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). Observation of Light-by-Light Scattering in Ultraperipheral Pb plus Pb Collisions with the ATLAS Detector. Phys. Rev. Lett., 123(5), 052001–21pp.
Abstract: This Letter describes the observation of the light-by-light scattering process, gamma gamma -> gamma gamma, in Pb + Pb collisions at root S-NN = 5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb(-1), collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E-T(gamma) > 3 GeV and pseudorapidity vertical bar eta(gamma)vertical bar < 2.4, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 +/- 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 +/- 13(stat) +/- 7(syst) +/- 3(lumi) nb.
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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). Search for high-mass dilepton resonances using 139 fb(-1) of pp collision data collected at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 796, 68–87.
Abstract: A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-ofmass energy of root s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb(-1). A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E-6-motivated Z(psi)' boson. Also presented are limits on Heavy Vector Triplet model couplings.
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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). Evidence for the production of three massive vector bosons with the ATLAS detector. Phys. Lett. B, 798, 134913–22pp.
Abstract: A search for the production of three massive vector bosons in proton-proton collisions is performed using data at root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in the years 2015-2017, corresponding to an integrated luminosity of 79.8 fb(-1). Events with two same-sign leptons l (electrons or muons) and at least two reconstructed jets are selected to search for WWW -> l nu l nu qq. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for WWW > l nu l nu l nu, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for WWZ -> l nu qqll. Finally, events with four leptons are analysed to search for WWZ > l nu l nu ll and WZZ > qqllll. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.1 standard deviations, where the expectation is 3.1 standard deviations.
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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. (2022). Two-particle Bose-Einstein correlations in pp collisions at root s=13 TeV measured with the ATLAS detector at the LHC. Eur. Phys. J. C, 82(7), 608–38pp.
Abstract: This paper presents studies of Bose-Einstein correlations (BEC) in proton-proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 μb(-1) and 8.4 nb(-1), respectively. The BEC are measured for pairs of like-sign charged particles, each with vertical bar eta vertical bar < 2.5, for two kinematic ranges: the first with particle pr > 100 MeV and the second with particle pr > 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.
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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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