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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 109 records · Page 6Linked to original sources

arXiv2026

Precise measurement of the $t\bar{t}$ production cross-section and lepton differential distributions in $eμ$ dilepton events from $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

The inclusive top quark pair ($t\bar{t}$) cross-section $σ_{t\bar{t}}$ has been measured in $\sqrt{s}=13$ TeV proton-proton collisions, using 140 fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge $eμ$ pair and $b$-tagged jets, the cross-section is measured to be: $\begin{equation}\nonumber σ_{t\bar{t}} = 829.3 \pm 1.3\,\mathrm{(stat)}\ \pm 8.0\,\mathrm{(syst)}\ \pm 7.3\,\mathrm{(lumi)}\ \pm 1.9\,\mathrm{(beam)}\,\mathrm{pb}, \end{equation}$ where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the proton beam energy, giving a total uncertainty of 1.3%. The result is used to determine the top quark pole mass via the dependence of the predicted cross-section on $m_t^\mathrm{pole}$, giving $m_t^\mathrm{pole}=172.8^{+1.5}_{-1.7}$ GeV. The same event sample is used to measure absolute and normalised differential cross-sections for the $t\bar{t}\rightarrow eμν\barνb\bar{b}$ process as a function of single-lepton and dilepton kinematic variables. Complementary measurements of $eμb\bar{b}$ production, treating both $t\bar{t}$ and $Wt$ events as signal, are also provided. Both sets of differential cross-sections are compared to the predictions of various Monte Carlo event generators, demonstrating that the state-of-the-art generators Powheg MiNNLO and Powheg $bb4l$ describe the data better than Powheg hvq. The sensitivity of some of the measured differential distributions to quasi-bound-state formation near the $t\bar{t}$ threshold is investigated in an addendum.

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arXiv2026

Observation of $t\bar{t}γγ$ production at $\sqrt{s}=$13 TeV with the ATLAS detector

This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{t}γγ$). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb$^{-1}$ recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{t}γγ$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be $2.42^{+0.58}_{-0.53}\, \text{fb}$, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{t}γγ$ to top-quark pair production in association with one photon is determined, yielding $(3.30^{+0.70}_{-0.65})\times 10^{-3}$.

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arXiv2026

Search for squarks and gluinos in $pp$ collisions at $\sqrt{s} = 13$ TeV and $13.6$ TeV in events with $τ$-leptons, jets and missing transverse momentum using the ATLAS detector

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying $τ$-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a $τ$-slepton or a $τ$-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying $τ$-lepton and no other leptons, exactly one hadronically decaying $τ$-lepton and at least one other lepton, or two or more hadronically decaying $τ$-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 $\mathrm{fb}^{-1}$ and 51.8 $\mathrm{fb}^{-1}$ of $pp$ collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015-2018 at $\sqrt{s} = 13$ TeV and 2022-2023 at $\sqrt{s} = 13.6$ TeV, respectively. Gluino masses below $2.25$ TeV and squark masses up to $1.7$ TeV are excluded.

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arXiv2026

Measurement of the top-quark Yukawa coupling from $t\overline{t}$ production in the lepton+jets final state using $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

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 one 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.

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arXiv2026

Precision calibration of calorimeter signals in the ATLAS experiment using an uncertainty-aware neural network

The ATLAS experiment at the Large Hadron Collider explores the use of modern neural networks for a multi-dimensional calibration of its calorimeter signal defined by clusters of topologically connected cells (topo-clusters). The Bayesian neural network (BNN) approach not only yields a continuous and smooth calibration function that improves performance relative to the standard calibration but also provides uncertainties on the calibrated energies for each topo-cluster. The results obtained by using a trained BNN are compared to the standard local hadronic calibration and to a calibration provided by training a deep neural network. The uncertainties predicted by the BNN are interpreted in the context of a fractional contribution to the systematic uncertainties of the trained calibration. They are also compared to uncertainty predictions obtained from an alternative estimator employing repulsive ensembles.

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arXiv2026

Precision measurement of the $B^{0}$ meson lifetime using $B^{0} \rightarrow J/ψK^{*0}$ decays with the ATLAS detector

A measurement of the $B^0$ meson lifetime using $B^0 \to J/ψK^{*0}$ decays in data from 13 TeV proton-proton collisions with an integrated luminosity of 140 fb$^{-1}$ recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is $$ τ= 1.5053 \pm 0.0012 ~\mathrm{(stat.)} \pm 0.0035 ~\mathrm{(syst.)~ps}. $$ The average decay width, extracted from the effective lifetime and using a parameter from external sources, is $$ Γ_d = 0.6643 \pm 0.0005 ~\mathrm{(stat.)} \pm 0.0016 ~\mathrm{(syst.)}~\mathrm{ps}^{-1}, $$ where the uncertainties are statistical and systematic. The uncertainty due to external sources is negligible at the given precision. The earlier ATLAS measurement of $Γ_s$ in the $B^0_s \to J/ψϕ$ decay was used to derive a value for the ratio of the average decay widths $Γ_d$ and $Γ_s$ for $B^0$ and $B^0_s$ mesons respectively, of $$ \frac{Γ_d}{Γ_s} = 0.9910 \pm 0.0022 ~\mathrm{(stat.)} \pm 0.0036 ~\mathrm{(syst.)}. $$ The measured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the $B^0$ meson to date.

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arXiv2026

Transforming jet flavour tagging at ATLAS

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 in 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.

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arXiv2026

Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector

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, experimental evidence for the global and collective nature of radial flow has been lacking. This Letter presents the first measurement of transverse momentum ($p_{\mathrm{T}}$) dependence of radial flow fluctuations ($v_0(p_{\mathrm{T}})$) over $0.5<p_{\mathrm{T}}<10$ GeV, using a two-particle correlation method in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in $p_{\mathrm{T}}$, and centrality-independent shape in $p_{\mathrm{T}}$. The comparison with a hydrodynamic model demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ 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.

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arXiv2026

Searches for unusual signatures from dark sectors with the ATLAS Experiment

Various theories beyond the Standard Model predict unusual signatures, including new, long-lived particles decaying at a significant distance from the collision point. These unique signatures are difficult to reconstruct and face unusual and challenging backgrounds. This contribution will focus on recent results from searches for unusual signatures motivated by dark sector models, utilising pp collision data collected by the ATLAS experiment at the Large Hadron Collider.

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arXiv2026

Search for single production of vector-like quarks decaying into $W(\ellν)b$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for single production of a vector-like quark $Q$, which could be either a singlet $T$, with charge $\tfrac23$, or a $Y$ from a $(T,B,Y)$ triplet, with charge $-\tfrac43$, 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 $Q \to Wb$ decays where the $W$ boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are 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.

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arXiv2026

Test of $CP$ Invariance in Higgs Boson Vector-Boson-Fusion Production Using the $H\toγγ$ Channel with the ATLAS Detector

A test of $CP$ invariance in Higgs boson production via vector-boson fusion has been performed in the $H\rightarrowγγ$ channel using 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13\,\mathrm{TeV}$ collected by the ATLAS detector at the LHC. The Optimal Observable method is used to probe the $CP$ structure of interactions between the Higgs boson and electroweak gauge bosons, as described by an effective field theory. No sign of $CP$ violation is observed in data. Constraints are set on the parameters describing the strength of the $CP$-odd component in the coupling between the Higgs boson and the electroweak gauge bosons in two effective field theory bases: $\tilde{d}$ in the HISZ basis and $c_{H\tilde{W}}$ in the Warsaw basis. The results presented are the most stringent constraints on $CP$ violation in the coupling between Higgs and weak bosons. The 95% CL constraint on $\tilde{d}$ is derived for the first time and the 95% CL constraint on $c_{H\tilde{W}}$ has been improved by a factor of 5 compared to the previous measurement.

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arXiv2026

Search for a new pseudoscalar decaying into a pair of bottom and antibottom quarks in top-associated production in $\sqrt{s}$=13 TeV proton-proton collisions with the ATLAS detector

A search for a pseudoscalar $a$ produced in association with a top-quark pair, or in association with a single top quark plus a $W$ boson, with the pseudoscalar decaying into $b$-quarks ($a\rightarrow b\bar{b}$), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12-100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard $b$-tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio BR($a\rightarrow b\bar{b}$)=100$\% $, the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95$\% $ confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95$\% $ confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.

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arXiv2026

Search for resonant leptoquark production via lepton-jet signatures in $pp$ collisions at $\sqrt{s} = 13$ TeV and $\sqrt{s} = 13.6$ TeV with the ATLAS detector

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 additional 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 $\tilde{S}_1$ 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.

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arXiv2026

Weakly supervised anomaly detection for resonant new physics in the dijet final state using proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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 estimate from data. The 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.

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arXiv2025

Measurement of inclusive dijet cross-sections in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Inclusive dijet cross-sections have been measured in proton-proton collisions at a centre-of-mass energy of 13 TeV using data with an integrated luminosity of 140 fb$^{-1}$, recorded by the ATLAS detector at the Large Hadron Collider during 2015-2018. Jets are identified using the anti-$k_{t}$ algorithm with a radius parameter of $R = 0.4$. The inclusive dijet double-differential cross-sections are measured first as a function of the invariant dijet mass and the half absolute rapidity separation between the two leading jets, $(m_{\mathrm{jj}}$, $y^{\ast})$, and second as a function of the invariant dijet mass and the total longitudinal boost of the dijet system, $(m_{\mathrm{jj}}$, $y_{\mathrm{boost}})$. The measured dijet system covers the invariant mass range from 240 GeV to almost 10 TeV, with dijet separation $y^{\ast} < 3.0$ and dijet boost $y_{\mathrm{boost}} < 3.0$. The results are unfolded to the particle level and compared with state-of-the-art next-to-next-to-leading-order full colour perturbative QCD calculations, corrected for non-perturbative and electroweak effects.

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arXiv2025

Estimation of backgrounds from jets misidentified as $τ$-leptons using the Universal Fake Factor method with the ATLAS detector

Processes with $τ$-leptons in the final state are important for Standard Model measurements and searches for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider observes $τ$-leptons produced in proton-proton collisions only through their decay products. Data analyses involving hadronically decaying $τ$-leptons face challenges due to backgrounds from jets misidentified as $τ$-leptons that are not modelled reliably by Monte Carlo simulations. Data-driven methods such as the fake-factor method allow such misidentified backgrounds to be predicted by measuring transfer factors, known as fake factors, in data from dedicated regions. This paper describes a refined technique for determining the fake factors, the Universal Fake Factor method. It evaluates the fake factors for a signal region by using fake factors from samples enriched in different sources of jets misidentified as $τ$-leptons (light-quark, gluon, $b$-quark, and pile-up jets). Each fake factor is calculated as a linear combination of fake factors measured in these different enriched samples. For the full Run 2 data set, the systematic uncertainty of the calculated fake factors, evaluated using $W(μν)$ enriched event sample, ranges from 15% to 35% depending on the $τ$-lepton's transverse momentum and charged-particle decay multiplicity.

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arXiv2025

Calibration of the jet energy scale and resolution of small-radius jets using semileptonic $t\bar{t}$ events with the ATLAS detector

A measurement of correction factors for the hadronic jet energy scale and resolution in the ATLAS detector is presented. These correction factors account for differences between simulated and observed data. They are obtained by analysing a selection of top quark events collected in proton-proton collisions by ATLAS between the years 2015 and 2018 at a centre-of-mass energy $\sqrt{s} = 13$ TeV as well as in 2022 and 2023 at $\sqrt{s} = 13.6$ TeV. The forward-folding technique is used to quantify the impact of different jet energy scale or resolution corrections on the reconstructed mass of the hadronically decaying $W$ boson from top-quark decays in simulation. The correction factors are extracted from a fit to the parameterised reconstructed $W$-boson mass distribution to data. The energy scale and resolution corrections are measured as a function of the jet transverse momentum between 20 GeV and 200 GeV and absolute pseudorapidity less than 0.8. The uncertainties in the energy scale range from about 0.93% to about 1.7% for jets between 35 and 200 GeV, while for the energy resolution the uncertainties range from about 14% to 28%. The method presented will be used in conjunction with other techniques to further improve ATLAS jet energy scale and resolution precision.

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arXiv2025

Search for ttbar resonances in final states with exactly one or two leptons using 140 fb$^{-1}$ of pp collision data at $\sqrt{s}=13$ TeV with the ATLAS experiment

A search for heavy spin-1 and spin-2 resonances decaying into a top-antitop-quark pair has been performed with 140 fb$^{-1}$ of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Final states with either exactly one electron or muon, or exactly two leptons ($ee$, $μμ$ or $eμ$), large missing transverse momentum, and two jets, at least one of which must be identified as likely containing a b-hadron decay, are considered. The search targets resonances with both narrow and broad widths relative to the detector resolution, and with masses in the range of 0.4-5.0 TeV. No significant deviation from the Standard Model prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for hypothetical $Z'$ bosons, Kaluza-Klein gravitons, and Kaluza-Klein gluons that decay into top-quark pairs.

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