11 research outputs found
Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in √s=13 Te pp collisions with the ATLAS detector
Get PDFA search for new phenomena in final states containing an e+e-or μ+μ-pair, jets, and large missing transverse momentum is presented. This analysis makes use of proton–proton collision data with an integrated luminosity of 36.1fb-1, collected during 2015 and 2016 at a centre-of-mass energy s=13TeV with the ATLAS detector at the Large Hadron Collider. The search targets the pair production of supersymmetric coloured particles (squarks or gluinos) and their decays into final states containing an e+e-or μ+μ-pair and the lightest neutralino (χ~10) via one of two next-to-lightest neutralino (χ~20) decay mechanisms: χ~20→Zχ~10, where the Z boson decays leptonically leading to a peak in the dilepton invariant mass distribution around the Z boson mass; and χ~20→l+l-χ~10 with no intermediate l+l-resonance, 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 using simplified models, and exclude gluinos and squarks with masses as large as 1.85 and 1.3 Te at 95% confidence level, respectively
Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at s√=8 TeV with the ATLAS detector
Get PDFSee paper for full list of authors - 27 pages plus author list + cover pages (45 pages total), 8 figures, 4 tables, submitted to Physical Review D, All figures including auxiliary figures are available at this http URLInternational audienceMeasurements of the top--antitop quark pair production charge asymmetry in the dilepton channel are presented using data corresponding to an integrated luminosity of 20.3 fb−1 from pp collisions at a center-of-mass energy of s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the tt¯ system are performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: AℓℓC based on the selected leptons and Att¯C based on the reconstructed tt¯ final state. The inclusive asymmetries are measured in the full phase space to be AℓℓC=0.008±0.006 and Att¯C=0.021±0.016, which are in agreement with the Standard Model predictions of AℓℓC=0.0064±0.0003 and Att¯C=0.0111±0.0004
Search for scalar leptoquarks in pp collisions at s√=13 TeV with the ATLAS experiment
Get PDFSee paper for full list authors -25 pages + author list (45 pages in total) , 7 figures and 4 tables, submitted to New Journal of Physics, all figures including auxiliary figures are available at this https URLInternational audienceAn inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in pp collisions at s√ = 13 TeV at the Large Hadron Collider have been considered. An integrated luminosity of 3.2 fb−1, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed yield in each channel is consistent with Standard Model background expectations. The observed (expected) leptoquark mass limits at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for the higher range of accessible leptoquark masses, and the observed exclusion limits confirm and extend the published results
Performance of top-quark and W -boson tagging with ATLAS in Run 2 of the LHC
Get PDFThe performance of identification algorithms (“taggers”) for hadronically decaying top quarks and W bosons in pp collisions at s = 13 TeV recorded by the ATLAS experiment at the Large Hadron Collider is presented. A set of techniques based on jet shape observables are studied to determine a set of optimal cut-based taggers for use in physics analyses. The studies are extended to assess the utility of combinations of substructure observables as a multivariate tagger using boosted decision trees or deep neural networks in comparison with taggers based on two-variable combinations. In addition, for highly boosted top-quark tagging, a deep neural network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is presented. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1 fb - 1 for the tt¯ and γ+ jet and 36.7 fb - 1 for the dijet event topologies. © 2019, CERN for the benefit of the ATLAS collaboration
Search for doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector
Get PDFA search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton-proton collisions corresponding to an integrated luminosity of 36.1fb-1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H +/-+/-. Those bosons are produced in pairs in proton-proton collisions and decay predominantly into electroweak gauge bosons H +/-+/- W +/- W +/-. Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H +/-+/- bosons with masses between 200 and 220 GeV.<p>For complete list of authors see http://dx.doi.org/10.1140/epjc/s10052-018-6500-y</p
Measurements of W and Z boson production in pp collisions at √s=5.02 TeV with the ATLAS detector
Get PDFMeasurements of fiducial integrated and differential cross sections for
inclusive , and boson production are reported. They are based on
pb of collision data at TeV
collected with the ATLAS detector at the CERN Large Hadron Collider. Electron
and muon decay channels are analysed, and the combined , and
integrated cross sections are found to be = 2266 9 (stat)
29 (syst) 43 (lumi) pb, = 1401 7 (stat)
18 (syst) 27 (lumi) pb, and = 374.5 3.4 (stat) 3.6
(syst) 7.0 (lumi) pb, in good agreement with
next-to-next-to-leading-order QCD cross-section calculations. These
measurements serve as references for Pb+Pb interactions at the LHC at this
nucleon--nucleon centre-of-mass energy
In situ calibration of large-radius jet energy and mass in 13 TeV proton–proton collisions with the ATLAS detector
Get PDFThe response of the ATLAS detector to large-radius jets is measured in situ using 36.2 fb - 1 of s=13 TeV proton–proton collisions provided by the LHC and recorded by the ATLAS experiment during 2015 and 2016. The jet energy scale is measured in events where the jet recoils against a reference object, which can be either a calibrated photon, a reconstructed Z boson, or a system of well-measured small-radius jets. The jet energy resolution and a calibration of forward jets are derived using dijet balance measurements. The jet mass response is measured with two methods: using mass peaks formed by W bosons and top quarks with large transverse momenta and by comparing the jet mass measured using the energy deposited in the calorimeter with that using the momenta of charged-particle tracks. The transverse momentum and mass responses in simulations are found to be about 2–3% higher than in data. This difference is adjusted for with a correction factor. The results of the different methods are combined to yield a calibration over a large range of transverse momenta (p T ). The precision of the relative jet energy scale is 1–2% for 200GeV<pT<2TeV, while that of the mass scale is 2–10%. The ratio of the energy resolutions in data and simulation is measured to a precision of 10–15% over the same p T range.</p
A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment
Get PDFThis paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo-experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a statistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2 fb- 1 of proton–proton collision data at a centre-of-mass energy of 13 TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 10 5 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined
Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run 2 data collected in 2015 and 2016
Get PDFThe efficiency of the photon identification criteria in the ATLAS detector is measured using 36.1 fb1 to
36.7 fb1 of pp collision data at √s = 13 TeV collected in
2015 and 2016. The efficiencies are measured separately for
converted and unconverted isolated photons, in four different pseudorapidity regions, for transverse momenta between
10 GeV and 1.5 TeV. The results from the combination of
three data-driven techniques are compared with the predictions from simulation after correcting the variables describing the shape of electromagnetic showers in simulation for
the average differences observed relative to data. Data-tosimulation efficiency ratios are determined to account for
the small residual efficiency differences. These factors are
measured with uncertainties between 0.5% and 5% depending on the photon transverse momentum and pseudorapidity.
The impact of the isolation criteria on the photon identification efficiency, and that of additional soft pp interactions, are
also discussed. The probability of reconstructing an electron
as a photon candidate is measured in data, and compared with
the predictions from simulation. The efficiency of the reconstruction of photon conversions is measured using a sample
of photon candidates from Z → μμγ events, exploiting the
properties of the ratio of the energies deposited in the first
and second longitudinal layers of the ATLAS electromagnetic calorimeter
Measurement of the top quark mass in the tt¯→ lepton+jets channel from √s=8 TeV ATLAS data and combination with previous results
Get PDFThe top quark mass is measured using a template method in the tt¯→ lepton+jets channel (lepton is e or μ) using ATLAS data recorded in 2012 at the LHC. The data were taken at a proton-proton centre-of-mass energy of √s=8 TeV and correspond to an integrated luminosity of 20.2 fb−1. The tt¯→ lepton+jets channel is characterized by the presence of a charged lepton, a neutrino and four jets, two of which originate from bottom quarks (b). Exploiting a three-dimensional template technique, the top quark mass is determined together with a global jet energy scale factor and a relative b-to-light-jet energy scale factor. The mass of the top quark is measured to be mtop=172.08±0.39(stat)±0.82(syst) GeV. A combination with previous ATLAS mtop measurements gives mtop=172.69±0.25(stat)±0.41(syst) GeV
