14 research outputs found

    Search for single vector-like B quark production and decay via B → bH(b¯b) in pp collisions at √s = 13 TeV with the ATLAS detector

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    A search is presented for single production of a vector-like B quark decaying into a Standard Model b-quark and a Standard Model Higgs boson, which decays into a b¯b pair. The search is carried out in 139 fb−1 of √s = 13 TeV proton-proton collision data collected by the ATLAS detector at the LHC between 2015 and 2018. No significant deviation from the Standard Model background prediction is observed, and mass-dependent exclusion limits at the 95% confidence level are set on the resonance production cross-section in several theoretical scenarios determined by the couplings cW, cZ and cH between the B quark and the Standard Model W, Z and Higgs bosons, respectively. For a vector-like B occurring as an isospin singlet, the search excludes values of cW greater than 0.45 for a B resonance mass (mB) between 1.0 and 1.2 TeV. For 1.2 TeV < mB < 2.0 TeV, cW values larger than 0.50–0.65 are excluded. If the B occurs as part of a (B, Y) doublet, the smallest excluded cZ coupling values range between 0.3 and 0.5 across the investigated resonance mass range 1.0 TeV < mB < 2.0 TeV

    Saturation of Tungsten Surfaces with Hydrogen: A Density Functional Theory Study Complemented by Low Energy Ion Scattering and Direct Recoil Spectroscopy Data

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    International audienceHerein, we investigate the saturation limits of hydrogen on the (110) and (100) surfaces of tungsten via Density Functional Theory (DFT) and complement our findings with experimental measurements. We present a detailed study of the various stable configurations that hydrogen can adopt upon the surfaces at coverage ratios starting below 1.0, up to the point of their experimental coverage ratios, and beyond. Our findings allow us to estimate that the saturation limit on each surface exists with one monolayer of hydrogen atoms adsorbed. In the case of (110) this corresponds to a coverage ratio of one hydrogen atom per tungsten atom, while in the case of (100) a full monolayer is present at a coverage ratio of 2.0. Preliminary Low Energy Ion Scattering (LEIS) and Direct Recoil Spectroscopy (DRS) measurements complement these results and tend to confirm the findings obtained by DFT. In particular, the preferred adsorption sites on both surfaces at any coverage, the reconstruction of the (100) surface and the saturation limits agree well. We show that depending on the coverage, hydrogen surface binding energies can be of the same magnitude as binding energies to defects like vacancies. As a consequence, surface effects should be included in models aiming to simulate retention and desorption of hydrogen from the bulk

    Hydrogen and oxygen on tungsten (110) surface: adsorption, absorption and desorption investigated by density functional theory

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    International audienceIn this work we investigated the adsorption of oxygen and the co-adsorption of oxygen and hydrogen on the (110) surface of tungsten by means of Density Functional calculations. The absorption, recombination and release mechanisms of hydrogen across the (110) surface with oxygen are further established at saturation and above saturation of the surface. It is found that hydrogen and oxygen both adsorb preferentially at three-fold sites. The saturation limit was determined to one monolayer in adsorbate. Oxygen is found to lower the binding energy of hydrogen on the surface and to lower the activation barrier for the recombination of molecular hydrogen. Finally, as on the clean surface, oversaturation in adsorbate is shown to lower both activation barriers for hydrogen absorption and for molecular hydrogen recombination on the (110) surface of tungsten

    Measurement of the nuclear modification factor for muons from charm and bottom hadrons in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

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    Heavy-flavour hadron production provides information about the transport properties and microscopic structure of the quark–gluon plasma created in ultra-relativistic heavy-ion collisions. A measurement of the muons from semileptonic decays of charm and bottom hadrons produced in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV with the ATLAS detector at the Large Hadron Collider is presented. The Pb+Pb data were collected in 2015 and 2018 with sampled integrated luminosities of 208ÎŒb−1 and 38ÎŒb−1, respectively, and pp data with a sampled integrated luminosity of 1.17pb−1 were collected in 2017. Muons from heavy-flavour semileptonic decays are separated from the light-flavour hadronic background using the momentum imbalance between the inner detector and muon spectrometer measurements, and muons originating from charm and bottom decays are further separated via the muon track's transverse impact parameter. Differential yields in Pb+Pb collisions and differential cross sections in pp collisions for such muons are measured as a function of muon transverse momentum from 4 GeV to 30 GeV in the absolute pseudorapidity interval |η|<2. Nuclear modification factors for charm and bottom muons are presented as a function of muon transverse momentum in intervals of Pb+Pb collision centrality. The bottom muon results are the most precise measurement of b quark nuclear modification at low transverse momentum where reconstruction of B hadrons is challenging. The measured nuclear modification factors quantify a significant suppression of the yields of muons from decays of charm and bottom hadrons, with stronger effects for muons from charm hadron decays

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

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    A 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 ÎŒ=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

    Search for heavy, long-lived, charged particles with large ionisation energy loss in pppp collisions at s=13 TeV\sqrt{s} = 13~\text{TeV} using the ATLAS experiment and the full Run 2 dataset

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    International audienceThis paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb−1^{−1} of proton–proton collisions at s \sqrt{s} = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to O \mathcal{O} (1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.[graphic not available: see fulltext

    Search for new phenomena in three- or four-lepton events in pp collisions at root s=13 TeV with the ATLAS detector

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    We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Ar-menia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbai-jan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; ANID, Chile; CAS, MOST and NSFC, China; Min-ciencias, Colombia; MSMT CR, MPO CR and VSC CR, Czech Repub-lic; DNRF and DNSRC, Denmark; IN2P3-CNRS and CEA-DRF/IRFU, France; SRNSFG, Georgia; BMBF, HGF and MPG, Germany; GSRI, Greece; RGC and Hong Kong SAR, China; ISF and Benoziyo Cen-ter, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; RCN, Norway; MEiN, Poland; FCT, Portugal; MNE/IFA, Romania; JINR; MES of Russia and NRC KI, Russian Fed-eration; MESTD, Serbia; MSSR, Slovakia; ARRS and MIZĆ , Slovenia; DSI/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Founda-tion, Sweden; SERI, SNSF and Cantons of Bern and Geneva, Switzer-land; MOST, Taiwan; TAEK, Turkey; STFC, United Kingdom; DOE and NSF, United States of America. In addition, individual groups and members have received support from BCKDF, CANARIE, Com-pute Canada and CRC, Canada; COST, ERC, ERDF, Horizon 2020 and Marie SkƂodowska-Curie Actions, European Union; Investisse-ments d’Avenir Labex, Investissements d’Avenir Idex and ANR, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Greek NSRF, Greece; BSF-NSF and GIF, Israel; Norwegian Financial Mecha-nism 2014-2021, Norway; NCN and NAWA, Poland; La Caixa Bank-ing Foundation, CERCA Programme Generalitat de Catalunya and PROMETEO and GenT Programmes Generalitat Valenciana, Spain; Göran Gustafssons Stiftelse, Sweden; The Royal Society and Lever-hulme Trust, United Kingdom. The crucial computing support from all WLCG partners is ac-knowledged gratefully, in particular from CERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA), the Tier-2 facilities worldwide and large non-WLCG resource providers. Major contributors of computing resources are listed in Ref.[91].A search with minimal model dependence for physics beyond the Standard Model in events featuring three or four charged leptons (3l and 4l, l = e, mu) is presented. The analysis aims to be sensitive to a wide range of potential new-physics theories simultaneously. This analysis uses data from pp collisions delivered by the Large Hadron Collider at a centre-of-mass energy of root s = 13 TeV and recorded with the ATLAS detector, corresponding to the full Run 2 dataset of 139 fb(-1). The 3l and 4l phase space is divided into 22 event categories according to the number of leptons in the event, the missing transverse momentum, the invariant mass of the leptons, and the presence of leptons originating from a Z-boson candidate. These event categories are analysed independently for the presence of deviations from the Standard Model. No statistically significant deviations from the Standard Model predictions are observed. Upper limits for all signal regions are reported in terms of the visible cross-section.ANPCyT, ArgentinaYerPhI, ArmeniaARC, AustraliaBMWFW and FWF, AustriaANAS, AzerbaijanSSTC, BelarusCNPq and FAPESP, BrazilNSERC, NRC and CFI, CanadaCERNANID, ChileCAS, MOST and NSFC, ChinaMinciencias, ColombiaMSMT CR, MPO CR and VSC CR, Czech RepublicDNRF and DNSRC, DenmarkIN2P3-CNRS and CEA-DRF/IRFU, FranceSRNSFG, GeorgiaBMBF, HGF and MPG, GermanyGSRI, GreeceRGC and Hong Kong SAR, ChinaISF and Benoziyo Center, IsraelINFN, ItalyMEXT and JSPS, JapanCNRST, MoroccoNWO, NetherlandsRCN, NorwayMEiN, PolandFCT, PortugalMNE/IFA, RomaniaJINRMES of Russia and NRC KI, Russian FederationMESTD, SerbiaMSSR, SlovakiaARRS and MIZĆ , SloveniaDSI/NRF, South AfricaMICINN, SpainSRC and Wallenberg Foundation, SwedenSERI, SNSF and Cantons of Bern and Geneva, SwitzerlandMOST, TaiwanTAEK, TurkeySTFC, United KingdomDOE and NSF, United States of AmericaBCKDF, CANARIE, Compute Canada and CRC, CanadaCOST, ERC, ERDF, Horizon 2020 and Marie SkƂodowska-Curie Actions, European UnionInvestissements d’Avenir Labex, Investissements d’Avenir Idex and ANR, FranceDFG and AvH Foundation, GermanyHerakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Greek NSRF, GreeceBSF-NSF and GIF, IsraelNorwegian Financial Mechanism 2014-2021, NorwayNCN and NAWA, PolandLa Caixa Banking Foundation, CERCA Programme Generalitat de Catalunya and PROMETEO and GenT Programmes Generalitat Valenciana, SpainGöran Gustafssons Stiftelse, SwedenThe Royal Society and Leverhulme Trust, United Kingdo

    Search for new phenomena in final states with photons, jets and missing transverse momentum in pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

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    International audienceA search for new phenomena has been performed in final states with at least one isolated high-momentum photon, jets and missing transverse momentum in proton–proton collisions at a centre-of-mass energy of s \sqrt{s} = 13 TeV. The data, collected by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb−1^{−1}. The experimental results are interpreted in a supersymmetric model in which pair-produced gluinos decay into neutralinos, which in turn decay into a gravitino, at least one photon, and jets. No significant deviations from the predictions of the Standard Model are observed. Upper limits are set on the visible cross section due to physics beyond the Standard Model, and lower limits are set on the masses of the gluinos and neutralinos, all at 95% confidence level. Visible cross sections greater than 0.022 fb are excluded and pair-produced gluinos with masses up to 2200 GeV are excluded for most of the NLSP masses investigated.[graphic not available: see fulltext
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