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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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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). Properties of g -> b(b)over-bar at small opening angles in pp collisions with the ATLAS detector at root s=13 TeV. Phys. Rev. D, 99(5), 052004–26pp.
Abstract: The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to b-quark pairs is a unique probe into the properties of gluon fragmentation because identified b-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the g -> b (b) over bar process are measured using 33 fb(-1) of root s = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-k(t) jet algorithm with radius parameter R = 0.2, are used to probe angular scales below the R = 0.4 jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into b-quarks.
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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), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2020). Performance of the missing transverse momentum triggers for the ATLAS detector during Run-2 data taking. J. High Energy Phys., 08(8), 080–53pp.
Abstract: The factor of four increase in the LHC luminosity, from 0.5x10(34)cm(-2)s(-1) to 2.0x10(34)cm(-2)s(-1), and the corresponding increase in pile-up collisions during the 2015-2018 data-taking period, presented a challenge for the ATLAS trigger, particularly for those algorithms that select events with missing transverse momentum. The output data rate at fixed threshold typically increases exponentially with the number of pile-up collisions, so the legacy algorithms from previous LHC data-taking periods had to be tuned and new approaches developed to maintain the high trigger efficiency achieved in earlier operations. A study of the trigger performance and comparisons with simulations show that these changes resulted in event selection efficiencies of >98% for this period, meeting and in some cases exceeding the performance of similar triggers in earlier run periods, while at the same time keeping the necessary bandwidth within acceptable limits.
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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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