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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 919 records · Page 51Linked to original sources

arXiv2016

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

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 $\sqrt{s}$ = 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 $-0.8 \pm 2.3$ 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.

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arXiv2016

Luminosity determination in pp collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector at the LHC

The luminosity determination for the ATLAS detector at the LHC during pp collisions at $\sqrt{s}$ = 8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of dL/L = +/- 1.9% is obtained for the 22.7 fb$^{-1}$ of pp collision data delivered to ATLAS at $\sqrt{s}$ = 8 TeV in 2012.

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arXiv2016

Performance of pile-up mitigation techniques for jets in $pp$ collisions at $\sqrt{s} = 8$ TeV using the ATLAS detector

The large rate of multiple simultaneous proton--proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 fb$^{-1}$ data sample collected at a centre-of-mass energy of $\sqrt{s} = 8$ TeV. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

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arXiv2016

Measurement of the $t\bar{t}$ production cross-section using $eμ$ events with $b$-tagged jets in $pp$ collisions at $\sqrt{s}=7$ and 8 TeV with the ATLAS detector

The inclusive top quark pair ($t\bar{t}$) production cross-section $σ_{t\bar{t}}$ has been measured in $pp$ collisions at $\sqrt{s}=7$ TeV and $\sqrt{s}=8$ TeV with the ATLAS experiment at the LHC, using $t\bar{t}$ events with an opposite-charge $eμ$ pair in the final state. The measurement was performed with the 2011 7 TeV dataset corresponding to an integrated luminosity of 4.6 fb$^{-1}$ and the 2012 8 TeV dataset of 20.3 fb$^{-1}$. The cross-section was measured to be: $σ_{t\bar{t}}=182.9\pm 3.1\pm 4.2\pm 3.6 \pm 3.3$ pb ($\sqrt{s}=7$ TeV) and $σ_{t\bar{t}}=242.9\pm 1.7\pm 5.5\pm 5.1\pm 4.2$ pb ($\sqrt{s}=8$ TeV, updated as described in the Addendum), where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the knowledge of the integrated luminosity and of the LHC beam energy. The results are consistent with recent theoretical QCD calculations at next-to-next-to-leading order. Fiducial measurements corresponding to the experimental acceptance of the leptons are also reported, together with the ratio of cross-sections measured at the two centre-of-mass energies. The inclusive cross-section results were used to determine the top quark pole mass via the dependence of the theoretically-predicted cross-section on $m_t^{\rm pole}$, giving a result of $m_t^{\rm pole}=172.9^{+2.5}_{-2.6}$ GeV. By looking for an excess of $t\bar{t}$ production with respect to the QCD prediction, the results were also used to place limits on the pair-production of supersymmetric top squarks $\tilde{t}_1$ with masses close to the top quark mass decaying via $\tilde{t}_1\rightarrow t\tildeχ^0_1$ to predominantly right-handed top quarks and a light neutralino $\tildeχ_0^1$, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 177 GeV are excluded at the 95% confidence level.

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arXiv2016

Search for new resonances in events with one lepton and missing transverse momentum in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for $W^\prime$ bosons in events with one lepton (electron or muon) and missing transverse momentum is presented. The search uses 3.2 fb$^{-1}$ of $pp$ collision data collected at $\sqrt{s} = 13$ TeV by the ATLAS experiment at the LHC in 2015. The transverse mass distribution is examined and no significant excess of events above the level expected from Standard Model processes is observed. Upper limits on the $W^\prime$ boson cross-section times branching ratio to leptons are set as a function of the $W^\prime$ mass. Assuming a $W^\prime$ boson as predicted by the Sequential Standard Model, $W^\prime$ masses below 4.07 TeV are excluded at the 95% confidence level. This extends the limit set using LHC data at $\sqrt{s}=8$ TeV by around 800 GeV.

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arXiv2016

Search for heavy resonances decaying to a $Z$ boson and a photon in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a $Z$ boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 13$ TeV with the ATLAS detector at the Large Hadron Collider. The $Z$ bosons are identified through their decays either to charged, light, lepton pairs ($e^+e^-$, $μ^+μ^-$) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to $Zγ$ of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.

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arXiv2016

Study of hard double-parton scattering in four-jet events in $pp$ collisions at $\sqrt{s} = 7$ TeV with the ATLAS experiment

Inclusive four-jet events produced in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV are analysed for the presence of hard double-parton scattering using data corresponding to an integrated luminosity of 37.3 pb$^{-1}$, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum $p_{\mathrm{T}} \geq 20$ GeV and pseudorapidity $η\leq 4.4$, and at least one having $p_{\mathrm{T}} \geq 42.5$ GeV, the contribution of hard double-parton scattering is estimated to be $f_{\mathrm{DPS}} = 0.092 ^{+0.005}_{-0.011} (\mathrm{stat.}) ^{+0.033}_{-0.037} (\mathrm{syst.})$. After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective overlap area between the interacting protons, $σ_{\mathrm{eff}}$, was determined to be $σ_{\mathrm{eff}} = 14.9 ^{+1.2}_{-1.0} (\mathrm{stat.}) ^{+5.1}_{-3.8} (\mathrm{syst.})$ mb. This result is consistent within the quoted uncertainties with previous measurements of $σ_{\mathrm{eff}}$, performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to $21^{+7}_{-6}$% of the total inelastic cross-section measured at $\sqrt{s} = 7$ TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.

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arXiv2016

Search for the Higgs boson produced in association with a $W$ boson and decaying to four $b$-quarks via two spin-zero particles in $pp$ collisions at 13 TeV with the ATLAS detector

This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, $H \rightarrow aa$, where the particle $a$ decays to $b$-quarks and has a mass in the range of 20-60 GeV. The search is performed in events where the Higgs boson is produced in association with a $W$ boson, giving rise to a signature of a lepton (electron or muon), missing transverse momentum, and multiple jets from $b$-quark decays. The analysis is based on the full dataset of $pp$ collisions at $\sqrt{s} = 13$ TeV recorded in 2015 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb$^{-1}$. No significant excess of events above the Standard Model prediction is observed, and a $95\%$ confidence-level upper limit is derived for the product of the production cross section for $pp \rightarrow WH$ times the branching ratio for the decay $H \rightarrow aa \rightarrow 4b$. The upper limit ranges from 6.2 pb for an $a$-boson mass $m_a = 20$ GeV to 1.5 pb for $m_a = 60$ GeV.

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arXiv2016

Search for Minimal Supersymmetric Standard Model Higgs bosons $H/A$ and for a $Z^{\prime}$ boson in the $ττ$ final state produced in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

A search for neutral Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) and for a heavy neutral $Z^{\prime}$ boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb$^{-1}$ from proton--proton collisions at $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a $τ^+ τ^-$ pair with at least one $τ$ lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2--1.2 TeV for the MSSM neutral Higgs bosons and 0.5--2.5 TeV for the heavy neutral $Z^{\prime}$ boson. The data are in good agreement with the background predicted by the Standard Model and hence results are given as upper limits on the production cross section times branching fraction of the boson decay to $τ^+τ^-$ as a function of the boson mass. The results are interpreted in MSSM and $Z^{\prime}$ benchmark scenarios. The most stringent MSSM parameter space constraints for the Higgs boson search exclude at 95\% confidence level (CL) $\tanβ> 7.6$ for $m_A = 200$ GeV in the $m_{h}^{\textrm{mod+}}$ MSSM scenario. This analysis extends the MSSM limits from previous searches for the mass range $m_A > 500$ GeV. For the Sequential Standard Model, a $Z^{\prime}_\mathrm{SSM}$ mass up to 1.90 TeV is excluded at 95% CL and masses up to 1.82--2.17 TeV are excluded for a $Z^{\prime}_{\mathrm{SFM}}$ of the Strong Flavour Model.

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arXiv2016

Searches for heavy diboson resonances in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Searches for new heavy resonances decaying to $WW$, $WZ$, and $ZZ$ bosons are presented, using a data sample corresponding to 3.2 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV collected with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting $ννqq$, $\ellνqq$, $\ell\ell qq$ and $qqqq$ final states are combined, searching for a narrow-width resonance with mass between 500 and 3000 GeV. The discriminating variable is either an invariant mass or a transverse mass. No significant deviations from the Standard Model predictions are observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 graviton. Cross-section limits are set at the 95% confidence level and are compared to theoretical cross-section predictions for a variety of models. The data exclude a scalar singlet with mass below 2650 GeV, a heavy vector-boson triplet with mass below 2600 GeV, and a graviton with mass below 1100 GeV. These results significantly extend the previous limits set using $pp$ collisions at $\sqrt{s}=8$ TeV.

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arXiv2016

Measurement of the charge asymmetry in highly boosted top-quark pair production in $\sqrt{s} =$ 8 TeV $pp$ collision data collected by the ATLAS experiment

In the $pp \rightarrow t\bar{t}$ process the angular distributions of top and anti-top quarks are expected to present a subtle difference, which could be enhanced by processes not included in the Standard Model. This Letter presents a measurement of the charge asymmetry in events where the top-quark pair is produced with a large invariant mass. The analysis is performed on 20.3 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} =$ 8 TeV collected by the ATLAS experiment at the LHC, using reconstruction techniques specifically designed for the decay topology of highly boosted top quarks. The charge asymmetry in a fiducial region with large invariant mass of the top-quark pair ($m_{t\bar{t}} > $ 0.75 TeV) and an absolute rapidity difference of the top and anti-top quark candidates within $-$2 $ < |y_t| - |y_{\bar{t}}| <$ 2 is measured to be 4.2 $\pm$ 3.2%, in agreement with the Standard Model prediction at next-to-leading order. A differential measurement in three $t\bar{t}$ mass bins is also presented.

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arXiv2016

Charged-particle distributions in $\sqrt{s}=13$ TeV $pp$ interactions measured with the ATLAS detector at the LHC

Charged-particle distributions are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using a data sample of nearly 9 million events, corresponding to an integrated luminosity of 170 $μ$b$^{-1}$, recorded by the ATLAS detector during a special Large Hadron Collider fill. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on the charged-particle multiplicity are presented. The measurements are performed with charged particles with transverse momentum greater than 500 MeV and absolute pseudorapidity less than 2.5, in events with at least one charged particle satisfying these kinematic requirements. Additional measurements in a reduced phase space with absolute pseudorapidity less than 0.8 are also presented, in order to compare with other experiments. The results are corrected for detector effects, presented as particle-level distributions and are compared to the predictions of various Monte Carlo event generators.

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arXiv2016

Search for dark matter produced in association with a hadronically decaying vector boson in pp collisions at $\sqrt{s}$=13 TeV with the ATLAS detector

A search is presented for dark matter produced in association with a hadronically decaying W or Z boson using 3.2 /fb of pp collisions at $\sqrt{s}$=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet compatible with a W or Z boson and with large missing transverse momentum are analysed. The data are consistent with the Standard Model predictions and are interpreted in terms of both an effective field theory and a simplified model containing dark matter.

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arXiv2016

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in $\sqrt{s_{NN}}=5.02$ TeV p+Pb collisions measured by the ATLAS experiment

Measurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using $p+$Pb collision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of $\sqrt{s_{NN}}=5.02$ TeV. Charged particles are reconstructed over pseudorapidity $|η|<2.3$ and transverse momentum between $0.1$ GeV and $22$ GeV in a dataset corresponding to an integrated luminosity of $1$ $μb^{-1}$. The results are presented in the form of charged-particle nuclear modification factors, where the $p+$Pb charged-particle multiplicities are compared between central and peripheral $p+$Pb collisions as well as to charged-particle cross sections measured in pp collisions. The $p+$Pb collision centrality is characterized by the total transverse energy measured in $-4.9<η<-3.1$, which is in the direction of the outgoing lead beam. Three different estimations of the number of nucleons participating in the $p+$Pb collision are carried out using the Glauber model and two Glauber-Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. A broad peak is observed for all centralities and rapidities in the nuclear modification factors for charged-particle transverse momentum values around $3$ GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus.

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arXiv2016

Search for bottom squark pair production in proton--proton collisions at $\sqrt{s}=$13 TeV with the ATLAS detector

The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark ($\tilde{b}_1$) is reported. The search uses 3.2 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=$13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from $b$-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95% confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric $R$-parity-conserving models in which the $\tilde{b}_1$ is the lightest squark and is assumed to decay exclusively via $\tilde{b}_1 \rightarrow b \tildeχ_1^0$, where $\tildeχ_1^0$ is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the $\tildeχ_1^0$ mass below 360 (100) GeV whilst differences in mass above 100 GeV between the $\tilde{b}_1$ and the $\tildeχ_1^0$ are excluded up to a $\tilde{b}_1$ mass of 500 GeV.

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arXiv2016

Measurement of top quark pair differential cross-sections in the dilepton channel in $pp$ collisions at $\sqrt{s}$ = 7 and 8 TeV with ATLAS

Measurements of normalized differential cross-sections of top quark pair ($t\bar t$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $t\bar t$ system in proton-proton collisions at center-of-mass energies of $\sqrt{s}$ = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb$^{-1}$ at 7 TeV and 20.2 fb$^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a $b$-hadron. The measured distributions are corrected for detector effects and selection efficiency to cross-sections at the parton level. The differential cross-sections are compared with different Monte Carlo generators and theoretical calculations of $t\bar t$ production. The results are consistent with the majority of predictions in a wide kinematic range.

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arXiv2016

Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The results of a search for gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton--proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV are presented. The dataset used was recorded in 2015 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 3.2 fb$^{-1}$. Six signal selections are defined that best exploit the signal characteristics. The data agree with the Standard Model background expectation in all six signal selections, and the largest deviation is a 2.1 standard deviation excess. The results are interpreted in a simplified model where pair-produced gluinos decay via the lightest chargino to the lightest neutralino. In this model, gluinos are excluded up to masses of approximately 1.6 TeV depending on the mass spectrum of the simplified model, thus surpassing the limits of previous searches.

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arXiv2016

Measurement of $W^+W^-$ production in association with one jet in proton--proton collisions at $\sqrt{s} =8$ TeV with the ATLAS detector

The production of $W$ boson pairs in association with one jet in $pp$ collisions at $\sqrt{s} = 8$ TeV is studied using data corresponding to an integrated luminosity of 20.3 fb$^{-1}$ collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The cross section is measured in a fiducial phase-space region defined by the presence of exactly one electron and one muon, missing transverse momentum and exactly one jet with a transverse momentum above 25 GeV and a pseudorapidity of $|η|<4.5$. The leptons are required to have opposite electric charge and to pass transverse momentum and pseudorapidity requirements. The fiducial cross section is found to be $σ^{\mathrm{fid,1\textrm{-}jet}}_{WW}=136\pm6($stat$)\pm14($syst$)\pm3($lumi$)$ fb. In combination with a previous measurement restricted to leptonic final states with no associated jets, the fiducial cross section of $WW$ production with zero or one jet is measured to be $σ^{\mathrm{fid,}\leq\mathrm{1\textrm{-}jet}}_{WW}=511\pm9($stat$)\pm26($syst$)\pm10($lumi$)$ fb. The ratio of fiducial cross sections in final states with one and zero jets is determined to be $0.36\pm0.05$. Finally, a total cross section extrapolated from the fiducial measurement of $WW$ production with zero or one associated jet is reported. The measurements are compared to theoretical predictions and found in good agreement.

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