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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). Measurement of long-range two-particle azimuthal correlations in Z-boson tagged pp collisions at root s=8 and 13 TeV. Eur. Phys. J. C, 80(1), 64–32pp.
Abstract: Results are presented from the measurement by ATLAS of long-range (|Delta eta|>2) dihadron angular correlations in root s=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 fb-1 of root s=8 TeV data recorded during Run 1 of the LHC and 36.1 fb-1 of root s=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2<|Delta eta|<5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v2,2, which in turn is used to obtain the single-particle anisotropy coefficient v2. The v2 values in the Z-tagged events, integrated over 0.5<pT<5 GeV, are found to be independent of multiplicity and <mml:msqrt>s</mml:msqrt>, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle pT, the Z-tagged and inclusive v2 values are consistent within uncertainties for pT<3 GeV.
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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 longitudinal flow decorrelations in Pb plus Pb collisions at root s(NN)=2.76 and 5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 78(2), 142–37pp.
Abstract: Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range vertical bar eta vertical bar < 2.4 using 7 μb(-1) and 470 μb(-1) of Pb+Pb collisions at root s(NN) = 2.76 and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients v(n) measured in two separated eta intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the eta separation between the intervals. The flow decorrelation is stronger at 2.76 TeVthan at 5.02 TeV. Higher-order moments of the correlations are also measured, and the corresponding linear coefficients for the kth-moment of the v(n) are found to be proportional to k for v(3), but not for v(2). The decorrelation effect is separated into contributions from the magnitude of v(n) and the event-plane orientation, each as a function of eta. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between v(n) of different order in n. The decorrelations of v(2) and v(3) are found to be independent of each other, while the decorrelations of v(4) and v(5) are found to be driven by the nonlinear contribution from v(2)(2) and v(2)v(3), respectively.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Escobar, C., et al. (2011). Measurement of multi-jet cross sections in proton-proton collisions at a 7 TeV center-of-mass energy. Eur. Phys. J. C, 71(11), 1763–27pp.
Abstract: Inclusive multi-jet production is studied in proton-proton collisions at a center-of-mass energy of 7 TeV, using the ATLAS detector. The data sample corresponds to an integrated luminosity of 2.4 pb(-1). Results on multi-jet cross sections are presented and compared to both leading-order plus parton-shower Monte Carlo predictions and to next-to-leading-order QCD calculations.
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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 multi-particle azimuthal correlations in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector. Eur. Phys. J. C, 77(6), 428–40pp.
Abstract: Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in pp collisions at root S = 5.02 and 13 TeV and in p + Pb collisions at root S-NN = 5.02 TeV, and compared to the results obtained for low-multiplicity Pb + Pb collisions at root S-NN = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in p + Pb and low-multiplicity Pb + Pb collisions. On the other hand, the pp results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in Pb + Pb, smaller in p + Pb and smallest in pp collisions. The pp results show no dependence on the collision energy, nor on the multiplicity.
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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). Measurement of muon pairs produced via gamma gamma scattering in nonultraperipheral Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector. Phys. Rev. C, 107(5), 054907–42pp.
Abstract: Results of a measurement of dimuon photoproduction in nonultraperipheral Pb + Pb collisions at root sNN = 5.02 TeV are presented. The measurement uses ATLAS data from the 2015 and 2018 Pb + Pb data-taking periods at the LHC with an integrated luminosity of 1.94 nb-1. The gamma gamma -> mu+mu- pairs are identified via selections on pair momentum asymmetry and acoplanarity. Differential cross sections for dimuon production are measured in different centrality, average muon momentum, and pair rapidity intervals as functions of acoplanarity and k perpendicular to, the transverse momentum kick of one muon relative to the other. Measurements are also made as a function of the rapidity separation of the muons and the angle of the muon pair relative to the second-order event plane to test whether magnetic fields generated in the quark-gluon plasma affect the measured muons. A prior observation of a centrality-dependent broadening of the acoplanarity distribution is confirmed. Furthermore, the improved precision of the measurement reveals a depletion in the number of pairs having small acoplanarity or k perpendicular to values in more central collisions. The acoplanarity distributions in a given centrality interval are observed to vary with the mean pT of the muons in the pair, but the k perpendicular to distributions do not. Comparisons with recent theoretical predictions are made. The predicted trends associated with effects of magnetic fields on the dimuons are not observed.
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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 photon-jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS. Phys. Lett. B, 789, 167–190.
Abstract: Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb(-1) of Pb + Pb collision data at root(NN)-N-s = 5.02 TeV and 25 pb(-1) of pp collision data at. root s= 5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1 < p(T)(gamma) < 200 GeV and vertical bar eta(gamma vertical bar) < 2.37 are paired with all jets in the event that have p(T)(jet) > 31.6 GeV and pseudorapidity vertical bar eta(Jet)vertical bar < 2.8. The transverse momentum balance given by the jet-to-photon p(T) ratio, x(j gamma), is measured for pairs with azimuthal opening angle Delta phi > 7 pi/8. Distributions of the per-photon jet yield as a function of x(j gamma), (1/N-gamma)(dN/dx(j gamma)), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the x(j gamma) distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.
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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 prompt photon production in root s(NN)=8.16 TeV p Pb collisions with ATLAS. Phys. Lett. B, 796, 230–252.
Abstract: The inclusive production rates of isolated, prompt photons in p Pb collisions at root s(NN) = 8.16 TeV are studied with the ATLAS detector at the Large Hadron Collider using a dataset with an integrated luminosity of 165 nb(-1) recorded in 2016. The cross-section and nuclear modification factor R-p pb are measured as a function of photon transverse energy from 20 GeV to 550 GeV and in three nucleon-nucleon centre-of-mass pseudorapidity regions, (-2.83, -2.02), (-1.84, 0.91), and (1.09, 1.90). The cross-section and R-p pb values are compared with the results of a next-to-leading-order perturbative QCD calculation, with and without nuclear parton distribution function modifications, and with expectations based on a model of the energy loss of partons prior to the hard scattering. The data disfavour a large amount of energy loss and provide new constraints on the parton densities in nuclei.
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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 quarkonium production in proton-lead and proton-proton collisions at 5.02 TeV with the ATLAS detector. Eur. Phys. J. C, 78(3), 171–32pp.
Abstract: The modification of the production of J/psi, psi(2S), and gamma (nS) (n = 1, 2, 3) in p+Pb collisions with respect to their production in pp collisions has been studied. The p+Pb and pp datasets used in this paper correspond to integrated luminosities of 28 nb(-1) and 25 pb(-1) respectively, collected in 2013 and 2015 by the ATLAS detector at the LHC, both at a centre-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of J/psi and psi(2S) are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor, R-pPb for J/psi and psi (nS). No significant modification of the J/psi production is observed while gamma (nS) production is found to be suppressed at low transverse momentum in p+Pb collisions relative to pp collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central p+Pb collisions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2021). Measurement of single top-quark production in association with a W boson in the single-lepton channel at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 81(8), 720–29pp.
Abstract: The production cross-section of a top quark in association with a W boson is measured using proton-proton collisions at root s = 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb(-1,) and was collected in 2012 by the ATLAS detector at the Large Hadron Collider at CERN. The analysis is performed in the single-lepton channel. Events are selected by requiring one isolated lepton (electron or muon) and at least three jets. A neural network is trained to separate the tW signal from the dominant t (t) over bar background. The cross-section is extracted from a binned profile maximum-likelihood fit to a two-dimensional discriminant built from the neural-network output and the invariant mass of the hadronically decaying W boson. The measured crosssection is sigma(tW) = 26 +/- 7 pb, in good agreement with the Standard Model expectation.
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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). Measurement of single top-quark production in the s-channel in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 191–47pp.
Abstract: A measurement of single top-quark production in the s-channel is performed in proton-proton collisions at a centre-of-mass energy of 13TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb(-1). The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two b-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is sigma = 8.2(-2.9)(+3.5) pb, consistent with the Standard Model prediction of sigma(SM) = 10.32(-0.36)(+0.40) pb.
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