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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.
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 charged-particle 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.
This paper presents a measurement of the triple-differential cross section for the Drell-Yan process \({Z/γ^*}\) → ℓ\(^+\)ℓ\(^-\) where ℓ is an electron or a muon. The measurement is performed for invariant masses of the lepton pairs, \(m_{ℓℓ}\) , between 46 and 200 GeV using a sample of 20.2 fb\(^{−1}\) of \(pp\) collisions data at a centre-of-mass energy of \(\sqrt{s}\) = 8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented in bins of invariant mass, absolute dilepton rapidity, |\(y_{ℓℓ}\)|, and the angular variable cos \(θ^*\) between the outgoing lepton and the incoming quark in the Collins-Soper frame. The measurements are performed in the range |\(y_{ℓℓ}\)| < 2.4 in the muon channel, and extended to |\(y_{ℓℓ}\)| < 3.6 in the electron channel. The cross sections are used to determine the \(Z\) boson forward-backward asymmetry as a function of |\(y_{ℓℓ}\)| and \(m_{ℓℓ}\) . The measurements achieve high-precision, below the percent level in the pole region, excluding the uncertainty in the integrated luminosity, and are in agreement with predictions. These precision data are sensitive to the parton distribution functions and the effective weak mixing angle.
Advanced LIGO detected a significant gravitational wave signal (GW170104) originating from the coalescence of two black holes during the second observation run on January 4th, 2017. An all-sky high-energy neutrino follow-up search has been made using data from the Antares neutrino telescope, including both upgoing and downgoing events in two separate analyses. No neutrino candidates were found within ±500 s around the GW event time nor any time clustering of events over an extended time window of ±3 months. The non-detection is used to constrain isotropic-equivalent high-energy neutrino emission from GW170104 to less than ∼ 1.2 × \(10^{55}\) erg for a \(E^{−2}\) spectrum. This constraint is valid in the energy range corresponding to the 5–95% quantiles of the neutrino flux [3.2 TeV; 3.6 PeV], if the GW emitter was below the Antares horizon at the alert time.
A search for massive coloured resonances which are pair-produced and decay into two jets is presented. The analysis uses 36.7 fb(-1) of root s = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the background prediction is observed. Results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is the top squark, (t) over tilde, which decays promptly into two quarks through R-parity-violating couplings. Top squarks with masses in the range 100 GeV < m((T) over tilde) < 410 GeV are excluded at 95% confidence level. If the decay is into a b-quark and a light quark, a dedicated selection requiring two b-tags is used to exclude masses in the ranges 100 GeV < m((t) over tilde) < 470 GeV and 480 GeV < m(<(t)over tilde>) < 610 GeV. Additional limits are set on the pair-production of massive colour-octet resonances.
The results of a search for new heavy W' bosons decaying to an electron or muon and a neutrino using proton-proton collision data at a centre-of-mass energy of root s = 13 TeV are presented. The dataset was collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb(-1). As no excess of events above the Standard Model prediction is observed, the results are used to set upper limits on the W' boson cross-section times branching ratio to an electron or muon and a neutrino as a function of the W' mass. Assuming a W' boson with the same couplings as the Standard Model W boson, W' masses below 5.1 TeV are excluded at the 95% confidence level.
A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 tb(-1) of integrated luminosity at root s = 13 TeV. Events with same flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.
The inclusive and fiducial t (t) over bar production cross sections are measured in the lepton+jets channel using 20.2 fb(-1) of proton proton collision data at a centre-of mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W+jets process is modelled using Z+jets events in a data-driven approach. The inclusive t (t) over bar cross-section is measured with a precision of 5.7% to be (sigma(inc) (t (t) over bar) = 248.3 +/- 0.7 (stat.) +/- 13.4 (syst.) +/- 4.7 (lumi.) pb, assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is sigma(fid) (t (t) over bar) = 48.8 +/- 0.1 (stat.) +/- 2.0 (syst.) +/- 0.9 (lumi.) pb with a precision of 4.5%.
A search for pair production of up-type vector-like quarks (T) with a significant branching ratio into a top quark and either a Standard Model Higgs boson or a Z boson is presented. The same analysis is also used to search for four-top-quark production in several new physics scenarios. The search is based on a dataset of pp collisions at root s = 13TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb(-1). Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets, as well as the jets+E-T(miss) final state, characterised by multiple jets and large missing transverse momentum. The search exploits the high multiplicity of jets identified as originating from b-quarks, and the presence of boosted, hadronically decaying top quarks and Higgs bosons reconstructed as large-radius jets, characteristic of signal events. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross sections for the different signal processes considered. These cross-section limits are used to derive lower limits on the mass of a vector-like T quark under several branching ratio hypotheses assuming contributions from T -> Wb, Zt, Ht decays. The 95% CL observed lower limits on the T quark mass range between 0.99TeV and 1.43TeV for all possible values of the branching ratios into the three decay modes considered, significantly extending the reach beyond that of previous searches. Additionally, upper limits on anomalous four-top-quark production are set in the context of an effective field theory model, as well as in an universal extra dimensions model.
A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of root s = 13 TeV corresponding to an integrated luminosity of 36.1 fb(-1). Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z' bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the Z' boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1-3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.
A search for Higgs boson pair production in the bbbb final state is carried out with up to 36.1 fb(-1) of LHC proton-proton collision data collected at s=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260-3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bbbb are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.
Jet substructure observables have significantly extended the search program for physics beyond the standard model at the Large Hadron Collider. The state-of-the-art tools have been motivated by theoretical calculations, but there has never been a direct comparison between data and calculations of jet substructure observables that are accurate beyond leading-logarithm approximation. Such observables are significant not only for probing the collinear regime of QCD that is largely unexplored at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. This Letter documents a measurement of the first jet substructure quantity at a hadron collider to be calculated at next-to-next-to-leading-logarithm accuracy. The normalized, differential cross section is measured as a function of log(10)rho(2), where rho is the ratio of the soft-drop mass to the ungroomed jet transverse momentum. This quantity is measured in dijet events from 32.9 fb(-1) of root s = 13 TeV proton-proton collisions recorded by the ATLAS detector. The data are unfolded to correct for detector effects and compared to precise QCD calculations and leading-logarithm particle-level Monte Carlo simulations.
A search for weakly interacting massive dark matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1 fb(-1) of proton proton collision data recorded by the ATLAS experiment at root s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements.
Measurements of di ff erential cross sections of top quark pair production in association with jets by the ATLAS experiment at the LHC are presented. The measurements are performed as functions of the top quark transverse momentum, the transverse momentum of the top quark-antitop quark system and the out-of-plane transverse momentum using data from pp collisions at p s = 13TeV collected by the ATLAS detector at the LHC in 2015 and corresponding to an integrated luminosity of 3.2 fb. The top quark pair events are selected in the lepton (electron or muon) + jets channel. The measured cross sections, which are compared to several predictions, allow a detailed study of top quark production.
A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of root s = 13TeV, corresponding to an integrated luminosity of 36.1 fb(-1), are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t -> qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t -> bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 x 10(-4) (2.4 x 10(-4)) on the t -> uZ branching ratio and 2.4 x 10(-4) (3.2 x 10(-4)) on the t -> cZ branching ratio, constituting the most stringent limits to date.
A measurement of the production of three isolated photons in proton-proton collisions at a centre-of-mass energy root s = 8 TeV is reported. The results are based on an integrated luminosity of 20.2 fb(-1) collected with the ATLAS detector at the LHC. The differential cross sections are measured as functions of the transverse energy of each photon, the difference in azimuthal angle and in pseudorapidity between pairs of photons, the invariant mass of pairs of photons, and the invariant mass of the triphoton system. A measurement of the inclusive fiducial cross section is also reported. Next-to-leading-order perturbative QCD predictions are compared to the cross-section measurements. The predictions underestimate the measurement of the inclusive fiducial cross section and the differential measurements at low photon transverse energies and invariant masses. They provide adequate descriptions of the measurements at high values of the photon transverse energies, invariant mass of pairs of photons, and invariant mass of the triphoton system. (C) 2018 The Author. Published by Elsevier B.V.
White Paper on Crowdsourced Network and QoE Measurements – Definitions, Use Cases and Challenges
(2020)
The goal of the white paper at hand is as follows. The definitions of the terms build a framework for discussions around the hype topic ‘crowdsourcing’. This serves as a basis for differentiation and a consistent view from different perspectives on crowdsourced network measurements, with the goal to provide a commonly accepted definition in the community. The focus is on the context of mobile and fixed network operators, but also on measurements of different layers (network, application, user layer). In addition, the white paper shows the value of crowdsourcing for selected use cases, e.g., to improve QoE or regulatory issues. Finally, the major challenges and issues for researchers and practitioners are highlighted.
This white paper is the outcome of the Würzburg seminar on “Crowdsourced Network and QoE Measurements” which took place from 25-26 September 2019 in Würzburg, Germany. International experts were invited from industry and academia. They are well known in their communities, having different backgrounds in crowdsourcing, mobile networks, network measurements, network performance, Quality of Service (QoS), and Quality of Experience (QoE). The discussions in the seminar focused on how crowdsourcing will support vendors, operators, and regulators to determine the Quality of Experience in new 5G networks that enable various new applications and network architectures. As a result of the discussions, the need for a white paper manifested, with the goal of providing a scientific discussion of the terms “crowdsourced network measurements” and “crowdsourced QoE measurements”, describing relevant use cases for such crowdsourced data, and its underlying challenges. During the seminar, those main topics were identified, intensively discussed in break-out groups, and brought back into the plenum several times. The outcome of the seminar is this white paper at hand which is – to our knowledge – the first one covering the topic of crowdsourced network and QoE measurements.
Ratios of top-quark pair to \(Z\)-boson cross sections measured from proton-proton collisions at the LHC centre-of-mass energies of \(\sqrt{s}\) = 13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given \(\sqrt{s}\) for the two processes and at different \(\sqrt{s}\) for each process, as well as double ratios of the two processes at different \(\sqrt{s}\), are evaluated. The ratios are constructed using previously published ATLAS measurements of the \({t\overline{t}}\) and \(Z\)-boson production cross sections, corrected to a common phase space where required, and a new analysis of \(Z\) → ℓ\(^+\)ℓ\(^-\) where ℓ = \(e, µ\) at \(\sqrt{s}\) = 13 TeV performed with data collected in 2015 with an integrated luminosity of 3.2 fb\(^−1\). Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the \(Z\) → \(e\)\(^+\)\(e\)\(^−\) and the \(Z\) → \(µ\)\(^+\)\(µ\)\(^−\) channels for each \(\sqrt{s}\) value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-\(x\) values near 0.1 and the light-quark sea for \(x\) < 0.02.
A measurement of the \(ZZ\) production cross section in the \(ℓ^−ℓ^+ℓ^{′−}ℓ^{′+}\) and \(ℓ^−ℓ^+{ν\overline{ν}}\) channels (ℓ = e, µ) in proton-proton collisions at \(\sqrt{s}\) = 8TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb\(^{−1}\) collected by the ATLAS experiment in 2012 is presented. The fiducial cross sections for \(ZZ\) → \(ℓ^−ℓ^+ℓ^{′−}ℓ^{′+}\) and \(ZZ\) → \(ℓ^−ℓ^+{ν\overline{ν}}\) are measured in selected phase-space regions. The total cross section for \(ZZ\) events produced with both \(Z\) bosons in the mass range 66 to 116GeV is measured from the combination of the two channels to be 7.3 ± 0.4(stat) ± 0.3 (syst)\(^{−0.2}_{−0.1}\) (lumi) pb, which is consistent with the Standard Model prediction of 6.6\(^{+0.7}_{−0.6}\) pb. The differential cross sections in bins of various kinematic variables are presented. The differential event yield as a function of the transverse momentum of the leading \(Z\) boson is used to set limits on anomalous neutral triple gauge boson couplings in \(ZZ\) production.
A measurement of the \({t\overline{t}}Z\) and \({t\overline{t}}W\) production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton–proton collisions at \(\sqrt{s}\) = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb\(^{−1}\). The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in \(\sigma_{{t\overline{t}}Z}\) = 0.9 ± 0.3 pb and \(\sigma_{{t\overline{t}}W}\) = 1.5 ± 0.8 pb, in agreement with the Standard Model predictions.