances in the dijet invariant mass spectrum with one or two jets identified as b-jets are performed using an integrated luminosity of 3.2 fb(-1) of proton-proton collisions with a centre-of-mass energy of root S = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% credibility level, excited b* quarks with masses from 1.1 TeV to 2.1 TeV and leptophobic Z' bosons with masses from 1.1 TeV to 1.5 TeV. Contributions of a Gaussian signal shape with effective cross sections ranging from approximately 0.4 to 0.001 pb are also excluded in the mass range 1.5-5.0 TeV.
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