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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2020). Observation and Measurement of Forward Proton Scattering in Association with Lepton Pairs Produced via the Photon Fusion Mechanism at ATLAS. Phys. Rev. Lett., 125(26), 261801–21pp.
Abstract: The observation of forward proton scattering in association with lepton pairs (e(+)e(-) + p or mu(+)mu(-) + p) produced via photon fusion is presented. The scattered proton is detected by the ATLAS Forward Proton spectrometer, while the leptons are reconstructed by the central ATLAS detector. Proton-proton collision data recorded in 2017 at a center-of-mass energy of root s = 13 TeV are analyzed, corresponding to an integrated luminosity of 14.6 fb(-1). A total of 57 (123) candidates in the ee + p (mu μ+ p) final state arc selected, allowing the background-only hypothesis to be rejected with a significance exceeding 5 standard deviations in each channel. Proton-tagging techniques are introduced for cross-section measurements in the fiducial detector acceptance, corresponding to sigma(ee)(+p) = 11.0 +/- 2.6(stat) 1.2(syst) +/- 0.3(lumi) and sigma(mu)(mu+)(p) = 7.2 +/- 1.6(stat) +/- 0.9(syst) 0.2(lumi) fb in the dielectron and dimuon channel, respectively.
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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). Resolution of the ATLAS muon spectrometer monitored drift tubes in LHC Run 2. J. Instrum., 14, P09011–35pp.
Abstract: The momentum measurement capability of the ATLAS muon spectrometer relies fundamentally on the intrinsic single-hit spatial resolution of the monitored drift tube precision tracking chambers. Optimal resolution is achieved with a dedicated calibration program that addresses the specific operating conditions of the 354 000 high-pressure drift tubes in the spectrometer. The calibrations consist of a set of timing offsets and drift time to drift distance transfer relations, and result in chamber resolution functions. This paper describes novel algorithms to obtain precision calibrations from data collected by ATLAS in LHC Run 2 and from a gas monitoring chamber, deployed in a dedicated gas facility. The algorithm output consists of a pair of correction constants per chamber which are applied to baseline calibrations, and determined to be valid for the entire ATLAS Run 2. The final single-hit spatial resolution, averaged over 1172 monitored drift tube chambers, is 81.7 +/- 2.2 μm.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2020). Determination of jet calibration and energy resolution in proton-proton collisions at s=8 TeV using the ATLAS detector. Eur. Phys. J. C, 80(12), 1104–81pp.
Abstract: The jet energy scale, jet energy resolution, and their systematic uncertainties are measured for jets reconstructed with the ATLAS detector in 2012 using proton-proton data produced at a centre-of-mass energy of 8 TeV with an integrated luminosity of 20 fb-1. Jets are reconstructed from clusters of energy depositions in the ATLAS calorimeters using the anti-kt algorithm. A jet calibration scheme is applied in multiple steps, each addressing specific effects including mitigation of contributions from additional proton-proton collisions, loss of energy in dead material, calorimeter non-compensation, angular biases and other global jet effects. The final calibration step uses several in situ techniques and corrects for residual effects not captured by the initial calibration. These analyses measure both the jet energy scale and resolution by exploiting the transverse momentum balance in gamma + jet, Z + jet, dijet, and multijet events. A statistical combination of these measurements is performed. In the central detector region, the derived calibration has a precision better than 1% for jets with transverse momentum 150 GeV<pT< 1500 GeV, and the relative energy resolution is (8.4 +/- 0.6)% for pT=100 GeV and (23 +/- 2)% for pT=20 GeV. The calibration scheme for jets with radius parameter R=1.0, for which jets receive a dedicated calibration of the jet mass, is also discussed.
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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 the jet mass in high transverse momentum Z(-> b(b)over-bar)gamma production at root s=13 TeV using the ATLAS detector. Phys. Lett. B, 812, 135991–23pp.
Abstract: The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z -> b (b) over bar decays are measured in Z gamma events in proton-proton collisions at root s = 13 TeV. The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb(-1). Photons are required to have a transverse momentum p(T) > 175 GeV. The Z -> b (b) over bar decay is reconstructed using a jet with p(T) > 200 GeV, found with the anti-k(t) R = 1.0 jet algorithm, and groomed to remove soft and wide-angle radiation and to mitigate contributions from the underlying event and additional proton-proton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the Z -> b (b) over bar decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify Zbosons, b-tagged R = 0.2 track-jets matched to the groomed large-R calorimeter jet are used as a proxy for the b-quarks. The signal yield is determined from fits of the data-driven background templates to the different jet mass distributions for the two grooming methods. Integrated fiducial cross-sections and unfolded jet mass spectra for each grooming method are compared with leading-order theoretical predictions. The results are found to be in good agreement with Standard Model expectations within the current statistical and systematic 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 ttbar production cross-section and lepton differential distributions in e mu dilepton events from pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(6), 528–70pp.
Abstract: The inclusive top quark pair (tt<overbar></mml:mover>) production cross-section sigma tt<overbar></mml:mover> has been measured in proton-proton collisions at <mml:msqrt>s</mml:msqrt>=13<mml:mspace width=“0.166667em”></mml:mspace>TeV, using 36.1 fb-1 of data collected in 2015-2016 by the ATLAS experiment at the LHC. Using events with an opposite-charge e μpair and b-tagged jets, the cross-section is measured to be: <disp-formula id=“Equ10”><mml:mtable><mml:mtr><mml:mtd columnalign=“right”>sigma tt<overbar></mml:mover>=826.4 +/- 3.6<mml:mspace width=“0.166667em”></mml:mspace>(stat)<mml:mspace width=“4pt”></mml:mspace>+/- 11.5<mml:mspace width=“0.166667em”></mml:mspace>(syst)<mml:mspace width=“4pt”></mml:mspace>+/- 15.7<mml:mspace width=“0.166667em”></mml:mspace>(lumi)<mml:mspace width=“4pt”></mml:mspace>+/- 1.9<mml:mspace width=“0.166667em”></mml:mspace>(beam)<mml:mspace width=“0.166667em”></mml:mspace>pb,</mml:mtd></mml:mtr></mml:mtable><graphic xmlns:xlink=“http://www.w3.org/1999/xlink” xlink:href=“1005220207907ArticleEqu10.gif” position=“anchor”></graphic></disp-formula>where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on mtpole, giving mtpole=173.1-2.1+2.0<mml:mspace width=“0.166667em”></mml:mspace>GeV. It is also combined with measurements at <mml:msqrt>s</mml:msqrt><mml:mo>=7<mml:mspace width=“0.166667em”></mml:mspace>TeV and <mml:msqrt>s</mml:msqrt><mml:mo>=8<mml:mspace width=“0.166667em”></mml:mspace>TeV to derive ratios and double ratios of t<mml:mover accent=“true”>t<mml:mo stretchy=“false”><overbar></mml:mover> and Z cross-sections at different energies. The same event sample is used to measure absolute and normalised differential cross-sections as functions of single-lepton and dilepton kinematic variables, and the results are compared with predictions from various Monte Carlo event generators.
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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., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Study of the hard double-parton scattering contribution to inclusive four-lepton production in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 790, 595–614.
Abstract: The inclusive production of four isolated charged leptons in pp collisions is analysed for the presence of hard double-parton scattering, using 20.2 fb(-1) of data recorded in the ATLAS detector at the LHC at centre-of-mass energy root s = 8 TeV. In the four-lepton invariant-mass range of 80 < m(4l) < 1000 GeV, an artificial neural network is used to enhance the separation between single- and double-parton scattering based on the kinematics of the four leptons in the final state. An upper limit on the fraction of events originating from double-parton scattering is determined at 95% confidence level to be f(DPS) = 0.042, which results in an estimated lower limit on the effective cross section at 95% confidence level of 1.0 mb.
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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 inclusive cross-section for the production of jets in association with a Z boson in proton-proton collisions at 8 TeV using the ATLAS detector. Eur. Phys. J. C, 79(10), 847–47pp.
Abstract: The inclusive cross-section for jet production in association with a Z boson decaying into an electronpositron pair is measured as a function of the transverse momentum and the absolute rapidity of jets using 19.9 fb(-1) of root s = 8 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The measured Z + jets cross-section is unfolded to the particle level. The cross-section is compared with state-of-the-art Standard Model calculations, including the next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations, corrected for non-perturbative and QED radiation effects. The results of the measurements cover final-state jets with transverse momenta up to 1 TeV, and show good agreement with fixed-order calculations.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2021). Measurement of b-quark fragmentation properties in jets using the decay B-+/- -> J/psi K-+/- in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 12(12), 131–46pp.
Abstract: The fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139fb(-1) of pp collisions at root s = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B-+/- into J/psi K-+/-, with the J/psi decaying into a pair of muons. Jets are reconstructed using the anti-k(t) algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets.
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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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