Measurement of the inclusive cross-sections of single top-quark and top-antiquark t -channel production in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

April 14, 2017, 12:00 am

≫ Next: Probing the W tb vertex structure in t -channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

≪ Previous: Performance of algorithms that reconstruct missing transverse momentum in $$\sqrt{s}$$ s = 8 TeV proton–proton collisions in the ATLAS detector

Abstract

A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be σ(tq) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and $ \sigma \left(\overline{t}q\right)=91\pm 4 $ (stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be $ {R}_t=\sigma (tq)/\sigma \left(\overline{t}q\right)=1.72\pm 0.09 $ (stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.

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Probing the W tb vertex structure in t -channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

April 20, 2017, 12:00 am

≫ Next: Measurement of the W boson polarisation in $$t\bar{t}$$ t t ¯ events from pp collisions at $$\sqrt{s}$$ s = 8 TeV in the lepton + jets channel with ATLAS

≪ Previous: Measurement of the inclusive cross-sections of single top-quark and top-antiquark t -channel production in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

To probe the W tb vertex structure, top-quark and W -boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb⁻¹, recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling g_R are also set from model-independent measurements.

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Measurement of the W boson polarisation in $$t\bar{t}$$ t t ¯ events from pp collisions at $$\sqrt{s}$$ s = 8 TeV in the lepton + jets channel with ATLAS

April 26, 2017, 12:00 am

≫ Next: Measurements of top-quark pair differential cross-sections in the $$e\mu $$ e μ channel in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV using the ATLAS detector

≪ Previous: Probing the W tb vertex structure in t -channel single-top-quark production and decay in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

This paper presents a measurement of the polarisation of W bosons from $t\bar{t}$ decays, reconstructed in events with one high-$p_{\text{ T }}$ lepton and at least four jets. Data from pp collisions at the LHC were collected at $\sqrt{s}$ = 8 TeV and correspond to an integrated luminosity of 20.2 fb$^{-1}$. The angle $\theta ^{*}$ between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of $\cos {\theta ^{*}}$ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are $F_{\mathrm {0}}=~0.709~\pm ~{0.019}$, $F_{\mathrm {L}}=~0.299~\pm ~{0.015}$ and $F_{\mathrm {R}}=~-0.008~\pm ~{0.014}$, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.

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Measurements of top-quark pair differential cross-sections in the $$e\mu $$ e μ channel in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV using the ATLAS detector

May 8, 2017, 12:00 am

≫ Next: Performance of the ATLAS trigger system in 2015

≪ Previous: Measurement of the W boson polarisation in $$t\bar{t}$$ t t ¯ events from pp collisions at $$\sqrt{s}$$ s = 8 TeV in the lepton + jets channel with ATLAS

Abstract

This article presents measurements of $t\bar{t}$ differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb$^{-1}$ of proton–proton data at a centre-of-mass energy of $\sqrt{s} = 13$ TeV recorded by the ATLAS experiment at the LHC in 2015. Differential cross-sections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the $t\bar{t}$ system. The $t\bar{t}$ events are selected by requiring one electron and one muon of opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box$+$ Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the $t\bar{t}$ system.

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Performance of the ATLAS trigger system in 2015

May 18, 2017, 12:00 am

≫ Next: Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC

≪ Previous: Measurements of top-quark pair differential cross-sections in the $$e\mu $$ e μ channel in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV using the ATLAS detector

Abstract

During 2015 the ATLAS experiment recorded $3.8\,{\mathrm{fb}}^{-1}$ of proton–proton collision data at a centre-of-mass energy of $13\,{\mathrm{TeV}}$. The ATLAS trigger system is a crucial component of the experiment, responsible for selecting events of interest at a recording rate of approximately 1 kHz from up to 40 MHz of collisions. This paper presents a short overview of the changes to the trigger and data acquisition systems during the first long shutdown of the LHC and shows the performance of the trigger system and its components based on the 2015 proton–proton collision data.

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Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC

May 19, 2017, 12:00 am

≫ Next: Measurements of the production cross section of a $$Z$$ Z boson in association with jets in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV with the ATLAS detector

≪ Previous: Performance of the ATLAS trigger system in 2015

Abstract

This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of $\sqrt{s} = 8$ TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about $30\;\upmu {\text {m}}$ is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than $20\;\upmu {\text {m}}$ and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.

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Measurements of the production cross section of a $$Z$$ Z boson in association with jets in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV with the ATLAS detector

May 31, 2017, 12:00 am

≫ Next: Precision measurement and interpretation of inclusive $$W^+$$ W + , $$W^-$$ W - and $$Z/\gamma ^*$$ Z / γ ∗ production cross sections with the ATLAS detector

≪ Previous: Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC

Abstract

Measurements of the production cross section of a $Z$ boson in association with jets in proton–proton collisions at $\sqrt{s} = 13$ TeV are presented, using data corresponding to an integrated luminosity of 3.16 fb$^{-1}$ collected by the ATLAS experiment at the CERN Large Hadron Collider in 2015. Inclusive and differential cross sections are measured for events containing a $Z$ boson decaying to electrons or muons and produced in association with up to seven jets with $p_{\text {T}} > 30$ GeV and $|y| <2.5$. Predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements for up to two additional partons interfaced with parton shower and fixed-order predictions at next-to-leading order and next-to-next-to-leading order are compared with the measured cross sections. Good agreement within the uncertainties is observed for most of the modelled quantities, in particular with the generators which use next-to-leading-order matrix elements and the more recent next-to-next-to-leading-order fixed-order predictions.

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Precision measurement and interpretation of inclusive $$W^+$$ W + , $$W^-$$ W - and $$Z/\gamma ^*$$ Z / γ ∗ production cross sections with the ATLAS detector

June 2, 2017, 12:00 am

≫ Next: Measurement of jet fragmentation in Pb+Pb and pp collisions at $$\sqrt{{s_\mathrm{NN}}} = 2.76$$ s NN = 2.76 TeV with the ATLAS detector at the LHC

≪ Previous: Measurements of the production cross section of a $$Z$$ Z boson in association with jets in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV with the ATLAS detector

Abstract

High-precision measurements by the ATLAS Collaboration are presented of inclusive $W^+ \rightarrow \ell ^+\nu $, $W^- \rightarrow \ell ^-\bar{\nu }$ and $Z/\gamma ^* \rightarrow \ell \ell $ ($\ell =e,\mu $) Drell–Yan production cross sections at the LHC. The data were collected in proton–proton collisions at $\sqrt{s} = 7\,\text {TeV}$ with an integrated luminosity of $4.6\,\mathrm {fb}^{-1}$. Differential $W^+$ and $W^-$ cross sections are measured in a lepton pseudorapidity range $|\eta _{\ell }|<2.5$. Differential $Z/\gamma ^*$ cross sections are measured as a function of the absolute dilepton rapidity, for $|y_{\ell \ell }| < 3.6$, for three intervals of dilepton mass, $m_{\ell \ell }$, extending from 46 to $150\,\,\text {GeV}$. The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive $e^{\pm }p$ scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element $\vert V_{cs} \vert $ is also provided.

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Measurement of jet fragmentation in Pb+Pb and pp collisions at $$\sqrt{{s_\mathrm{NN}}} = 2.76$$ s NN = 2.76 TeV with the ATLAS detector at the LHC

June 8, 2017, 12:00 am

≫ Next: Search for dark matter at $$\sqrt{s}=13~\mathrm{TeV}$$ s = 13 TeV in final states containing an energetic photon and large missing transverse momentum with the ATLAS detector

≪ Previous: Precision measurement and interpretation of inclusive $$W^+$$ W + , $$W^-$$ W - and $$Z/\gamma ^*$$ Z / γ ∗ production cross sections with the ATLAS detector

Abstract

The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb$^{-1}$ of Pb+Pb data and 4.0 pb$^{-1}$ of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet $p_\mathrm{T}$ and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.

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Search for dark matter at $$\sqrt{s}=13~\mathrm{TeV}$$ s = 13 TeV in final states containing an energetic photon and large missing transverse momentum with the ATLAS detector

June 14, 2017, 12:00 am

≫ Next: Measurement of multi-particle azimuthal correlations in pp, p + Pb and low-multiplicity Pb + Pb collisions with the ATLAS detector

≪ Previous: Measurement of jet fragmentation in Pb+Pb and pp collisions at $$\sqrt{{s_\mathrm{NN}}} = 2.76$$ s NN = 2.76 TeV with the ATLAS detector at the LHC

Abstract

Results of a search for physics beyond the Standard Model in events containing an energetic photon and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. As the number of events observed in data, corresponding to an integrated luminosity of 36.1 fb$^{-1}$ of proton–proton collisions at a centre-of-mass energy of $13~\mathrm{TeV}$, is in agreement with the Standard Model expectations, model-independent limits are set on the fiducial cross section for the production of events in this final state. Exclusion limits are also placed in models where dark-matter candidates are pair-produced. For dark-matter production via an axial-vector or a vector mediator in the s-channel, this search excludes mediator masses below 750–$1200~\mathrm{GeV}$ for dark-matter candidate masses below 230–$480~\mathrm{GeV}$ at 95% confidence level, depending on the couplings. In an effective theory of dark-matter production, the limits restrict the value of the suppression scale $M_{*}$ to be above $790~\mathrm{GeV}$ at 95% confidence level. A limit is also reported on the production of a high-mass scalar resonance by processes beyond the Standard Model, in which the resonance decays to $Z\gamma $ and the Z boson subsequently decays into neutrinos.

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Measurement of multi-particle azimuthal correlations in pp, p + Pb and low-multiplicity Pb + Pb collisions with the ATLAS detector

June 26, 2017, 12:00 am

≫ Next: Jet reconstruction and performance using particle flow with the ATLAS Detector

≪ Previous: Search for dark matter at $$\sqrt{s}=13~\mathrm{TeV}$$ s = 13 TeV in final states containing an energetic photon and large missing transverse momentum with the ATLAS detector

Abstract

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in $pp$ collisions at $\sqrt{s}$ = 5.02 and 13 TeV and in $p$ + Pb collisions at $\sqrt{s_{_\text {NN}}}$ = 5.02 TeV, and compared to the results obtained for low-multiplicity $\mathrm{Pb}~+~\mathrm{Pb}$ collisions at $\sqrt{s_{_\text {NN}}}$ = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in $p$ + Pb and low-multiplicity $\mathrm{Pb}~+~\mathrm{Pb}$ collisions. On the other hand, the $pp$ results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in $\mathrm{Pb}~+~\mathrm{Pb}$, smaller in $p$ + Pb and smallest in $pp$ collisions. The $pp$ results show no dependence on the collision energy, nor on the multiplicity.

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Jet reconstruction and performance using particle flow with the ATLAS Detector

July 13, 2017, 12:00 am

≫ Next: Measurements of electroweak $$Wjj $$ W j j production and constraints on anomalous gauge couplings with the ATLAS detector

≪ Previous: Measurement of multi-particle azimuthal correlations in pp, p + Pb and low-multiplicity Pb + Pb collisions with the ATLAS detector

Abstract

This paper describes the implementation and performance of a particle flow algorithm applied to 20.2 fb$^{-1}$ of ATLAS data from 8 TeV proton–proton collisions in Run 1 of the LHC. The algorithm removes calorimeter energy deposits due to charged hadrons from consideration during jet reconstruction, instead using measurements of their momenta from the inner tracker. This improves the accuracy of the charged-hadron measurement, while retaining the calorimeter measurements of neutral-particle energies. The paper places emphasis on how this is achieved, while minimising double-counting of charged-hadron signals between the inner tracker and calorimeter. The performance of particle flow jets, formed from the ensemble of signals from the calorimeter and the inner tracker, is compared to that of jets reconstructed from calorimeter energy deposits alone, demonstrating improvements in resolution and pile-up stability.

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Measurements of electroweak $$Wjj $$ W j j production and constraints on anomalous gauge couplings with the ATLAS detector

July 17, 2017, 12:00 am

≫ Next: Studies of Zγ production in association with a high-mass dijet system in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

≪ Previous: Jet reconstruction and performance using particle flow with the ATLAS Detector

Abstract

Measurements of the electroweak production of a W boson in association with two jets at high dijet invariant mass are performed using $\sqrt{s} =$ 7 and 8 $\text {TeV}$ proton–proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb$^{-1}$ of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of a W boson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.

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Studies of Zγ production in association with a high-mass dijet system in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

July 21, 2017, 12:00 am

≫ Next: Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

≪ Previous: Measurements of electroweak $$Wjj $$ W j j production and constraints on anomalous gauge couplings with the ATLAS detector

Abstract

The production of a Z boson and a photon in association with a high-mass dijet system is studied using 20.2 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of $ \sqrt{s}=8 $ TeV recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total pp→ Zγjj cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes VV→ Zγ. No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.

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Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

July 24, 2017, 12:00 am

≫ Next: Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in s = 13 $$ \sqrt{s}=13 $$ TeV pp collisions with the ATLAS detector

≪ Previous: Studies of Zγ production in association with a high-mass dijet system in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

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Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in s = 13 $$ \sqrt{s}=13 $$ TeV pp collisions with the ATLAS detector

August 1, 2017, 12:00 am

≫ Next: Fiducial, total and differential cross-section measurements of t -channel single top-quark production in pp collisions at 8 TeV using data collected by the ATLAS detector

≪ Previous: Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

Abstract

A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at $ \sqrt{s}=13 $ TeV collected with the ATLAS detector at the LHC in 2015–2016, corresponding to an integrated luminosity of 36.1 fb⁻¹. No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ($ {\tilde{t}}_1 $) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ($ {\tilde{t}}_2 $) decaying via $ {\tilde{t}}_2\to Z{\tilde{t}}_1 $ or $ {\tilde{t}}_2\to h{\tilde{t}}_1 $. The results exclude at 95% confidence level $ {\tilde{t}}_2 $ and $ {\tilde{t}}_1 $ masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.

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Fiducial, total and differential cross-section measurements of t -channel single top-quark production in pp collisions at 8 TeV using data collected by the ATLAS detector

August 9, 2017, 12:00 am

≫ Next: Measurement of the k t splitting scales in Z → ℓℓ events in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

≪ Previous: Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in s = 13 $$ \sqrt{s}=13 $$ TeV pp collisions with the ATLAS detector

Abstract

Detailed measurements of t-channel single top-quark production are presented. They use 20.2 fb$^{-1}$ of data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be $\sigma _{\text {tot}} (tq) = 56.7^{+4.3}_{-3.8}\;\mathrm{pb}$ for top-quark production and $\sigma _{\text {tot}} (\bar{t} q) = 32.9^{+3.0}_{-2.7}\;\mathrm{pb}$ for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be $R_t=1.72 \pm 0.09$. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.

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Measurement of the k t splitting scales in Z → ℓℓ events in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

August 8, 2017, 12:00 am

≫ Next: Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in s = 13 $$ \sqrt{s}=13 $$ TeV pp collisions with the ATLAS detector

≪ Previous: Fiducial, total and differential cross-section measurements of t -channel single top-quark production in pp collisions at 8 TeV using data collected by the ATLAS detector

Abstract

A measurement of the splitting scales occuring in the k_t jet-clustering algorithm is presented for final states containing a Z boson. The measurement is done using 20.2 fb⁻¹ of proton-proton collision data collected at a centre-of-mass energy of $ \sqrt{s}=8 $ TeV by the ATLAS experiment at the LHC in 2012. The measurement is based on chargedparticle track information, which is measured with excellent precision in the p_T region relevant for the transition between the perturbative and the non-perturbative regimes. The data distributions are corrected for detector effects, and are found to deviate from state-of-the-art predictions in various regions of the observables.

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Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in s = 13 $$ \sqrt{s}=13 $$ TeV pp collisions with the ATLAS detector

August 16, 2017, 12:00 am

≫ Next: Erratum to: Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at $$\sqrt{s}=8$$ s = 8 TeV with the ATLAS detector

≪ Previous: Measurement of the k t splitting scales in Z → ℓℓ events in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

The results of a search for vector-like top quarks using events with exactly one lepton, at least four jets, and large missing transverse momentum are reported. The search is optimised for pair production of vector-like top quarks in the Z(→νν) t + X decay channel. LHC pp collision data at a centre-of-mass energy of $ \sqrt{s}=13 $ TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb⁻¹. No significant excess over the Standard Model expectation is seen and upper limits on the production cross-section of a vector-like T quark pair as a function of the T quark mass are derived. The observed (expected) 95% CL lower limits on the T mass are 870 GeV (890 GeV) for the weak-isospin singlet model, 1.05 TeV (1.06 TeV) for the weak-isospin doublet model and 1.16 TeV (1.17 TeV) for the pure Zt decay mode. Limits are also set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV.