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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 1,009 records · Page 56Linked to original sources

arXiv2016

Evidence for single top-quark production in the $s$-channel in proton-proton collisions at $\sqrt{s}=$8 TeV with the ATLAS detector using the Matrix Element Method

This Letter presents evidence for single top-quark production in the $s$-channel using proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two $b$-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb$^{-1}$. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark $s$-channel events from the main background contributions, which are top-quark pair production and $W$ boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of $σ_s=4.8\pm0.8$(stat.)$^{+1.6}_{-1.3}$(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.

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arXiv2016

Identification of boosted, hadronically decaying W bosons and comparisons with ATLAS data taken at $\sqrt{s} = 8$ TeV

This paper reports a detailed study of techniques for identifying boosted, hadronically decaying $W$ bosons using 20fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy $\sqrt{s} =$ 8 TeV. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50% efficiency for identifying $W$ bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2-4% for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.

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arXiv2016

Search for new phenomena with photon+jet events in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search is performed for the production of high-mass resonances decaying into a photon and a jet in 3.2 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV collected by the ATLAS detector at the Large Hadron Collider. Selected events have an isolated photon and a jet, each with transverse momentum above 150 GeV. No significant deviation of the $γ+$jet invariant mass distribution from the background-only hypothesis is found. Limits are set at 95% confidence level on the cross sections of generic Gaussian-shaped signals and of a few benchmark phenomena beyond the Standard Model: excited quarks with vector-like couplings to the Standard Model particles, and non-thermal quantum black holes in two models of extra spatial dimensions. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 2% decrease from about 6 fb for a mass of 1.5 TeV to about 0.8 fb for a mass of 5 TeV. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 15% decrease from about 50 fb for a mass of 1.5 TeV to about 1.0 fb for a mass of 5 TeV. Excited quarks are excluded below masses of 4.4 TeV, and non-thermal quantum black holes are excluded below masses of 3.8 (6.2) TeV for Randall--Sundrum (Arkani-Hamed--Dimopoulous--Dvali) models with one (six) extra dimensions.

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arXiv2016

Search for strong gravity in multijet final states produced in pp collisions at $\sqrt{s} =$ 13 TeV using the ATLAS detector at the LHC

A search is conducted for new physics in multijet final states using 3.6 inverse femtobarns of data from proton-proton collisions at $\sqrt{s}$ = 13 TeV taken at the CERN Large Hadron Collider with the ATLAS detector. Events are selected containing at least three jets with scalar sum of jet transverse momenta ($H_T$) greater than 1 TeV. No excess is seen at large $H_T$ and limits are presented on new physics: models which produce final states containing at least three jets and having cross sections larger than 1.6 fb with $H_T$ > 5.8 TeV are excluded. Limits are also given in terms of new physics models of strong gravity that hypothesize additional space-time dimensions.

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arXiv2016

Search for the electroweak production of supersymmetric particles in $\sqrt{s}$=8 TeV $pp$ collisions with the ATLAS detector

The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross-sections. Results are based on 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

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arXiv2016

Search for Dark Matter in Events with Missing Transverse Momentum and a Higgs Boson Decaying to Two Photons in $pp$ Collisions at $\sqrt{s}=8$ TeV with the ATLAS Detector

Results of a search for new phenomena in events with large missing transverse momentum and a Higgs boson decaying to two photons are reported. Data from proton-proton collisions at a center-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb$^{-1}$ have been collected with the ATLAS detector at the LHC. The observed data are well described by the expected Standard Model backgrounds. Upper limits on the cross section of events with large missing transverse momentum and a Higgs boson candidate are also placed. Exclusion limits are presented for models of physics beyond the Standard Model featuring dark-matter candidates.

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arXiv2016

Search for direct top squark pair production in final states with two tau leptons in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95% confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 GeV and 650 GeV are placed on the top squark mass within the model considered.

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arXiv2016

Measurement of the dependence of transverse energy production at large pseudorapidity on the hard-scattering kinematics of proton-proton collisions at $\sqrt{s} = 2.76$ TeV with ATLAS

The relationship between jet production in the central region and the underlying-event activity in a pseudorapidity-separated region is studied in 4.0 pb$^{-1}$ of $\sqrt{s} = 2.76$ TeV $pp$ collision data recorded with the ATLAS detector at the LHC. The underlying event is characterised through measurements of the average value of the sum of the transverse energy at large pseudorapidity downstream of one of the protons, which are reported here as a function of hard-scattering kinematic variables. The hard scattering is characterised by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. The dijet kinematics are used to estimate, on an event-by-event basis, the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons moving toward and away from the region measuring transverse energy, respectively. Transverse energy production at large pseudorapidity is observed to decrease with a linear dependence on the longitudinal momentum fraction in the target proton and to depend only weakly on that in the projectile proton. The results are compared to the predictions of various Monte Carlo event generators, which qualitatively reproduce the trends observed in data but generally underpredict the overall level of transverse energy at forward pseudorapidity.

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arXiv2016

Measurements of $W^\pm Z$ production cross sections in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector and limits on anomalous gauge boson self-couplings

This paper presents measurements of $W^\pm Z$ production in $pp$ collisions at a center-of-mass energy of 8 TeV. The gauge bosons are reconstructed using their leptonic decay modes into electrons and muons. The data were collected in 2012 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The measured inclusive cross section in the detector fiducial region is $σ_{W^\pm Z \rightarrow \ell^{'} ν \ell \ell} = 35.1 \pm$ 0.9 (stat.) $\pm 0.8$ (sys.) $\pm 0.8$ (lumi.) fb, for one leptonic decay channel. In comparison, the next-to-leading-order Standard Model expectation is 30.0 $\pm$ 2.1 fb. Cross sections for $W^+Z$ and $W^-Z$ production and their ratio are presented as well as differential cross sections for several kinematic observables. Limits on anomalous triple gauge boson couplings are derived from the transverse mass spectrum of the $W^\pm Z$ system. From the analysis of events with a $W$ and a $Z$ boson associated with two or more forward jets an upper limit at 95% confidence level on the $W^\pm Z$ scattering cross section of 0.63 fb, for each leptonic decay channel, is established, while the Standard Model prediction at next-to-leading order is 0.13 fb. Limits on anomalous quartic gauge boson couplings are also extracted.

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arXiv2016

Search for an additional, heavy Higgs boson in the $H\rightarrow ZZ$ decay channel at $\sqrt{s}$ = 8 TeV in $pp$ collision data with the ATLAS detector

A search is presented for a high-mass Higgs boson in the $H\rightarrow\ell\ell\ell\ell$, $H\rightarrow\ell\ellνν$, $H\rightarrow\ell\ell qq$, and $H\rightarrowννqq$ decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to 1 TeV for all four decay modes and down to as low as 140 GeV, depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to Z boson pairs. 95% confidence level upper limits range from 0.53 pb at $m_H=195$ GeV to 0.008 pb at $m_H=950$ GeV for the gluon-fusion production mode and from 0.31 pb at $m_H=195$ GeV to 0.009 pb at $m_H=950$ GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.

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arXiv2016

Search for the production of single vector-like and excited quarks in the $Wt$ final state in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

A search for vector-like quarks and excited quarks in events containing a top quark and a $W$ boson in the final state is reported here. The search is based on $20.3 {fb}^{-1}$ of proton--proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a $b$-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-$p_\mathrm{T}$ $W$ boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to $Wt$ with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.

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arXiv2016

Search for New Phenomena in Dijet Mass and Angular Distributions from $pp$ Collisions at $\sqrt{s}$ = 13 TeV with the ATLAS Detector

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb$^{-1}$ of proton-proton collisions with a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.3 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a $W^\prime$ model, a range of masses starting from $m_{Z^\prime}=1.5$ TeV and couplings from $g_q = 0.2$ in a $Z^\prime$ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between the new interaction and QCD processes. These results significantly extend the ATLAS limits obtained from 8 TeV data. Gaussian-shaped contributions to the mass distribution are also excluded if the effective cross-section exceeds values ranging from approximately 50-300 fb for masses below 2 TeV to 2-20 fb for masses above 4 TeV.

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arXiv2016

Measurement of the differential cross-section of highly boosted top quarks as a function of their transverse momentum in $\sqrt{s}$ = 8 TeV proton-proton collisions using the ATLAS detector

The differential cross-section for pair production of top quarks with high transverse momentum is measured in 20.3 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. The measurement is performed for $t\bar{t}$ events in the lepton+jets channel. The cross-section is reported as a function of the hadronically decaying top quark transverse momentum for values above 300 GeV. The hadronically decaying top quark is reconstructed as an anti-$k_t$ jet with radius parameter $R=1.0$ and identified with jet substructure techniques. The observed yield is corrected for detector effects to obtain a cross-section at particle level in a fiducial region close to the event selection. A parton-level cross-section extrapolated to the full phase space is also reported for top quarks with transverse momentum above 300 GeV. The predictions of a majority of next-to-leading-order and leading-order matrix-element Monte Carlo generators are found to agree with the measured cross-sections.

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arXiv2016

A search for prompt lepton-jets in $pp$ collisions at $\sqrt{s}=$ 8 TeV with the ATLAS detector

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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arXiv2016

Search for massive supersymmetric particles decaying to many jets using the ATLAS detector in $pp$ collisions at $\sqrt{s} = 8$ TeV

Results of a search for decays of massive particles to fully hadronic final states are presented. This search uses 20.3 fb$^{-1}$ of data collected by the ATLAS detector in $\sqrt{s} = 8$ TeV proton--proton collisions at the LHC. Signatures based on high jet multiplicities without requirements on the missing transverse momentum are used to search for $R$-parity-violating supersymmetric gluino pair production with subsequent decays to quarks. The analysis is performed using a requirement on the number of jets, in combination with separate requirements on the number of $b$-tagged jets, as well as a topological observable formed from the scalar sum of the mass values of large-radius jets in the event. Results are interpreted in the context of all possible branching ratios of direct gluino decays to various quark flavors. No significant deviation is observed from the expected Standard Model backgrounds estimated using jet-counting as well as data-driven templates of the total-jet-mass spectra. Gluino pair decays to ten or more quarks via intermediate neutralinos are excluded for a gluino with mass $m_{\tilde{g}} < 1$ TeV for a neutralino mass $m_{\tildeχ^0_1} = 500$ GeV. Direct gluino decays to six quarks are excluded for $m_{\tilde{g}} < 917$ GeV for light-flavor final states, and results for various flavor hypotheses are presented.

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arXiv2016

Search for invisible decays of a Higgs boson using vector-boson fusion in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search for a Higgs boson produced via vector-boson fusion and decaying into invisible particles is presented, using 20.3 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a mass of 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28 is set on the branching fraction of $H\to$ invisible at 95% confidence level, where the expected upper limit is 0.31. The results are interpreted in models of Higgs-portal dark matter where the branching fraction limit is converted into upper bounds on the dark-matter--nucleon scattering cross section as a function of the dark-matter particle mass, and compared to results from the direct dark-matter detection experiments.

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arXiv2016

Constraints on non-Standard Model Higgs boson interactions in an effective Lagrangian using differential cross sections measured in the $H \rightarrow γγ$ decay channel at $\sqrt{s} = 8$ TeV with the ATLAS detector

The strength and tensor structure of the Higgs boson's interactions are investigated within an effective field theory framework, which allows new CP-even and CP-odd interactions that can lead to changes in the kinematic properties of the Higgs boson and associated jet spectra. The parameters of the effective field theory are probed using a fit to five differential cross sections previously measured by the ATLAS experiment in the $H \rightarrow γγ$ decay channel with an integrated luminosity of 20.3 fb$^{-1}$ at $\sqrt{s}=8$ TeV. In order to perform a simultaneous fit to the five distributions, the statistical correlations between them are determined by re-analysing the $H \rightarrow γγ$ candidate events in the proton-proton collision data. No significant deviations from the Standard Model are observed and limits on the effective field theory parameters are derived. The statistical correlations are made publicly available to allow for future analysis of theories with non-Standard Model interactions.

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arXiv2016

Dijet production in $\sqrt{s}=7$ TeV $pp$ collisions with large rapidity gaps at the ATLAS experiment

A $6.8 \ {\rm nb^{-1}}$ sample of $pp$ collision data collected under low-luminosity conditions at $\sqrt{s} = 7$ TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events containing at least two jets with $p_\mathrm{T} > 20$ GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in $Δη^F$, the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, $\tildeξ$, of the fractional momentum loss of the proton assuming single diffractive dissociation ($pp \rightarrow pX$). Model comparisons indicate a dominant non-diffractive contribution up to moderately large $Δη^F$ and small $\tildeξ$, with a diffractive contribution which is significant at the highest $Δη^F$ and the lowest $\tildeξ$. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.

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