63 Search Results

A measurement of the top-quark pole mass $$m$$$^{pole}_{t}$$ is presented in $$t\bar{t}$$ events with an additional jet, $$t\bar{t}$$+ 1-jet, produced in pp collisions at $$\sqrt{s} = 13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb⁻¹. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $$t\bar{t}$ + 1-jet differential cross-section as a function of the inverse of the invariant mass of the $$t\bar{t}$$ + 1-jet system. Iterative Bayesian Unfolding is used to correct the datamore » to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp → $$t\bar{t}$$j(2 → 3), where top quarks are taken as stable particles, and the process pp → $$b\bar{b}$$l⁺ vl^–$$\overline{ν}$$j (2 → 7), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ² fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check.« less

A search for single production of a vector-like quark Q, which could be either a singlet T, with charge $$\frac{2}{3}$$, or a Y from a (T, B, Y) triplet, with charge $$-\frac{4}{3}$$, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb⁻¹ recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis targets Q → Wb decays where the W boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits aremore » set on the cross-section times branching ratio, and on the coupling of the Q to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet T, the 95% confidence level limit on the coupling strength κ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the (T, B, Y) triplet, the limits on κ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.« less

This Letter reports the observation of W⁺W^–γ triboson production in 140 fb^–1 of data collected by the ATLAS detector from proton–proton collisions at a centre-of-mass energy of $$\sqrt{s}$$ = 13 TeV at the LHC. Events with an opposite-charge eμ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W⁺W^–γ → e^±μ^∓$$ν\bar{ν}$$γ final state is 6.2  ±  0.8 (stat.)  ±  0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1$$^{+1.0}_{–0.7}$$ fb. Constraints on the Wilson coefficientsmore » of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.« less

This article reports on a search for dijet resonances using 132  fb⁻¹ of 𝑝⁢𝑝 collision data recorded at $$\sqrt{𝑠}$$ = 13  TeV by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of 𝑝_T ≥ 85  GeV and jet rapidity separation of |𝑦*| <0.6 are analysed for dijet resonances with invariant masses from 375 to 1800  GeV. A data-driven background estimate is used tomore » model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at 95% confidence level on coupling values for a benchmark leptophobic axial-vector 𝑍′ model and on the production cross section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.« less

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark (U₁) as predicted by an ultraviolet-complete extension of the SM, referred to as the ‘4321’ model. Given the assumed decay of U₁ into third-generation SM fermions, the final state can contain multiple τ-leptons and b-quarks. This search is based on a dataset of pp collisions at $$\sqrt{s}=13$$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to $$140~\textrm{fb}^{-1}$$. No significant excess above the SM backgroundmore » prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an R-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).« less

A calibration of the ATLAS flavour-tagging algorithms using a new calibration procedure based on optimal transportation maps is presented. Simultaneous, continuous corrections to the b-jet, c-jet, and light-flavour jet classification probabilities from jet-tagging algorithms in simulation are derived for b-jets using $$t\bar{t} \rightarrow e\mu \nu \nu bb$$ data. After application of the derived calibration maps, closure between simulation and observation is achieved for jet flavour observables used in ATLAS analyses of Large Hadron Collider (LHC) Run 2 proton-proton collision data. This continuous calibration opens up new possibilities for the future use of jet flavour information in LHC analyses and alsomore » serves as a guide for deriving high-dimensional corrections to simulation via transportation maps, an important development for a broad range of inference tasks.« less

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$$^{-1}$$ of Pb+Pb data and 255 pb$$^{-1}$$ of $pp$ data, both at $$\sqrt{s_{_\mathrm{NN}}} = 5.02$$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentummore » above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($$R_\mathrm{AA}$$). The jet $$R_\mathrm{AA}$$ is measured as a function of jet $$p_{\mathrm{T}}$$, the charged $$k_t$$ splitting scale ($$\sqrt{d_{12}}$$), and the angular separation ($$dR_{12}$$) of two leading sub-jets. The jet $$R_\mathrm{AA}$$ gradually decreases with increasing $$\sqrt{d_{12}}$$, implying significantly stronger suppression of large-radius jets with larger $$k_t$$ splitting scale. The jet $$R_\mathrm{AA}$$ gradually decreases for $$dR_{12}$$ in the range $$0.01{-}0.2$$ and then remains consistent with a constant for $$ΔR_{12} \gtrsim 0.2$$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.« less

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using 140 fb⁻¹ of proton-proton collision data at $$\sqrt{𝑠}$$ =13 TeV recorded in 2015–2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicatedmore » analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than 𝒪⁡(10) cm.« less

Measurements of integrated and differential cross-sections for W^±Z production in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of $$\sqrt{s}=13$$ TeV are used, corresponding to an integrated luminosity of 140 fb⁻¹. The W^±Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons or muons. The integrated cross-section per lepton flavour for the production of W^±Z is measured in the detector fiducial region with a relative precision of 4%. The measured value is compared with the Standard Model prediction atmore » a precision of up to next-to-next-to-leading-order in QCD and next-to-leading-order in electroweak. Cross-sections for W⁺Z and W⁻Z production and their ratio are presented. The W^±Z production is also measured differentially as functions of various kinematic variables, including new observables sensitive to CP-violation effects. All measurements are compared with state-of-the-art Standard Model predictions from fixed-order calculations or Monte Carlo generators based on next-to-leading-order matrix elements interfaced with parton showers. An effective field theory interpretation of the measurements is performed, considering both CP-conserving and CP-violating dimension-6 operators modifying the W^±Z production. In the absence of observed deviations from the Standard Model, limits on CP-conserving Wilson coefficients are extracted using the transverse mass of the W^±Z system. For CP-violating coefficients a machine learning approach is used to construct an observable with enhanced sensitivity to CP-violation effects.« less

An anomaly detection search for narrow-width resonances beyond the Standard Model that decay into a pair of jets is presented. The search is based on 139 fb⁻¹ of proton-proton collisions at $$\sqrt{s}$$ = 13 TeV recorded during 2015–2018 with the ATLAS detector at the Large Hadron Collider. The analysis is optimized without a particular signal model and aims to be sensitive to a broad range of new physics. It uses two different machine learning strategies to estimate the background in different signal regions. In each region, a weakly supervised classifier is trained to distinguish this background model from data. Themore » analysis focuses on events with high transverse momentum jets reconstructed as large-radius jets. The mass and substructure of these jets are used as inputs to the classifiers. After a classifier-based selection, the distribution of the invariant mass of the two jets is used to search for potential local excesses. The model-independent results of both the anomaly detection methods show no signs of significant local excesses. In addition to model-independent results, a representative set of signal models is injected into the data, and the sensitivity of the methods to these scenarios is reported.« less