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Alio, L.

Publications and source records attributed to Alio, L..

At least 217 records · Page 12
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DOE OSTI2014

Measurement of the parity-violating asymmetry parameter α b and the helicity amplitudes for the decay Λ b 0 → J / ψ Λ 0 with the ATLAS detector

A measurement of the parity-violating decay asymmetry parameter, α b , and the helicity amplitudes for the decay Λ$ ^0_b $→J/ψ(μ + μ - )Λ 0 (pπ - ) is reported. The analysis is based on 1400 Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ baryons selected in 4.6 fb -1 of proton-proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ samples under the assumption of CP conservation, the value of α b is measured to be 0.30 ± 0.16(stat) ± 0.06(syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS

A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the standard model expectation is observed in 4.5 fb -1 (20.3 fb -1 ) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the standard model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of an upper limit on the allowed dark matter-nucleon scattering cross section within a Higgs-portal dark matter scenario. Within the constraints of such a scenario, the results presented in this Letter provide the strongest available limits for low-mass dark matter candidates. Limits are also set on an additional neutral Higgs boson, in the mass range 110 < m H < 400 GeV, produced in association with a Z boson and decaying to invisible particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the production of a W boson in association with a charm quark in pp collisions at $ \sqrt{s} $ = 7 TeV with the ATLAS detector

The production of a W boson in association with a single charm quark is studied using 4.6 fb -1 of pp collision data at $ \sqrt{s} $ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96$ ^{+0.26}_{-0.30} $ at Q 2 = 1.9 GeV 2 , which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally the cross-section ratio σ(W + + $ \bar{c} $)/σ(W - +c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s–$ \bar{s} $ quark asymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in $pp$ collisions at $ \sqrt{s} $ = 8TeV with the ATLAS detector

Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb -1 of proton-proton collision data at $ \sqrt{s} $ = 8TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of dijet cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector

Double-differential dijet cross-sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and half the rapidity separation of the two highest-p T jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5 fb -1 , recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross-sections are presented at the particle level. Cross-sections are measured up to 5TeV dijet mass using jets reconstructed with the anti- k t algorithm for values of the jet radius parameter of 0.4 and 0.6. The cross-sections are compared with next-to-leading-order perturbative QCD calculations by NLOJet++ corrected to account for non-perturbative effects. Comparisons with POWHEG predictions, using a next-to-leading-order matrix element calculation interfaced to a partonshower Monte Carlo simulation, are also shown. Electroweak effects are accounted for in both cases. The quantitative comparison of data and theoretical predictions obtained using various parameterizations of the parton distribution functions is performed using a frequentist method. In general, good agreement with data is observed for the NLOJet++ theoretical predictions when using the CT10, NNPDF2.1 and MSTW 2008 PDF sets. Disagreement is observed when using the ABM11 and HERAPDF1.5 PDF sets for some ranges of dijet mass and half the rapidity separation. An example setting a lower limit on the compositeness scale for a model of contact interactions is presented, showing that the unfolded results can be used to constrain contributions to dijet production beyond that predicted by the Standard Model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in $ \sqrt{s} $ = 8 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb -1 of $ \sqrt{s} $ = 8TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R -parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (&sim;$ \tilde{X} ^± _1 $) and next-to-lightest neutralino ($ \tilde{X} ^0 _2 $) production with decays to lightest neutralino (&sim;$ \tilde{X} ^0 _1 $) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, $ \tilde{X} ^± _1 $ and $ \tilde{X} ^0 _2 $ masses are excluded up to 700GeV, 380GeV, 345GeV, or 148GeV respectively, for a massless $ \tilde{X} ^0 _1 $.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at $ \sqrt{s} $ = 7 TeV with the ATLAS detector

The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb -1 of $ \sqrt{s} $ = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → μν μ and J/ψ → μ + μ - . A yield of 27.4$ ^{+7.5} _{-6.5} $ W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at $ \sqrt{s} $ = 8 TeV using the ATLAS detector

Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 fb -1 of proton-proton collision data collected at a centre-of-mass energy of √s = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5σ level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the Sherpa and Powheg event generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Quantum Black Hole Production in High-Invariant-Mass Lepton + Jet Final States Using p p Collisions at s = 8 TeV and the ATLAS Detector

This Letter presents a search for quantum black-hole production using 20.3 fb -1 of data collected with the ATLAS detector in $pp$ collisions at the LHC at $ \sqrt{s}$ = 8 TeV. The quantum black holes are assumed to decay into a final state characterized by a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton + jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the top quark pair production charge asymmetry in proton-proton collisions at $ \sqrt{s} $ = 7 TeV using the ATLAS detector

This paper presents a measurement of the top quark pair ($ t\bar{t} $) production charge asymmetry AC using 4.7 fb -1 of proton-proton collisions at a centre-of-mass energy $ \sqrt{s} $ = 7 TeV collected by the ATLAS detector at the LHC. A $ t\bar{t} $-enriched sample of events with a single lepton (electron or muon), missing transverse momentum and at least four high transverse momentum jets, of which at least one is tagged as coming from a b-quark, is selected. A likelihood fit is used to reconstruct the $ t\bar{t} $ event kinematics. A Bayesian unfolding procedure is employed to estimate AC at the parton-level. The measured value of the $ t\bar{t} $ production charge asymmetry is AC = 0.006 ± 0.010, where the uncertainty includes both the statistical and the systematic components. Differential AC measurements as a function of the invariant mass, the rapidity and the transverse momentum of the $ t\bar{t} $ system are also presented. In addition, AC is measured for a subset of events with large $ t\bar{t} $ velocity, where physics beyond the Standard Model could contribute. All measurements are consistent with the Standard Model predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for a multi-Higgs-boson cascade in W + W - b b ¯ events with the ATLAS detector in p p collisions at s = 8 TeV

A search is presented for new particles in an extension to the Standard Model that includes a heavy Higgs boson (H 0 ), an intermediate charged Higgs-boson pair ( H &plusmn; ), and a light Higgs boson (h 0 ). The analysis searches for events involving the production of a single heavy neutral Higgs boson which decays to the charged Higgs boson and a W boson, where the charged Higgs boson subsequently decays into a W boson and the lightest neutral Higgs boson decaying to a bottom-antibottom-quark pair. Such a cascade results in a W -boson pair and a bottom-antibottom-quark pair in the final state. Events with exactly one lepton, missing transverse momentum, and at least four jets are selected from a data sample corresponding to an integrated luminosity of 20.3 fb -1 , collected by the ATLAS detector in proton-proton collisions at $ \sqrt{s}$ = 8 TeV at the LHC. The data are found to be consistent with Standard Model predictions, and 95% confidence-level upper limits are set on the product of cross section and branching ratio. Lastly, these limits range from 0.065 to 43 pb as a function of H 0 and H &plusmn; masses, with m h 0 fixed at 125 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Dark Matter in Events with a Hadronically Decaying W or Z Boson and Missing Transverse Momentum in p p Collisions at s = 8 TeV with the ATLAS Detector

A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb -1 of integrated luminosity at $ \sqrt{s} $ = 8 TeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗