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). Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS. Phys. Rev. Lett., 123(4), 042001–21pp.
Abstract: Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb(-1) of pp and 0.49 nb(-1) of Pb + Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb + Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb + Pb events.
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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). Search for scalar resonances decaying into mu(+)mu(-) in events with and without b-tagged jets produced in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 117–41pp.
Abstract: A search for a narrow scalar resonance decaying into an opposite-sign muon pair produced in events with and without b-tagged jets is presented in this paper. The search uses 36.1 fb(-1) of =13 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. No significant excess of events above the expected Standard Model background is observed in the investigated mass range of 0.2 to 1.0 TeV. The observed upper limits at 95% confidence level on the cross section times branching ratio for b-quark associated production and gluon-gluon fusion are between 1.9 and 41 fb and 1.6 and 44 fb respectively, which is consistent with expectations.
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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). Search for low-mass resonances decaying into two jets and produced in association with a photon using pp collisions root s=13 TeV with the ATLAS detector. Phys. Lett. B, 795, 56–75.
Abstract: A search is performed for localised excesses in dijet mass distributions of low-dijet-mass events produced in association with a high transverse energy photon. The search uses up to 79.8 fb(-1) of LHC proton-proton collisions collected by the ATLAS experiment at a centre-of-mass energy of 13 TeV during 2015-2017. Two variants are presented: one which makes no jet flavour requirements and one which requires both jets to be tagged as b-jets. The observed mass distributions are consistent with multi-jet processes in the Standard Model. The data are used to set upper limits on the production cross-section for a benchmark Z' model and, separately, on generic Gaussian-shape contributions to the mass distributions, extending the current ATLAS constraints on dijet resonances to the mass range between 225 and 1100 GeV.
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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). Search for chargino and neutralino production in final states with a Higgs boson and missing transverse momentum at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 100(1), 012006–37pp.
Abstract: A search is conducted for the electroweak pair production of a chargino and a neutralino pp -> (chi) over tilde (+/-)(1)(chi) over tilde (0)(2), where the chargino decays into the lightest neutralino and a W boson, (chi) over tilde (+/-)(1) -> (chi) over tilde W-0(1)+/- while the neutralino decays into the lightest neutralino and a Standard Model-like 125 GeV Higgs boson,(chi) over tilde (0)(2) -> (chi) over tilde (0)(1)h. Fully hadronic, semileptonic, diphoton, and multilepton (electrons, muons) final. states with missing transverse momentum are considered in this search. Higgs bosons in the final state are identified by either two jets originating from bottom quarks (h -> b (b) over bar), two photons (h -> gamma gamma), or leptons from the decay modes h -> WW, h -> ZZ or h -> tau tau. The analysis is based on 36.1 fb(-1) of s root s = 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. Observations are consistent with the Standard Model expectations, and 95% confidence-level limits of up to 680 GeV in (chi) over tilde (+/-)(1)/(chi) over tilde (0)(2) mass are set in the context of a simplified supersymmetric model.
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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). 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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