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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 55 records · Page 3Linked to original sources

arXiv2026

Evidence for longitudinally polarised $Z$ bosons in electroweak $W^{\pm}Z$ production in association with two jets from $pp$ collisions at $\sqrt{s} = $ 13 TeV and 13.6 TeV with the ATLAS detector

Measurements of polarisation states of $W$ and $Z$ gauge bosons in electroweak $W^{\pm}Z$ production in association with two jets ($WZjj\mathrm{-EW}$) in proton-proton collisions are presented. The data set used corresponds to integrated luminosities of 140 fb$^{-1}$ and 164 fb$^{-1}$ of proton-proton collisions at centre-of-mass energies of 13 TeV and 13.6 TeV, respectively, recorded by the ATLAS detector at the CERN Large Hadron Collider. The $W^{\pm}Z$ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The observed significance for the presence in data of $WZjj\mathrm{-EW}$ events containing a longitudinally polarised $Z$ boson is of 4.0 standard deviations. The measured fraction of such events, integrated over the fiducial region, is $f_{\mathrm{X0}} = 0.32 \pm 0.09$, in agreement with the Standard Model prediction at next-to-leading-order in electroweak corrections. Upper limits at 95% confidence level of 0.19 and 0.12 are set on the fraction of events with a $W$ boson longitudinally polarised and on the fraction of events with the two bosons longitudinally polarised, respectively. The integrated fiducial cross-section per lepton flavour of $WZjj\mathrm{-EW}$ production at $\sqrt{s} = 13.6$ TeV is also measured to be $σ_{WZjj\mathrm{-EW} \rightarrow \ell^{'} ν\ell \ell jj} = 0.374 \; \pm 0.077 (\mathrm{stat.}) \; \pm 0.053 (\mathrm{syst.}) \; \pm 0.009 (\mathrm{lumi.}) \; \mathrm{fb}$, where $\ell$ and $\ell^{'}$ are either an electron or a muon. This measurement is in agreement with the Standard Model expectation.

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arXiv2026

Measurement of the $|V_{cb}|$ element of the CKM matrix in $t\bar{t}$ decays with the ATLAS detector

The first measurement of the magnitude of the CKM quark mixing matrix element $|V_{cb}|$ using on-shell $W$-boson decays is presented. The measurement is made with $t\bar{t}$ events from $pp$ collision data corresponding to an integrated luminosity of $140$ fb$^{-1}$ at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. A value of: $$|V_{cb}| = (50^{+11}_{-14})\times10^{-3}$$ is measured, where the uncertainties have roughly equal contributions from the limited size of the data sample and from systematic effects. This is consistent with existing measurements made at much lower energy scales in $B$ hadron decays. While the sensitivity does not yet rival that of previous determinations, this novel approach probes a very different physical situation, namely the rare hadronic $t\rightarrow b\bar{b} c$ decay at high momentum-transfer, rather than semileptonic $B$ decays at low momentum-transfer.

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arXiv2026

Search for new scalars via $X \rightarrow SH \rightarrow b\bar{b}b\bar{b}$ in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search is performed for a scalar resonance X decaying into a Standard Model Higgs boson H and an additional scalar S, each decaying into a bottom-antibottom-quark pair, giving a $b\bar{b}b\bar{b}$ final state. Two event topologies are considered, depending on whether the Higgs boson is reconstructed from a single large-radius jet or from a pair of small-radius jets. The analysis uses proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2, corresponding to an integrated luminosity of 126-136 fb$^{-1}$ depending on the topology and the associated triggers. The search covers masses of 300 GeV-6 TeV for X and 70 GeV-5 TeV for S. The dominant multijet and $t\bar{t}$ backgrounds are estimated with a hierarchical model using Gaussian processes and normalizing flows. No significant excess over the Standard Model background expectation is observed. Upper limits at 95% confidence level are set on the production cross-section times branching ratio, ranging from 0.7 fb to 2.6 pb.

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arXiv2026

Measurement of the azimuthal anisotropy of charged particles in $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV $^{16}$O$+^{16}$O and $^{20}$Ne$+^{20}$Ne collisions with the ATLAS detector

This paper presents the first measurements of the azimuthal anisotropy coefficients $v_{n}$, which quantify the $n^{\mathrm{th}}$-order Fourier modulation of charged-particle azimuthal distributions, for $n=2$-4 in $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV $\mathrm{^{16}O}+\mathrm{^{16}O}$ and $\mathrm{^{20}Ne}+\mathrm{^{20}Ne}$ collisions recorded with the ATLAS detector at the Large Hadron Collider in 2025. The $v_{n}$ coefficients are measured as a function of transverse momentum ($p_{\mathrm{T}}$), collision centrality, and event multiplicity. They are extracted using two complementary methods: two-particle correlations with a template-fit subtraction of short-range non-flow contributions, and four-particle subevent cumulants, which intrinsically suppress non-flow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy $v_{2} > v_{3} > v_{4}$ and a non-monotonic dependence on $p_{\mathrm{T}}$, reaching a maximum around 2 GeV, consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced $v_{2}$ in central $\mathrm{^{20}Ne}+\mathrm{^{20}Ne}$ collisions, consistent with theory expectations based on the predicted prolate deformation of neon nuclei, in contrast to the slightly tetrahedral structure predicted for oxygen. The four-particle cumulant results highlight strong event-by-event fluctuations and provide the greatest sensitivity to nuclear shape effects. These measurements can place new constraints on the initial geometry and the hydrodynamic response in light-ion collisions, offering valuable input for models of nuclear structure.

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arXiv2026

Search for long-lived particles using displaced vertices of oppositely charged leptons in 140 fb$^{-1}$ of pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search is presented for long-lived particles decaying into an oppositely charged lepton pair, $μ^{+}μ^{-}$, $e^{+}e^{-}$, or $e^{\pm}μ^{\mp}$, that form a vertex within the inner tracking system of the ATLAS detector at the Large Hadron Collider, displaced from the primary proton-proton interaction region. The analysis uses the 140 fb$^{-1}$ of Run-2 data collected at $\sqrt{s}=13$ TeV by the ATLAS experiment in 2015-2018. The results of the analysis are interpreted in the context of three benchmark models covering masses from 0.1 to 2.2 TeV and a range of mean proper lifetimes times the speed of light from 1 to 10000 mm. The first model is a generic $Z'$ boson pair-produced by a new heavy scalar, with the $Z'$ decaying into lepton pairs. The remaining two models are $R$-parity violating supersymmetric models in which the lightest neutralino $\tildeχ^{0}_{1}$ decays into $\ell^{+}\ell^{'-}ν$ ($\ell, \ell^{'} = e$, $μ$). The models differ by the mode of production of the $\tildeχ^{0}_{1}$, which can be produced via the decay of pairs of gluinos or of pairs of charginos and neutralinos ($\tildeχ_{1}^{\pm}\tildeχ_{1}^{0}$, $\tildeχ_{1}^{\pm}\tildeχ_{2}^{0}$, or $\tildeχ_{2}^{0}\tildeχ_{1}^{0}$). Although each benchmark sample includes pair-produced LLPs, only a single vertex is required to be reconstructed. No dilepton displaced vertex candidate is observed and the results are presented as upper limits on the production cross-sections. This analysis sets leading limits on the production cross-sections for multiple models, including parameter space that has never been directly probed.

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arXiv2026

Search for decays of the Higgs boson into scalar particles decaying into four or six $b$-quarks using $pp$ collisions at $\sqrt{s}= 13\,\mathrm{TeV}$ with the ATLAS detector

A search for exotic decays of the Higgs boson $H$ into new scalar or pseudoscalar particles that subsequently decay into $b$-quarks is presented. The search considers $ZH$ production with several decay scenarios for the Higgs boson: first to a pair of identical scalars, $H\rightarrow 2a \rightarrow 4b$, second to a pair of scalars with different masses ($m_{a1}<m_{a2}$), either directly, $H\rightarrow a_1a_2 \rightarrow 4b$, or via a longer decay chain, $H \rightarrow a_1a_2 \rightarrow 3a_1 \rightarrow 6b$. The analysis uses proton-proton collision data at $\sqrt{s} = 13$ TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140~\mathrm{fb}^{-1}$. No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross-section to the SM prediction times the branching ratio of Higgs bosons decaying into $4b$ or $6b$, between 4% and 25% for $σ(ZH)/σ_{\text{SM}}(ZH) \times \mathcal{B}$($H \rightarrow 2a \rightarrow 4b$), between 24% and 38% for $σ(ZH)/σ_{\text{SM}}(ZH) \times \mathcal{B}$($H \rightarrow a_1a_2 \rightarrow 4b$), and between 10% and 20% for $σ(ZH)/σ_{\text{SM}}(ZH) \times \mathcal{B}$($H \rightarrow a_1a_2 \rightarrow 3a_1 \rightarrow 6b$), depending on the masses of the scalar particles.

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arXiv2026

Electroweak diboson production in association with a high-mass dijet system in semileptonic final states from $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

This paper reports the observation of electroweak diboson ($WW/WZ/ZZ$) production in association with a high-mass dijet system, in which final states with one boson decaying leptonically and the other boson decaying hadronically are studied. The hadronically decaying $W/Z$ boson is reconstructed as either two small-radius jets or one large-radius jet with jet substructure requirements. The data analyzed correspond to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV collected with the ATLAS detector during the 2015-2018 data taking at the Large Hadron Collider. The electroweak production of $WW/WZ/ZZ$ in association with two jets is observed in a phase space dominated by vector-boson scattering with a significance of $7.4σ$ (expected $6.1σ$) and the signal strength is determined to be $1.28^{+0.23}_{-0.21}$. The corresponding production cross section in a fiducial phase space is measured in addition. The signal strengths of both electroweak and QCD associated diboson productions are furthermore measured in a two-dimensional fit, the result of which agrees with the Standard Model prediction. The data are interpreted in the context of a dimension-8 effective field theory to probe anomalous quartic gauge couplings resulting in the first set of exclusion limits on the Wilson coefficients in the semileptonic channel reported by the ATLAS Collaboration. The observed limits for the S02, T0 and M0 operators are $(-3.96 < f_{S02} / Λ^4 < 3.96)$ TeV$^{-4}$, $(-0.25 < f_{T0} / Λ^4 < 0.22)$ TeV$^{-4}$, $(-1.26 < f_{M0} / Λ^4 < 1.25)$ TeV$^{-4}$.

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arXiv2026

Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

A search is presented for emerging jets using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the ATLAS experiment between 2015 and 2018. The search looks for the existence of a dark sector with symmetries similar to those in quantum chromodynamics. This dark sector is populated with dark quarks, which undergo showering similar to quarks in the Standard Model, leading to a high multiplicity of long-lived dark hadrons within a dark jet. These dark hadrons subsequently decay to Standard Model particles via a new heavy scalar mediating particle $ϕ$. This results in jets which contain multiple displaced vertices, known as emerging jets. This analysis targets four-jet topologies, with two emerging jets and two Standard Model jets, resulting from the decay of pair-produced scalar mediators. No significant excess above the Standard Model background is observed. For dark pion proper decay lengths of 20 mm, mediator masses are excluded between 1 TeV and 2 TeV assuming a dark pion mass of 20 GeV.

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arXiv2026

Observation of pair production of longitudinally polarized $Z$ bosons in four-lepton final state with the ATLAS detector

The production of two longitudinally polarized $Z$ bosons is measured using proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy $\sqrt{s}=13.6$ TeV and with an integrated luminosity of 164 fb$^{-1}$. The $ZZ$ candidate events are reconstructed using two same-flavor opposite-charge electron or muon pairs. The polarization fractions of the $Z$ boson pair are measured inclusively in the fiducial phase space and also in two regions of the four-lepton invariant mass, and the results are consistent with the Standard Model predictions. After a statistical combination with a previous measurement at $\sqrt{s}=13$ TeV, the first observation of the production of two simultaneously longitudinally polarized $Z$ bosons is obtained, with observed and expected significances of 6.5 and 5.6 standard deviations, respectively.

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arXiv2026

Combined effective field theory interpretation of measurements sensitive to quartic gauge boson couplings in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A combination of measurements sensitive to anomalous quartic electroweak gauge boson couplings is presented using proton-proton collision data collected by the ATLAS detector at $\sqrt{s} = 13$ TeV at the LHC. Contributing analyses include measurements of vector-boson scattering in numerous final states as well as a tri-boson measurement. The combined measurement is used to constrain anomalous electroweak boson quartic self-couplings that result from dimension-8 operators in the Éboli model using an effective field theory. Results are presented as 68% and 95% confidence level intervals parameterised by one or two Wilson coefficients, both with and without unitarity constraints applied. Theoretical bounds from unitarity and positivity are overlaid where relevant. Confidence intervals obtained from simultaneous profiled fits to all Wilson coefficients are also presented.

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arXiv2026

Measurement of high-mass $t\bar{t}\ell^{+}\ell^{-}$ production and lepton flavour universality-inspired effective field theory interpretations at $\sqrt{s}=13$ TeV with the ATLAS detector

Measurements of $t\bar{t}\ell^{+}\ell^{-}$ production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on $\sqrt{s} = 13$ TeV proton-proton collision data with an integrated luminosity of $140\,\mathrm{fb}^{-1}$, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the $t\bar{t}\ell^{+}\ell^{-}$ signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including to test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on relevant operators.

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arXiv2026

Measurements of differential cross-sections of $WbWb$ production in the dilepton channel in $pp$ collisions at $\sqrt{s}$ = 13 TeV using the ATLAS detector

At the Large Hadron Collider, the $WbWb$ final state is expected to be dominated by $t\bar{t}$ production with a contribution from single-top processes. Differential cross-sections for $WbWb$ production in the dilepton decay channel are measured at the particle level as a function of various kinematic variables. The analysis is based on data from proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV, recorded by the ATLAS detector at the Large Hadron Collider over the period from 2015 to 2018, corresponding to an integrated luminosity of 140 fb$^{-1}$. Measurements are performed within the fiducial phase-space defined by the presence of two $b$-jets and one electron and one muon of opposite charges. The differential cross-sections are corrected for detector effects and unfolded to the particle level. Results are compared with predictions from Monte Carlo event generators at next-to-leading order in perturbative quantum chromodynamics. These measurements provide valuable constraints on the modelling of $WbWb$ production and the interference between doubly resonant and singly resonant $WbWb$ production.

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arXiv2026

Search for the Higgs boson decay to a $Z$ boson and a photon in $pp$ collisions at $\sqrt{s}=13$ TeV and $13.6$ TeV with the ATLAS detector

A search for the Higgs boson decay to a $Z$ boson and a photon in the $\ell\ellγ$ ($\ell = e, μ$) final state is performed using $pp$ collisions at $\sqrt{s}=13.6$ TeV recorded with the ATLAS detector at the Large Hadron Collider during 2022-2024, corresponding to an integrated luminosity of 165 fb$^{-1}$. The signal yield, normalised to the Standard Model prediction, is measured to be $μ=0.9^{+0.7}_{-0.6}$, compared to an expected value of $μ=1.0\pm0.7$. This corresponds to an observed (expected) signal significance of 1.4 (1.5) standard deviations for the background-only hypothesis. This result is combined with that of a similar search performed with 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collisions to provide the most stringent expected sensitivity to date to this rare decay, namely an observed (expected) signal strength of $μ= 1.3^{+0.6}_{-0.5}$ ($μ= 1.0^{+0.6}_{-0.5}$), corresponding to an observed (expected) significance of 2.5 (1.9) standard deviations. The measurement is consistent with the Standard Model expectation.

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arXiv2026

Simultaneous efficiency measurements of $b$- and $c$-jets in $t\bar{t}$ events from $\sqrt{s}=13.6$ TeV $pp$ collision data collected with the ATLAS detector

This paper reports simultaneous measurements of the efficiencies with which $b$-jets are identified, and $c$-jets are misidentified as $b$-jets, by the single-transformer-based GN2 tagger that is currently used in the ATLAS experiment at the LHC. The measurements use $t\bar{t}$ events where one of the $W$ bosons decays into an electron or muon and a neutrino, and the other decays into a quark-antiquark pair, together with the known $t\to bW$ and $W\to cs$ branching fractions. The data were collected from $\sqrt{s} = 13.6$ TeV proton-proton collisions by the ATLAS detector and correspond to an integrated luminosity of 56 fb$^{-1}$. Events are reconstructed using a kinematic likelihood technique which maps the jets to the $t\bar{t}$ decay products. The $b$-jet tagging efficiency is measured from jets arising from leptonically decaying top quarks and the $c$-jet mistagging efficiency is obtained from jets arising from hadronic $W$ decays. The efficiencies are measured as a function of jet transverse momentum in intervals of the tagging-variable distribution. The $b$-jet ($c$-jet) tagging efficiency measurements reach a precision of better than 2% (5%). Each $b$-jet efficiency is combined with an independent measurement using $t\bar{t}$ events where both $W$ bosons decay leptonically.

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arXiv2026

Measurement of $J/ψ$-jet correlations in $pp$ and Pb+Pb collisions at $\sqrt{s_{\text{NN}}}=5.02$ TeV with the ATLAS detector

Yields of charmonia, bound states of $c\bar{c}$ quarks, are observed to be strongly suppressed in heavy-ion collisions relative to proton-proton collisions as a result of their interaction with the quark-gluon plasma produced in such collisions. Understanding the mechanisms responsible for this suppression requires a detailed understanding of charmonium production. To constrain the production mechanisms, it is essential to quantify how charmonium production correlates with jet activity. Measurements of $J/ψ$ production in the dimuon decay channel are presented for inclusive $J/ψ$ mesons and for $J/ψ$ mesons that are either isolated or not isolated from jets, separately for prompt and non-prompt production. Non-isolated $J/ψ$ mesons are defined by matching $J/ψ$ candidates to anti-$k_t$ jets with $R=0.2$ and $p_\mathrm{T}>20$ GeV, reconstructed without signal from the $J/ψ$ meson, allowing the quantification of jet activity accompanying $J/ψ$ production. The analysis uses Pb+Pb and $pp$ collisions at $\sqrt{s_{\text{NN}}}=5.02$ TeV recorded by the ATLAS experiment at the LHC, corresponding to integrated luminosities of 1.82 nb$^{-1}$ and 0.26 fb$^{-1}$, respectively. The $J/ψ$ yields in Pb+Pb, cross sections in $pp$, and nuclear modification factors are presented for non-isolated, isolated, and inclusive $J/ψ$ up to $p_\mathrm{T}=60$ GeV. Isolated fractions and non-prompt fractions are reported in both $pp$ and Pb+Pb collisions. Strong $J/ψ$ suppression is observed to persist up to the highest measured $p_\mathrm{T}$ values, with differences observed between non-isolated, isolated, and inclusive $J/ψ$. These results provide new constraints on charmonium production mechanisms and their in-medium suppression.

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arXiv2026

Search for dimuon resonance in the 35 to 75 GeV mass range using 140 fb$^{-1}$ of 13 TeV $pp$ collisions with the ATLAS detector

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb$^{-1}$, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.

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arXiv2026

Search for higgsinos in compressed mass spectra using low-momentum tracks in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper presents two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events are required to feature an energetic jet, large missing transverse momentum, and at least one low-momentum charged particle that serves as a candidate higgsino decay product. In the first search, targeting higgsino mass splittings in the range of 0.3-1 GeV, the higgsinos are expected to predominantly decay into pions that are identified as low-momentum charged particles with large transverse impact parameters due to the long higgsino lifetime ($cτ\approx\mathcal{O}$(0.1-10 mm)), and neural networks are used to discriminate between signal and background processes. The second search targets larger mass splittings in the range of 1-3 GeV, where the higgsinos are expected to decay promptly into low-momentum leptons, one of which is identified by dedicated low-momentum electron or muon taggers based on neural networks utilising tracking and calorimeter information. No significant excess above the Standard Model prediction is observed in either search and the results are interpreted within simplified models, to set lower limits on the masses of the higgsino-like charginos and neutralinos. Together, these searches exclude chargino masses below 126 GeV at 95% confidence level for mass splittings between the chargino and lightest neutralino in the range of 0.3-2 GeV. This represents the first ATLAS constraints in a portion of this parameter space and surpasses the limits previously set by other experiments.

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arXiv2026

Observation of structures in the $J/ψ+ψ(2S)$ mass spectrum with the ATLAS detector

A search for resonant structures in the $J/ψ+ψ$(2S) mass spectrum is performed using proton--proton collision data at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 140 fb$^{-1}$, recorded by the ATLAS experiment at the LHC. The decay channels of $J/ψ+ψ\text{(2S)} \rightarrow 4μ$ and $J/ψ+ψ\text{(2S)} \rightarrow 4μ+2π$ are analyzed. An excess near 6.9 GeV is observed in both channels with a combined significance of 8.9$σ$. No significant signal is observed near 7.2 GeV, and an upper limit on its yield relative to $X$(6900) is provided. A simultaneous fit with the di-$J/ψ$ channel is carried out under assumptions regarding the resonance interferences, yielding a ratio of the partial decay widths between the $J/ψ+ψ\text{(2S)}$ and di-$J/ψ$ channels of $1.08\pm 0.20_{-0.17}^{+0.40}$ for the resonance near 6.9 GeV.

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