Search for new phenomena with photon+jet events in proton-proton collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Measurement of the charge asymmetry in dileptonic decays of top quark pairs in pp collisions at $ \sqrt{s}=7 $ TeV using the ATLAS detector
A measurement of the top-antitop (t$\bar{t}$) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb -1 of LHC pp collisions at a centreof-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A C ℓℓ based on the identified charged leptons, and A$t\bar{t}\atop{C}$, based on the reconstructed tt final state. The asymmetries are measured to be A $ℓℓ\atop{C}$ =0.024 ± 0.015 (stat.) ± 0.009 (syst.), A $t\bar{t}\atop{C}$ = 0.021 ± 0.025 (stat.) ± 0.017 (syst.). The measured values are in agreement with the Standard Model predictions.
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Measurement of the $$ t\overline{t}\gamma $$ production cross section in proton-proton collisions at $$ \sqrt{s}=8 $$ TeV with the ATLAS detector
The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$= 8 TeV with 20.2 fb -1 of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum p T > 15 GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b -quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat.) ± 17 (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of 151 ± 24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.
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A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp-collision data with the ATLAS detector
A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb –1 of proton–proton collision data at √s = 7 TeV from 2010 and 0.1 nb –1 of data at √s = 8 TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of Geant4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2–5% for jets with transverse momenta above 2 TeV, where this method provides the jet energy scale uncertainty for ATLAS.
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Search for direct stau production in events with two hadronic tau-leptons in root s=13 TeV pp collisions with the ATLAS detector
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Observation of an Excited B c ± Meson State with the ATLAS Detector
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Search for new resonances in mass distributions of jet pairs using 139 fb(-1) of pp collisions at root s=13TeV with the ATLAS detector
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of long-range pseudorapidity correlations and azimuthal harmonics in s N N = 5.02 TeV proton-lead collisions with the ATLAS detector
We present measurements of two-particle correlation functions and the first five azimuthal harmonics, v 1 to v 5 , are presented, using 28 nb -1 of p + Pb collisions at a nucleon-nucleon center-of-mass energy of s N N = 5.02 TeV measured with the ATLAS detector at the LHC. Significant long-range "ridgelike" correlations are observed for pairs with small (|Δϕ|<π/3) and back-to-back pairs (|Δϕ|>2π/3) over the transverse momentum range 0.44 GeV. The ν 2 ( p T ), ν 3 ( p T ), and ν 4 ( p T ) are compared to the ν n coefficients in Pb+Pb collisions at s N N = 2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average p T of particles produced in the two collision systems.
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Constraints on mediator-based dark matter and scalar dark energy models using $\sqrt{s}$ = 13 TeV pp collision data collected by the ATLAS detector
Constraints on selected mediator-based dark matter models and a scalar dark energy model using up to 37 fb ₋1 $\sqrt{s}$ = 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015-2016 are summarised in this paper. The conclusions of experimental searches in a variety of final states are interpreted in terms of a set of spin-1 and spin-0 single-mediator dark matter simplified models and a second set of models involving an extended Higgs sector plus an additional vector or pseudo-scalar mediator. The searches considered in this paper constrain spin-1 leptophobic and leptophilic mediators, spin-0 colour-neutral and colour-charged mediators and vector or pseudo-scalar mediators embedded in extended Higgs sector models. In this case, also $\sqrt{s}$ = 8 TeV pp collision data are used for the interpretation of the results. The results are also interpreted for the first time in terms of light scalar particles that could contribute to the accelerating expansion of the universe (dark energy).
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Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at s = 8 TeV with the ATLAS detector
We presented measurements of the top-antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high- pT leptons (electrons or muons), using data corresponding to an integrated luminosity of 20.3 fb $-$1 from pp collisions at a center-of-mass energy $\sqrt{s}$ = 8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the $t\bar{t}$ system are performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: A $ℓℓ\atop{C}$ based on the selected leptons and A$t\bar{t}\atop{C}$ based on the reconstructed $t\bar{t}$ final state. The inclusive asymmetries are measured in the full phase space to be A $ℓℓ\atop{C}$ = 0.008 ± 0.006 and A$t\bar{t}\atop{C}$= 0.021 ± 0.016 , which are in agreement with the Standard Model predictions of A $ℓℓ\atop{C}$ = 0.0064 ± 0.0003 and A$t\bar{t}\atop{C}$ = 0.0111 ± 0.0004 .
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Measurement of charged-particle distributions sensitive to the underlying event in $ \sqrt{s}=13 $ TeV proton-proton collisions with the ATLAS detector at the LHC
We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb –1 . The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.
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Search for single production of vector-like quarks decaying into Wb in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector
A search for singly produced vector-like quarks Q , 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 collision data at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 36.1 fb ₋1 , recorded with the ATLAS detector at the LHC in 2015 and 2016. The analysis targets Q → Wb decays where the W boson decays leptonically. No crtical deviation from the expected Standard Model background is observed. Upper limits are set on the QWb coupling strength and the mixing between the Standard Model sector and a singlet T quark or a Y quark from a ( B, Y ) doublet or a ( T, B, Y ) triplet, taking into account the interference effects with the Standard Model background. The upper limits set on the mixing angle are as small as |sin θ L | = 0.18 for a singlet T quark of mass 800 GeV, |sin θ R | = 0.17 for a Y quark of mass 800 GeV in a ( B, Y ) doublet model and |sin θL| = 0.16 for a Y quark of mass 800 GeV in a ( T, B, Y ) triplet model. Within a ( B, Y ) doublet model, the limits set on the mixing parameter |sin θ R | are comparable with the exclusion limits from electroweak precision observables in the mass range between about 900 GeV and 1250 GeV.
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Charged-particle distributions at low transverse momentum in $\sqrt{s} = 13$ TeV pp interactions measured with the ATLAS detector at the LHC
Here, measurements of distributions of charged particles produced in proton–proton collisions with a centre-of-mass energy of 13 TeV are presented. The data were recorded by the ATLAS detector at the LHC and correspond to an integrated luminosity of 151 μb –1 . The particles are required to have a transverse momentum greater than 100 MeV and an absolute pseudorapidity less than 2.5. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on multiplicity are measured in events containing at least two charged particles satisfying the above kinematic criteria. The results are corrected for detector effects and compared to the predictions from several Monte Carlo event generators.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Search for resonances in diphoton events at root s=13TeV with the ATLAS detector
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Measurement of the charged-particle multiplicity inside jets from $$\sqrt{s}=8$$ $${\mathrm{TeV}}$$ pp collisions with the ATLAS detector
The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with p T > 500 MeV inside high-momentum jets in dijet events using 20.3 fb -1 of data recorded with the ATLAS detector in pp collisions at √s=8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV . The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Lastly, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.
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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.
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Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at √s = 7 TeV with the ATLAS detector
A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a \(Z\)-boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of \(7\) TeV with an integrated luminosity of \(4.6\) fb\(^{-1}\). Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the \(Z\)-boson direction. As a result, the measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models.
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Search for top-quark decays t → Hq with 36 fb ₋1 of pp collision data at $\sqrt{s}$= 13 TeV with the ATLAS detector
A search for flavour-changing neutral current decays of a top quark into an up-type quark ( q = u, c ) and the Standard Model Higgs boson, t → Hq , is introduced. The search is based on a dataset of pp collisions at $\sqrt{s}$= 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb ₋1 . Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the H→$b\bar{b}$ and H → τ + τ ₋ decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1 × 10 ₋3 (8.3 × 10 ₋4 ) and 1.2 × 10 ₋3 (8.3 × 10 ₋4 ), respectively. The corresponding combined observed (expected) upper limits on the |λ tcH | and |λ tuH | couplings are 0.064 (0.055) and 0.066 (0.055), respectively.
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