Search for Dark Matter Produced in Association with a Dark Higgs Boson Decaying into W±W∓ or ZZ in Fully Hadronic Final States from sqrt(s) = 13 TeV pp Collisions Recorded with the ATLAS Detector

none12siSeveral extensions of the Standard Model predict the production of dark matter particles at the LHC. An uncharted signature of dark matter particles produced in association with VV=W±Wa"or ZZ pairs from a decay of a dark Higgs boson s is searched for using 139 fb-1 of pp collisions recorded by the ATLAS detector at a center-of-mass energy of 13 TeV. The s→V(qq)V(qq) decays are reconstructed with a novel technique aimed at resolving the dense topology from boosted VV pairs using jets in the calorimeter and tracking information. Dark Higgs scenarios with ms>160 GeV are excluded.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Performance of the upgraded PreProcessor of the ATLAS Level-1 Calorimeter Trigger

none13siThe 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.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, A Mirto, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Mirto, A; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Search for heavy resonances decaying to a photon and a hadronically decaying Z /W /H boson in pp collisions at sqrt(s) =13 TeV with the ATLAS detector

none12siMany extensions of the Standard Model predict new resonances decaying to a Z, W, or Higgs boson and a photon. This paper presents a search for such resonances produced in pp collisions at sqrt(s) = 13 TeV using a data set with an integrated luminosity of 36.1 fb-1 collected by the ATLAS detector at the LHC. The Z/W/H bosons are identified through their decays to hadrons. The data are found to be consistent with the Standard Model expectation in the entire investigated mass range. Upper limits are set on the production cross section times branching fraction for resonance decays to Z/W+γ in the mass range from 1.0 to 6.8 TeV and for the first time into H+γ in the mass range from 1.0 to 3.0 TeV.openM Aaboud, K Bachas, G Chiodini, E Gorini, L Longo, A Mirto, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAaboud, M; Bachas, K; Chiodini, G; Gorini, E; Longo, L; Mirto, A; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Evidence for electroweak production of two jets in association with a Zγ pair in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

none13siEvidence for electroweak production of two jets in association with a Zγ pair in sqrt(s) = 13 TeV proton–proton collisions at the Large Hadron Collider is presented. The analysis uses data collected by the ATLAS detector in 2015 and 2016 that corresponds to an integrated luminosity of 36.1fb−1. Events that contain a Z boson candidate decaying leptonically into either e+e− or μ+μ−, a photon, and two jets are selected. The electroweak component is measured with observed and expected significances of 4.1 standard deviations. The fiducial cross-section for electroweak production is measured to be σZγjj−EW=7.8±2.0fb, in good agreement with the Standard Model prediction.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, A Mirto, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Mirto, A; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Evidence for Higgs boson decays to a low-mass dilepton system and a photon in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

none12siA search for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass mℓℓ<30 GeV is presented. The analysis is performed using 139 fb−1 of proton–proton collision data, produced by the LHC at a centre-of-mass energy of 13 TeV and collected by the ATLAS experiment. Evidence for the H→ℓℓγ process is found with a significance of 3.2 over the background-only hypothesis, compared to an expected significance of 2.1 for the Standard Model prediction. The best-fit value of the signal-strength parameter, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is μ=1.5±0.5. The Higgs boson production cross-section times the H→ℓℓγ branching ratio for mℓℓ<30 GeV is determined to be 8.7−2.7+2.8 fb.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector

none12siA search for the dimuon decay of the Standard Model (SM) Higgs boson is performed using data corresponding to an integrated luminosity of 139 fb(-1) collected with the ATLAS detector in Run 2 pp collisions at sqrt(s) = 13 TeV at the Large Hadron Collider. The observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV is 2.0 sigma (1.7 sigma). The observed upper limit on the cross section times branching ratio for pp -> H -> mu mu is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a H -> mu mu signal assuming the absence (presence) of a SM signal is 1.1(2.0). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is mu = 1.2 +/- 0.6.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Measurement of the tt¯ production cross-section in the lepton+jets channel at sqrt(s) = 13 TeV with the ATLAS experiment

none12siThe 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 sqrt(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 tt¯ production cross-section through a profile-likelihood fit. The inclusive cross-section is measured to be σinc=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 tt¯ cross-section within the experimental acceptance is also measured.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Probing the Quantum Interference between Singly and Doubly Resonant Top-Quark Production in pp Collisions at sqrt(s) = 13 TeV with the ATLAS Detector

none13siThis Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a W boson and a b-quark are significant. Events with exactly two leptons (ee, μμ, or eμ) and two b-tagged jets that satisfy a multiparticle invariant mass requirement are selected from 36.1 fb-1 of proton-proton collision data taken at sqrt(s) = 13 TeV with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within 2σ of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.openM Aaboud, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, A Mirto, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAaboud, M; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Mirto, A; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Search for new phenomena in final states with large jet multiplicities and missing transverse momentum using sqrt(s) = 13 TeV proton-proton collisions recorded by ATLAS in Run 2 of the LHC

none13siResults 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 sqrt(s) = 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.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, A Mirto, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Mirto, A; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An

Measurements of Higgs bosons decaying to bottom quarks from vector boson fusion production with the ATLAS experiment at sqrt(s) = 13 TeV

none12siThe paper presents a measurement of the Standard Model Higgs Boson decaying to b-quark pairs in the vector boson fusion (VBF) production mode. A sample corresponding to 126 fb - 1 of sqrt(s) = 13 TeV proton–proton collision data, collected with the ATLAS experiment at the Large Hadron Collider, is analyzed utilizing an adversarial neural network for event classification. The signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model for VBF Higgs production, is measured to be 0.95-0.36+0.38 , corresponding to an observed (expected) significance of 2.6 (2.8) standard deviations from the background only hypothesis. The results are additionally combined with an analysis of Higgs bosons decaying to b-quarks, produced via VBF in association with a photon.openG Aad, K Bachas, G Chiodini, E Gorini, F Gravili, L Longo, M Primavera, M Reale, E Schioppa, S Spagnolo, A Ventura, and ATLAS CollaborationAad, G; Bachas, K; Chiodini, G; Gorini, E; Gravili, F; Longo, L; Primavera, M; Reale, M; Schioppa, E; Spagnolo, S; Ventura, A; ATLAS Collaboration, An