189 research outputs found

    Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in √s=8 TeV pp collisions with the ATLAS detector

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    A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb⁻¹ of LHC proton-proton collision data at √s=8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be −0.8 ± 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis

    Search for pair production of Higgs bosons in the bbbb final state using proton--proton collisions at √s = 13 TeV with the ATLAS detector

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    A search for Higgs-boson pair production in the bbbb final state is carried out with 3.2 fb⁻¹ of proton--proton collision data collected at √s=13 TeV with the ATLAS detector. The data are consistent with the estimated background and are used to set upper limits on the production cross section of Higgs-boson pairs times branching ratio to bbbb for both nonresonant and resonant production. In the case of resonant production of Kaluza--Klein gravitons within the Randall--Sundrum model, upper limits in the 24 to 91 fb range are obtained for masses between 600 and 3000 GeV, at the 95% confidence level. The production cross section times branching ratio for nonresonant Higgs-boson pairs is also constrained to be less than 1.22 pb, at the 95% confidence level

    Measurement of the bb dijet cross section in pp collisions at √s = 7 TeV with the ATLAS detector

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    The dijet production cross section for jets containing a b-hadron (b-jets) has been measured in proton–proton collisions with a centre-of-mass energy of √s =7 TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of 4.2fb⁻¹. The cross section is measured for events with two identified b-jets with a transverse momentum pT>20GeV and a minimum separation in the η–ϕ plane of ΔR=0.4. At least one of the jets in the event is required to have p T >270 GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b-jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization

    Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in √s=8 TeV proton-proton collisions

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    A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use 20fb⁻¹ of proton-proton collision data at √s=8 TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino--higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass m(χ0/1) of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger m(χ0/1) due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small

    Measurement of the transverse momentum and φ *η distributions of Drell–Yan lepton pairs in proton–proton collisions at √s=8 TeV with the ATLAS detector

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    Distributions of transverse momentum p ll T and the angular variable ϕ ∗ η of Drell--Yan lepton pairs are measured in 20.3 fb⁻¹ of proton--proton collisions at √s=8 TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton--proton collisions at √s=7 TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of ϕ ∗ η <1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of ϕ ∗ η this is not generally the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of p ll T and the fixed-order prediction of DYNNLO falls below the data at high values of p ll T. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the ϕ ∗ η and p ll T distributions as a function of lepton-pair mass and rapidity

    Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at √s=8 TeV using the ATLAS detector

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    Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tt system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of √s=8 TeV. The observables have been chosen to emphasize the tt 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⁻¹, 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 b-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. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. 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

    Search for charged Higgs bosons in the H± → tb decay channel in pp collisions at √s=8 TeV using the ATLAS detector

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    Charged Higgs bosons heavier than the top quark and decaying via H ± →tb are searched for in proton--proton collisions measured with the ATLAS experiment at √s=8 TeV corresponding to an integrated luminosity of 20.3 fb⁻¹. The production of a charged Higgs boson in association with a top quark, gb→tH ±, is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background-only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the gb→tH ± production cross section times the branching fraction BR(H ± →tb) . Additionally, the complementary s-channel production, qq ′ →H±, is investigated through a reinterpretation of W ′ →tb searches in ATLAS. Final states with one electron or muon are relevant for H ± masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the qq ′ →H ± production cross section times the branching fraction BR(H ± →tb)

    Search for single production of vector-like quarks decaying into Wb in pp collisions at √s = 8 TeV with the ATLAS detector

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    A search for singly produced vector-like Q quarks, where Q can be either a T quark with charge +2/3 or a Y quark with charge −4/3 , 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⁻¹ and was produced with a centre-of-mass energy of √s=8 TeV. This analysis targets Q→Wb decays where the W boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant tt ¯ background. The reconstructed Q -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 Q→Wb cross-section times branching ratio. The results are also interpreted as limits on the QWb coupling and the mixing with the Standard Model sector for a singlet T quark or a Y quark from a doublet. T quarks with masses below 0.95 TeV are excluded at 95% confidence level, assuming a unit coupling and a BR(T→Wb)=0.5 , whereas the expected limit is 1.10 TeV

    A search for prompt lepton-jets in pp collisions at √s= 8 TeV with the ATLAS detector

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    A search is presented for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb⁻¹ of data collected by the ATLAS detector at the Large Hadron Collider in proton--proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets
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