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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). Search for Higgs boson decays into pairs of light (pseudo)scalar particles in the gamma gamma jj final state in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 782, 750–767.
Abstract: This Letter presents a search for exotic decays of the Higgs boson to a pair of new (pseudo) scalar particles, H -> aa, where the a particle has a mass in the range 20-60 GeV, and where one of the a bosons decays into a pair of photons and the other to a pair of gluons. The search is performed in event samples enhanced in vector-boson fusion Higgs boson production by requiring two jets with large invariant mass in addition to the Higgs boson candidate decay products. The analysis is based on the full dataset of pp collisions at root s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.7 fb(-1). The data are in agreement with the Standard Model predictions and an upper limit at the 95% confidence level is placed on the production cross section times the branching ratio for the decay H -> aa -> gamma gamma gg. This limit ranges from 3.1 pb to 9.0 pb depending on the mass of the a boson.
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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. (2020). Measurements of top-quark pair spin correlations in the e mu channel at root s=13 TeV using pp collisions in the ATLAS detector. Eur. Phys. J. C, 80(8), 754–43pp.
Abstract: A measurement of observables sensitive to spin correlations in tt<overbar></mml:mover> production is presented, using 36.1 fb-1 of pp collision data at <mml:msqrt>s</mml:msqrt>=13 TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the t<mml:mover accent=“true”>t<mml:mo stretchy=“false”><overbar></mml:mover> system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Campabadal Segura, F., Castillo Gimenez, V., Costa, M. J., et al. (2010). Drift Time Measurement in the ATLAS Liquid Argon Electromagnetic Calorimeter using Cosmic Muons. Eur. Phys. J. C, 70(3), 755–785.
Abstract: The ionization signals in the liquid argon of the ATLAS electromagnetic calorimeter are studied in detail using cosmic muons. In particular, the drift time of the ionization electrons is measured and used to assess the intrinsic uniformity of the calorimeter gaps and estimate its impact on the constant term of the energy resolution. The drift times of electrons in the cells of the second layer of the calorimeter are uniform at the level of 1.3% in the barrel and 2.8% in the endcaps. This leads to an estimated contribution to the constant term of (0.29(-0.04)(+0.05))% in the barrel and (0.54(-0.04)(+0.06))% in the endcaps. The same data are used to measure the drift velocity of ionization electrons in liquid argon, which is found to be 4.61 +/- 0.07 mm/mu s at 88.5 K and 1 kV/mm.
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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. (2019). Measurement of the cross-section and charge asymmetry of W bosons produced in proton-proton collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 79(9), 760–25pp.
Abstract: This paper presents measurements of the W+->mu+nu and W-->mu-nu cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2fb(-1). The precision of the cross-section measurements varies between 0.8 and 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.
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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). New techniques for jet calibration with the ATLAS detector. Eur. Phys. J. C, 83(8), 761–41pp.
Abstract: A determination of the jet energy scale is presented using proton-proton collision data with a centre-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 140 fb(-1) collected using the ATLAS detector at the LHC. Jets are reconstructed using the ATLAS particle-flow method that combines charged-particle tracks and topo-clusters formed from energy deposits in the calorimeter cells. The anti-kt jet algorithm with radius parameter R = 0.4 is used to define the jet. Novel jet energy scale calibration strategies developed for the LHC Run 2 are reported that lay the foundation for the jet calibration in Run 3. Jets are calibrated with a series of simulation-based corrections, including state-of-the-art techniques in jet calibration such as machine learning methods and novel in situ calibrations to achieve better performance than the baseline calibration derived using up to 81 fb(-1) of Run 2 data. The performance of these new techniques is then examined in the in situ measurements by exploiting the transverse momentum balance between a jet and a reference object. The b-quark jet energy scale using particle flow jets is measured for the first time with around 1% precision using gamma+jet events.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., et al. (2018). Prompt and non-prompt J/psi and psi(2S) suppression at high transverse momentum in 5.02 TeV Pb+Pb collisions with the ATLAS experiment. Eur. Phys. J. C, 78(9), 762–28pp.
Abstract: A measurement of J/psi and psi(2S) production is presented. It is based on a data sample from Pb+Pb collisions at root s(NN) = 5.02 TeV and pp collisions at root s = 5.02 TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of 0.42 nb(-1) and 25 pb(-1) in Pb+Pb and pp, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for 9 < p(T)(mu mu) < 40 GeV in dimuon transverse momentum, and -2 < y(mu mu) < 2 in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt J/psi, increasing with event centrality. The suppression of prompt psi(2S) is observed to be stronger than that of J/psi, while the suppression of non-prompt psi(2S) is equal to that of the non-prompt J/psi within uncertainties, consistent with the expectation that both arise from b-quarks propagating through the medium. Despite prompt and non-prompt J/psi arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar.
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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 detector-corrected observables sensitive to the anomalous production of events with jets and large missing transverse momentum in pp collisions at root s=13 TeV using the ATLAS detector. Eur. Phys. J. C, 77(11), 765–31pp.
Abstract: Observables sensitive to the anomalous production of events containing hadronic jets and missing momentum in the plane transverse to the proton beams at the Large Hadron Collider are presented. The observables are defined as a ratio of cross sections, for events containing jets and large missing transverse momentum to events containing jets and a pair of charged leptons from the decay of a Z/gamma* boson. This definition minimises experimental and theoretical systematic uncertainties in the measurements. This ratio is measured differentially with respect to a number of kinematic properties of the hadronic system in two phase-space regions; one inclusive single-jet region and one region sensitive to vector-boson-fusion topologies. The data are found to be in agreement with the Standard Model predictions and used to constrain a variety of theoretical models for dark-matter production, including simplified models, effective field theory models, and invisible decays of the Higgs boson. The measurements use 3.2 fb(-1) of proton-proton collision data recorded by the ATLAS experiment at a centre-of-mass energy of 13TeV and are fully corrected for detector effects, meaning that the data can be used to constrain new-physics models beyond those shown in this paper.
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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). Measurements of differential cross sections of Higgs boson production through gluon fusion in the H → WW *→ eνμν final state at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(9), 774–40pp.
Abstract: Higgs boson production via gluon-gluon fusion is measured in the WW *-> e nu μnu decay channel. The dataset utilized corresponds to an integrated luminosity of 139 fb(-1) collected by the ATLAS detector from root s = 13TeV proton-proton collisions delivered by the Large Hadron Collider between 2015 and 2018. Differential cross sections are measured in a fiducial phase space restricted to the production of at most one additional jet. The results are consistent with Standard Model expectations, derived using different Monte Carlo generators.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb-1 of ATLAS √s=13 TeV proton-proton collision data. Eur. Phys. J. C, 83(9), 781–33pp.
Abstract: Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the mu(+)mu(-) gamma final state with a proton-proton collision data sample corresponding to an integrated luminosity of 139 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/psi gamma, psi(2S)gamma, and Upsilon(1S, 2S, 3S)gamma are found to be 2.0 x 10(-4), 10.5x10(-4), and (2.5, 4.2, 3.4) x10(-4), respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 x 10(-6), 2.4 x 10(-6), and (1.1, 1.3, 2.4) x10(-6). An observed 95% CL interval of (-133, 175) is obtained for the kappa(c)/kappa(gamma) ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for kappa(b)/kappa(gamma).
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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. (2018). Prompt and non-prompt J/psi elliptic flow in Pb plus Pb collisions at root S-NN=5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 78(9), 784–23pp.
Abstract: The elliptic flow of prompt and non-prompt J/psi was measured in the dimuon decay channel in Pb+Pb collisions at root S-NN = 5.02 TeV with an integrated luminosity of 0.42 nb(-1) with the ATLAS detector at the LHC. The prompt and non-prompt signals are separated using a two-dimensional simultaneous fit of the invariant mass and pseudo-proper decay time of the dimuon system from the J/psi decay. The measurement is performed in the kinematic range of dimuon transverse momentum and rapidity 9 < p(T) < 30 GeV, vertical bar y vertical bar < 2, and 0-60% collision centrality. The elliptic flow coefficient, v(2), is evaluated relative to the event plane and the results are presented as a function of transverse momentum, rapidity and centrality. It is found that prompt and non-prompt J/psi mesons have non-zero elliptic flow. Prompt J/psi v(2 )decreases as a function of p(T), while for non-prompt J/psi it is, with limited statistical significance, consistent with a flat behaviour over the studied kinematic region. There is no observed dependence on rapidity or centrality.
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