187 Search Results

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (𝑝_T) dependence of radial flow fluctuations (𝑣₀⁡(𝑝_T)) over 0.5 < 𝑝_T < 10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at $$\sqrt{𝑠_{NN}}$$ = 5.02 TeV. The data revealmore » three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in 𝑝_T, and centrality-independent shape in 𝑝_T. The comparison with a hydrodynamic model demonstrates the sensitivity of 𝑣₀⁡(𝑝_T) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.« less

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair ($$t\overline{t}$$) invariant mass in proton-proton collisions using 140 fb⁻¹ of data at $$\sqrt{s}=13$$ TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the $$t\overline{t}$$ invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The $$t\overline{t}$$ system is reconstructed in the single-lepton final state, requiring exactly onemore » isolated electron or muon and at least four jets with at least two identified as originating from b-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of Y_t < 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.« less

Jet flavour tagging enables the identification of jets originating from heavy-flavour quarks in proton–proton collisions at the Large Hadron Collider, playing a critical role in its physics programmes. This paper presents GN2, a transformer-based flavour tagging algorithm deployed by the ATLAS Collaboration that represents a different methodology compared to previous approaches. Designed to classify jets based on the flavour of their constituent particles, GN2 processes low-level tracking information in an end-to-end architecture and incorporates physics-informed auxiliary training objectives to enhance both interpretability and performance. Its performance is validated in both simulation and collision data. The measured c-jet (light-jet) rejection inmore » data is improved by a factor of 3.5 (1.8) for a 70% b-jet tagging efficiency, compared to the previous algorithm. GN2 provides substantial benefits for physics analyses involving heavy-flavour jets, such as measurements of Higgs boson pair production and the couplings of bottom and charm quarks to the Higgs boson, and demonstrates the impact of advanced machine learning methods in experimental particle physics.« less

This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks — a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: e+light jet, μ+light jet, e+b-jet, and μ+b-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additionalmore » same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb⁻¹ at a centre-of-mass energy of $$\sqrt{s}=13$$ TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb⁻¹ at $$\sqrt{s}=13.6$$ TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark ($$\tilde{S}$$₁) production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded $$\tilde{S}$$₁ $$\tilde{S}$$₁ mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings y_de = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for y_sμ, y_be, and y_bμ couplings set to 3.5, respectively.« less

Higgs boson production cross-sections via gluon–gluon fusion and vector-boson fusion in proton–proton collisions are measured in the $$H\rightarrow WW^*\rightarrow \ell \nu \ell \nu$$ decay channel. The Large Hadron Collider delivered proton–proton collisions at a centre-of-mass energy of 13 TeV between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $$140\,\text {fb}^{-1}.$$ The total cross-sections for Higgs boson production by gluon–gluon fusion and vector-boson fusion times the $$H\rightarrow WW^*$$ branching ratio are measured to be $$12.4^{+1.3}_{-1.2}\,\text {pb}$$ and $$0.79^{+0.18}_{-0.16}\,\text {pb},$$ respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterisedmore » through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory.« less

A search for the dimuon decay of the Higgs boson is presented based on 𝑝⁢𝑝 collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 165 fb⁻¹ at $$\sqrt{s}$$ = 13.6 TeV. To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of 3.4⁢𝜎 (2.5⁢𝜎 expected). The best-fit signal strength is 𝜇 = 1.4 ± 0.4. This result provides evidence for the 𝐻 → 𝜇⁢𝜇 decay with ATLAS data and offers a direct probe of themore » Higgs-boson Yukawa coupling to second-generation fermions.« less

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 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 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