986 research outputs found
Some bounds for the spectral radius of a coxeter transformation
Let âł be finite quiver (=oriented, connected graph) without oriented cycles. Let k be any field. The path albebra k[âł] is a hereditary algebra, see [7]. The study of this kind of algebras had played a central role in the development of the Representation Theory of Algebras, ..
Search for vector-boson resonances decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at s=13 TeV with the ATLAS detector
A search for new charged massive gauge bosons, W, is performed with the ATLAS detector at the LHC. Data were collected in protonâproton collisions at a center-of-mass energy of s=13 TeV and correspond to an integrated luminosity of 36.1 fb. This analysis searches for W bosons in the WâtbÂŻ decay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0 TeV and considers right-handed W bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the WâtbÂŻ cross section times branching ratio and the W boson effective couplings as a function of the W boson mass. For right-handed W bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15 TeV are excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for WâtbÂŻ in the fully hadronic final state. Using the combined searches, right-handed W bosons with masses below 3.25 TeV are excluded at the 95% confidence level.Peer Reviewe
Measurement of W±Z production cross sections and gauge boson polarisation in pp collisions at âs=13TeV with the ATLAS detector
This paper presents measurements of WZ production cross sections in pp collisions at a centre-of-mass energy of 13 TeV. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 36.1fb-1. The WZ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is ÏW±ZâââČÎœââfid.=63.7±1.0(stat.)±2.3(syst.)±1.4(lumi.) fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of 61.5-1.3+1.4 fb. Cross sections for WZ and WZ production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF and DNSRC, Denmark; IN2P3-CNRS, CEA-DRF/IRFU, France; SRNSFG, Georgia; BMBF, HGF, and MPG, Germany; GSRT, Greece; RGC, Hong Kong SAR, China; ISF and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; RCN, Norway; MNiSW and NCN, Poland; FCT, Portugal; MNE/IFA, Romania; MES of Russia and NRC KI, Russian Federation; JINR; MESTD, Serbia; MSSR, Slovakia; ARRS and MIZĆ , Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SERI, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, UK; DOE and NSF, USA. In addition, individual groups and members have received support from BCKDF, CANARIE, CRC and Compute Canada, Canada; COST, ERC, ERDF, Horizon 2020, and Marie SkĆodowska-Curie Actions, European Union; Investissements dâ Avenir Labex and Idex, 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; CERCA Programme Generalitat de Catalunya, Spain; The Royal Society and Leverhulme Trust, UK. The crucial computing support from all WLCG partners is acknowledged 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. [106].Peer Reviewe
Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in pp collisions at s =13 TeV with the ATLAS detector
A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of s=13 TeV corresponding to an integrated luminosity of 36.1 fb-1. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical ZâČ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the ZâČ boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1-3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.Peer Reviewe
Search for invisible Higgs boson decays in vector boson fusion at s=13TeV with the ATLAS detector
We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1)TeV and O(100)GeV missing transverse momentum. The analysis uses 36.1 fb of pp collision data at s=13TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on the WIMPânucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3â1.7pb.Peer Reviewe
Search for the Higgs boson decays HâŻââŻee and HâŻââŻeÎŒ in pp collisions at s=13TeV with the ATLAS detector
Searches for the Higgs boson decays Hâee and HâeÎŒ are performed using data corresponding to an integrated luminosity of 139fb collected with the ATLAS detector in pp collisions at s=13 TeV at the LHC. No significant signals are observed, in agreement with the Standard Model expectation. For a Higgs boson mass of 125 GeV, the observed (expected) upper limit at the 95% confidence level on the branching fraction B(Hâee) is 3.6Ă10 (3.5Ă10) and on B(HâeÎŒ) is 6.2Ă10 (5.9Ă10). These results represent improvements by factors of about five and six on the previous best limits on B(Hâee) and B(HâeÎŒ) respectively
Measurement of the t t Z and t t W cross sections in proton-proton collisions at s =13 TeV with the ATLAS detector
A measurement of the associated production of a top-quark pair (tt) with a vector boson (W, Z) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using 36.1 fb-1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The ttZ and ttW production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are ÏttZ=0.95±0.08stat±0.10syst pb and ÏttW=0.87±0.13stat±0.14syst pb in agreement with the Standard Model predictions. The measurement of the ttZ cross section is used to set constraints on effective field theory operators which modify the ttZ vertex.Peer Reviewe
Performance of electron and photon triggers in ATLAS during LHC Run 2
Electron and photon triggers covering transverse energies from 5Â GeV to several TeV are essential for the ATLAS experiment to record signals for a wide variety of physics: from Standard Model processes to searches for new phenomena in both protonâproton and heavy-ion collisions. To cope with a fourfold increase of peak LHC luminosity from 2015 to 2018 (Run 2), to 2.1Ă1034cm-2s-1, and a similar increase in the number of interactions per beam-crossing to about 60, trigger algorithms and selections were optimised to control the rates while retaining a high efficiency for physics analyses. For protonâproton collisions, the single-electron trigger efficiency relative to a single-electron offline selection is at least 75% for an offline electron of 31Â GeV, and rises to 96% at 60Â GeV; the trigger efficiency of a 25Â GeV leg of the primary diphoton trigger relative to a tight offline photon selection is more than 96% for an offline photon of 30Â GeV. For heavy-ion collisions, the primary electron and photon trigger efficiencies relative to the corresponding standard offline selections are at least 84% and 95%, respectively, at 5Â GeV above the corresponding trigger threshold
Search for the Production of a Long-Lived Neutral Particle Decaying within the ATLAS Hadronic Calorimeter in Association with a Z Boson from pp Collisions at s =13 TeV
This Letter presents a search for the production of a long-lived neutral particle (Zd) decaying within the ATLAS hadronic calorimeter, in association with a standard model (SM) Z boson produced via an intermediate scalar boson, where Zâ+ (=e, ÎŒ). The data used were collected by the ATLAS detector during 2015 and 2016 pp collisions with a center-of-mass energy of s=13 TeV at the Large Hadron Collider and correspond to an integrated luminosity of 36.1±0.8 fb-1. No significant excess of events is observed above the expected background. Limits on the production cross section of the scalar boson times its decay branching fraction into the long-lived neutral particle are derived as a function of the mass of the intermediate scalar boson, the mass of the long-lived neutral particle, and its cÏ from a few centimeters to one hundred meters. In the case that the intermediate scalar boson is the SM Higgs boson, its decay branching fraction to a long-lived neutral particle with a cÏ approximately between 0.1 and 7 m is excluded with a 95% confidence level up to 10% for mZd between 5 and 15 GeV
Measurement of azimuthal anisotropy of muons from charm and bottom hadrons in pp collisions at âs = 13 TeV with the ATLAS detector
The elliptic flow of muons from the decay of charm and bottom hadrons is measured in pp collisions at âs = 13 TeV using a data sample with an integrated luminosity of 150 pb recorded by the ATLAS detector at the LHC. The muons from heavy-flavor decay are separated from light-hadron decay muons using momentum imbalance between the tracking and muon spectrometers. The heavy-flavor decay muons are further separated into those from charm decay and those from bottom decay using the distance-of closest-approach to the collision vertex. The measurement is performed for muons in the transverse momentum range 4-7 GeV and pseudorapidity range |η| < 2.4. A significant nonzero elliptic anisotropy coefficient Ξ2 is observed for muons from charm decays, while the Ξ2 value for muons from bottom decays is consistent with zero within uncertainties
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