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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). Search for t(t)over-bar resonances in fully hadronic final states in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 061–43pp.
Abstract: This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb(-1) of proton-proton collision data recorded at a centre-of-mass energy of root s = 13TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a t (t) over bar pair with mass above 1.4TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new Z' boson in a topcolor-assisted-technicolor model. The Z0 boson masses below 3.9 and 4.7TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively.
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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). Search for pairs of scalar leptoquarks decaying into quarks and electrons or muons in root s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 10(10), 112–45pp.
Abstract: A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced inppcollisions at root s = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb(-1), corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation ofc- orb-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.
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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). Search for new phenomena in final states with large jet multiplicities and missing transverse momentum using root s=13 TeV proton-proton collisions recorded by ATLAS in Run 2 of the LHC. J. High Energy Phys., 10(10), 062–53pp.
Abstract: Results of a search for new particles decaying into eight or more jets and moderate missing transverse momentum are presented. The analysis uses 139 fb(-1) of proton-proton collision data at <mml:msqrt>s</mml:msqrt> = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The selection rejects events containing isolated electrons or muons, and makes requirements according to the number of b-tagged jets and the scalar sum of masses of large-radius jets. The search extends previous analyses both in using a larger dataset and by employing improved jet and missing transverse momentum reconstruction methods which more cleanly separate signal from background processes. No evidence for physics beyond the Standard Model is found. The results are interpreted in the context of supersymmetry-inspired simplified models, significantly extending the limits on the gluino mass in those models. In particular, limits on the gluino mass are set at 2 TeV when the lightest neutralino is nearly massless in a model assuming a two-step cascade decay via the lightest chargino and second-lightest neutralino.
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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). Measurement of the t(t)over-bar production cross-section in the lepton plus jets channel at root s=13 TeV with the ATLAS experiment. Phys. Lett. B, 810, 135797–22pp.
Abstract: The top anti-top quark production cross-section is measured in the lepton+jets channel using proton-proton collision data at a centre-of-mass energy of root s = 13 TeV collected with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 139 fb(-1). Events with exactly one charged lepton and four or more jets in the final state, with at least one jet containing b-hadrons, are used to determine the a production cross-section through a profile-likelihood t (t) over bar. The inclusive cross-section is measured to be alpha(inc) = 830 +/- 0.4 (stat.) +/- 36 (syst.) +/- 14 (lumi.) pb with a relative uncertainty of 4.6%. The result is consistent with theoretical calculations at next-to-next-to-leading order in perturbative QCD. The fiducial t (t) over bar cross-section within the experimental acceptance is also measured.
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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). Higgs boson production cross-section measurements and their EFT interpretation in the 4l decay channel at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(10), 957–54pp.
Abstract: Higgs boson properties are studied in the fourlepton decay channel (where lepton = e, mu) using 139 fb(-1) of proton-proton collision data recorded at v s =13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio for H -> ZZ * decay is measured to be 1.34 +/- 0.12 pb for a Higgs boson with absolute rapidity below 2.5, in good agreement with the Standard Model prediction of 1.33 +/- 0.08 pb. Crosssections times branching ratio are measured for the main Higgs boson production modes in several exclusive phasespace regions. The measurements are interpreted in terms of coupling modifiers and of the tensor structure of Higgs boson interactions using an effective field theory approach. Exclusion limits are set on the CP-even and CP-odd beyond the Standard Model couplings of the Higgs boson to vector bosons, gluons and top 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. (2020). Search for new non-resonant phenomena in high-mass dilepton final states with the ATLAS detector. J. High Energy Phys., 11(11), 005–41pp.
Abstract: A search for new physics with non-resonant signals in dielectron and dimuon final states in the mass range above 2 TeV is presented. This is the first search for non-resonant signals in dilepton final states at the LHC to use a background estimate from the data. The data, corresponding to an integrated luminosity of 139 fb(-1), were recorded by the ATLAS experiment in proton-proton collisions at a center-of-mass energy of root s= 13 TeV during Run 2 of the Large Hadron Collider. The benchmark signal signature is a two-quark and two-lepton contact interaction, which would enhance the dilepton event rate at the TeV mass scale. To model the contribution from background processes a functional form is fit to the dilepton invariant-mass spectra in data in a mass region below the region of interest. It is then extrapolated to a high-mass signal region to obtain the expected background there. No significant deviation from the expected background is observed in the data. Upper limits at 95% CL on the number of events and the visible cross-section times branching fraction for processes involving new physics are provided. Observed (expected) 95% CL lower limits on the contact interaction energy scale reach 35.8 (37.6) TeV.
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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. (2020). A search for the Z gamma decay mode of the Higgs boson in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 809, 135754–21pp.
Abstract: A search for the Z gamma decay of the Higgs boson, with Z boson decays into pairs of electrons or muons is presented. The analysis uses proton-proton collision data at root s = 13 TeV corresponding to an integrated luminosity of 139 fb(-1) recorded by the ATLAS detector at the Large Hadron Collider. The observed data are consistent with the expected background with a p-value of 1.3%. An upper limit at 95% confidence level on the production cross-section times the branching ratio for pp -> H -> Z gamma is set at 3.6 times the Standard Model prediction while 2.6 times is expected in the presence of the Standard Model Higgs boson. The best-fit value for the signal yield normalised to the Standard Model prediction is 2.0(-0.9)(+1.0) where the statistical component of the uncertainty is dominant.
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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). Search for Higgs Boson Decays into a Z Boson and a Light Hadronically Decaying Resonance Using 13 TeV pp Collision Data from the ATLAS Detector. Phys. Rev. Lett., 125(22), 221802–22pp.
Abstract: A search for Higgs boson decays into a Z boson and a light resonance in two-lepton plus jet events is performed, using a pp collision dataset with an integrated luminosity of 139 fb(-1) collected at root s = 13 TeV by the ATLAS experiment at the CERN LHC. The resonance considered is a light boson with a mass below 4 GeV from a possible extended scalar sector or a charmonium state. Multivariate discriminants are used for the event selection and for evaluating the mass of the light resonance. No excess of events above the expected background is found. Observed (expected) 95% confidence-level upper limits are set on the Riggs boson production cross section times branching fraction to a Z boson and the signal resonance, with values in the range 17-340 pb (16(-5)(+6)-320(-90)(+130) pb) for the different light spin-0 boson mass and branching fraction hypotheses, and with values of 110 and 100 pb (100(-30)(+40) and 100(-30)(+40) pb) for the eta(c) and J/psi hypotheses, respectively.
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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 upgraded PreProcessor of the ATLAS Level-1 Calorimeter Trigger. J. Instrum., 15(11), P11016–48pp.
Abstract: The PreProcessor of the ATLAS Level-1 Calorimeter Trigger prepares the analogue trigger signals sent from the ATLAS calorimeters by digitising, synchronising, and calibrating them to reconstruct transverse energy deposits, which are then used in further processing to identify event features. During the first long shutdown of the LHC from 2013 to 2014, the central components of the PreProcessor, the Multichip Modules, were replaced by upgraded versions that feature modern ADC and FPGA technology to ensure optimal performance in the high pile-up environment of LHC Run 2. This paper describes the features of the new Multichip Modules along with the improvements to the signal processing achieved.
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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). Search for top squarks in events with a Higgs or Z boson using 139 fb(-1) of pp collision data at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(11), 1080–33pp.
Abstract: This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton-proton collision data at root s = 13 TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb(-1). No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark ((t) over tilde (1)) or the heavier top squark ((t) over tilde (2)), excluding at 95% confidence level (t) over tilde (1) and (t) over tilde2 masses up to about 1220 and 875 GeV, respectively.
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