406 Search Results

This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton–proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb^–1 are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standardmore » Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.« less

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 fb^–1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sectionsmore » are measured to be 521 ± 9(stat.) ± 41(sys.) fb and 69 ± 3(stat.) ± 4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.« less

It has been found that the theoretical predictions for W and Z boson cross sections, and for the W boson charge asymmetry, which are labelled as NNPDF3.1 [1] have in fact been calculated using the NNPDF3.0 PDF set [2] instead.

The performance of identification algorithms (“taggers”) for hadronically decaying top quarks and W bosons in pp collisions at $$\sqrt{s}$$ = 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. Furthermore, for highly boosted top-quark tagging, a deep neuralmore » network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is introduced. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1 fb^–1 for the $$t\overline{t}$$ and γ+jet and 36.7 fb ^–1 for the dijet event topologies.« less

A study of the decays B_s⁰ → μ⁺μ^– and B⁰ → μ⁺μ^– has been performed using 26.3 fb^–1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in μ⁺μ^– invariant mass is comparable to the B_s⁰ -B⁰ mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B_s⁰ → μ⁺μ^–) = (3.2^+1.1_–1.0) × 10^–9 and an upper limit B(B⁰ → μ⁺μ^–) < 4.3×10^–10 at 95% confidence level. The result is combined with the Run 1 ATLAS result,more » yielding B(B_s⁰ → μ⁺μ^–) = (2.8^+0.8_–0.7) × 10^–9 and B(B⁰ → μ⁺μ^–) < 2.1 × 10^–10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B⁰ → μ⁺μ ^–) – B(B⁰_s → μ⁺μ^–) plane.« less

A search for Higgs boson pair production in the $$ b\overline{b}W{W}^{*} $$ decay mode is performed in the $$ b\overline{b}$$ℓνqq final state using 36.1 fb^–1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of events beyond the background expectation is found. Upper limits on the non-resonant pp → HH production cross section of 10 pb and on the resonant production cross section as a function of the HH invariant mass are obtained. Resonant production limits are set for scalar and spin-2 graviton hypotheses in themore » mass range 500 to 3000 GeV.« less

A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb^–1 of proton-proton collisions at √s = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single Z boson production, Higgs boson production, and Z boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with themore » data. These calculations are used to interpret the data in terms of gg → ZZ → 4ℓ and Z → 4ℓ subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.« less