17 research outputs found
Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC pp collision data at root s=7 and 8 TeV
70 pages plus author lists + cover page (104 pages total), 32 figures, 22 tables, submitted to JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2015-07/ and at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIG-15-002/Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a or a boson or a pair of top quarks, and of the six decay modes , , and . All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton--proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb at TeV and 20 fb at TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the decay of and standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.Peer reviewe
Search for single production of vector-like quarks decaying into in collisions at 8 TeV with the ATLAS detector
See paper for full list of authors - 24 pages plus author list (40 pages total), 8 figures, 3 tables, submitted to EPJC, all figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2014-12/International audienceA search for singly produced vector-like quarks, where can be either a quark with charge or a quark with charge , is performed in proton-proton collisions recorded with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb and was produced with a centre-of-mass energy of TeV. This analysis targets decays where the boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant background. The reconstructed -candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on the cross-section times branching ratio. The results are also interpreted as limits on the coupling and the mixing with the Standard Model sector for a singlet quark or a quark from a doublet. quarks with masses below 0.95 TeV are excluded at 95% confidence level, assuming a unit coupling and a BR, whereas the expected limit is 1.10 TeV
Measurement of internal structure of jets in Pb+Pb collisions at TeV with the ATLAS detector at the LHC
41 pages in total, author list starting page 25, 9 figures, 1 table, submitted to EPJC, All figures including auxiliary figures are available at this URL https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2015-05/ See paper for full list of authorsInternational audienceThe distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb of Pb+Pb data and 4.0 pb of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets
Measurements of top-quark pair differential cross-sections in the channel in collisions at TeV using the ATLAS detector
See paper for full list of authors - 43 pages in total, author list starting page 26, 5 figures, 5 tables, submitted to EPJC, all figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2016-04/International audienceThis article presents measurements of differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb of proton--proton data at a centre-of-mass energy of TeV recorded by the ATLAS experiment at the LHC in 2015. Differential cross-sections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the system. The events are selected by requiring one electron and one muon of opposite electric charge, and at least two jets, one of which must be tagged as containing a -hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the POWHEG-Box + HERWIG++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the system
Measurement of the boson polarisation in events from collisions at = 8 TeV in the lepton+jets channel with ATLAS
See paper for full list of authors - 42 pages in total, author list starting page 26, 10 figures, 5 tables, submitted to EPJC, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2016-02/International audienceThis paper presents a measurement of the polarisation of bosons from decays, reconstructed in events with one high-p_{\mbox{T}} lepton and at least four jets. Data from collisions at the LHC were collected at = 8 TeV and correspond to an integrated luminosity of 20.2 fb. The angle between the -quark from the top quark decay and a direct boson decay product in the boson rest frame is sensitive to the boson polarisation. Two different decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying boson, respectively. The most precise measurement of the boson polarisation via the distribution of is obtained using the leptonic analyser and events in which at least two of the jets are tagged as -quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are , and , and are the most precisely measured boson polarisation fractions to date. Limits on anomalous couplings of the vertex are set
Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in TeV pp collisions with the ATLAS detector
See paper for full list of authors - 46 pages plus author list (63 pages total), 15 figures, 11 tables, submitted to EPJC. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2016-05International audienceTwo searches for new phenomena in final states containing a same-flavour opposite-lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton--proton collision data, collected during 2015 and 2016 at a centre-of-mass energy TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an integrated luminosity of 14.7 fb. Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g. ), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 TeV (980 GeV)
Measurements of top-quark pair differential cross-sections in the lepton+jets channel in collisions at TeV using the ATLAS detector
See paper for full list of authors - 44 pages plus author list (61 pages total), 18 figures, 6 tables, submitted to Eur. Phys. J. C, all figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2015-06/International audienceMeasurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of TeV. The observables have been chosen to emphasize the production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a -quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. The agreement for this observable improves when next-to-next-to-leading order corrections are taken into account. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used