arXiv subjects

ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 19 recordsLinked to original sources

arXiv2026

Observation of single top-quark production in the $s$-channel in proton--proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The first single-experiment observation of single top-quark production in the $s$-channel is presented. Measurements of this process are essential to fully characterize the electroweak interactions of the top quark and probe for new physics, however its highly suppressed cross-section has so far caused it to evade direct detection at the Large Hadron Collider. The analysis uses proton--proton collision data collected at a center-of-mass energy of $\sqrt{s}=13$ TeV with the ATLAS detector, corresponding to an integrated luminosity of $140$ fb$^{-1}$. The selected events contain exactly one electron or muon, missing transverse momentum, and exactly two $b$-quark-initiated jets. A Graph Neural Network is utilized to separate the signal from the top-quark pair, $W$+jets, and $t$-channel single top-quark backgrounds. A likelihood fit to the network output yields an observed (expected) signal significance of $5.1$ ($5.2$) standard deviations. The measured cross-section of $σ_{t\bar{b}+\bar{t}b} = 10.8 \pm 1.5 \text{ (stat.)} \pm 1.6 \text{ (syst.)}\,\text{pb}$ is consistent with the Standard Model prediction.

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arXiv2026

Measurements of $Z$-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of $Z$ bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 TeV and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to $ZZ^*$ spin-density-matrix elements in the $H\rightarrow ZZ^* \rightarrow \ell^+\ell^-\ell^+\ell^-$ process yield coefficients $C_{2,1,2,-1} = -0.71 \pm 0.45$ and $C_{2,2,2,-2}=0.08 \pm 0.44$, consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected $4.9σ$) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.

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arXiv2026

Observation of a $B_c^{*+}$ meson with the ATLAS detector

The first observation of a new meson state decaying into $B_c^+$ and a photon is presented using a dataset of $pp$ collisions at a centre-of-mass energy $\sqrt{s} = 13$\,\TeV collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140\,fb$^{-1}$. The $B_c^+$ mesons are reconstructed from a three-muon final state from $B_c^+\to J/ψ(μ^+μ^-)μ^+ν_μX$ decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state $B_c^+$ meson is measured to be $64.5 \pm 1.4\text{(stat.)}^{+1.0}_{-1.4}\text{(syst.)}$ MeV. This corresponds to a mass for the observed state of $6339.0 \pm 1.4\text{(stat.)}^{+1.0}_{-1.4}\text{(syst.)}\pm 0.3\,\,(m_{B_c^+})$ MeV, where the last uncertainty is due to the precision of the best knowledge of the $B_c^+$ meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the $\bar{b}c$ system, the $B_c^{*+}$ meson.

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arXiv2026

Search for charged-lepton-flavour violating $eμtq$ interactions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for charged-lepton-flavour violation (cLFV) in top-quark production and decay is presented. The data analysed correspond to 140 fb$^{-1}$ of proton-proton collisions recorded at a centre-of-mass energy of $\sqrt{s}= 13$ TeV by the ATLAS detector at the Large Hadron Collider. The possible cLFV contribution is studied via two mechanisms. The first is via the production of a leptonically-decaying single-top quark in association with a pair of oppositely-charged different-flavour leptons. The second is the production of top-quark pairs, where one top quark decays into a pair of oppositely-charged different-flavour leptons and a $u$- or $c$-quark, while the other decays into a leptonically-decaying $W$ boson and a $b$-quark, according to the Standard Model. The signal processes contain three charged leptons, taken to be electrons or muons. No significant deviation from the Standard Model prediction is observed, and these results are interpreted within an Effective Field Theory framework. Upper limits are placed at 95% confidence level on six Wilson coefficients, ranging from $|C^{\mathrm{tensor}}_{eμtu}|/Λ^2 < 0.018$ TeV$^{-2}$ to $|C^{\mathrm{scalar}}_{eμtc}|/Λ^2 < 0.32$ TeV$^{-2}$ for the tensor $u$-quark and scalar $c$-quark couplings, respectively. These Wilson coefficient limits are translated into 95% confidence level upper limits on cLFV top-quark decay branching ratios, ranging from $B(t \to eμu) < 6 \cdot 10^{-9}$ to $B(t \to eμc) < 2.7 \cdot 10^{-7}$ for the scalar $u$-quark and tensor $c$-quark couplings, respectively.

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arXiv2026

Search for events with one displaced vertex from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in $pp$ collisions at $\sqrt{s}=13$ TeV

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 dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axion-like 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 photo-phobic axion-like particle model considered, this search produces the strongest limits to date for proper decay lengths greater than $\mathcal{O}(10)$ cm.

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arXiv2026

Search for compressed electroweak SUSY production in final states with two $τ$-leptons in $\sqrt{s} = 13$ and $13.6$ TeV $pp$ collisions with the ATLAS detector

Searches for the production of $τ$-sleptons ($\tildeτ$), charginos ($\tildeχ^{\pm}_1$), and neutralinos ($\tildeχ^0_2$) in events with at least one hadronically decaying $τ$-lepton are presented, targeting compressed mass spectra scenarios. For charginos and neutralinos, the cascade decays producing either a $τ$-slepton or a $τ$-sneutrino are considered. Two final selections are studied, requiring either one hadronically decaying $τ$-lepton and one other charged lepton, or two or more hadronically decaying $τ$-leptons. To enhance sensitivity to the compressed mass spectra, production modes with initial-state radiation (ISR) jets are considered. The searches use 140 $\mathrm{fb}^{-1}$ and 53 $\mathrm{fb}^{-1}$ of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015-2018 at a centre-of-mass energy of $\sqrt{s} = 13$ TeV and 2022-2023 at a centre-of-mass energy of $\sqrt{s} = 13.6$ TeV, respectively. The searches provide sensitivity to the compressed mass spectra scenarios with light $τ$-sleptons. The results are generally consistent with the Standard Model, although several deviations with local significances of $2.1-2.4 σ$ are observed. These deviations weaken the observed exclusion limits. For direct $τ$-slepton production, in the ISR channel alone the limits are set for $Δm(\tildeτ, \tildeχ^0_1) = 30$ GeV for $τ$-slepton masses up to $115$ GeV at 95% confidence level. Given the observed data, no exclusion limits are set for direct $τ$-sleptons production when statistically combining all considered channels. For electroweakino production, limits are set for $Δm(\tildeχ^0_2, \tildeχ^0_1) = 30$ GeV for $\tildeχ^0_2$ masses up to $200$ GeV. For pure chargino pair-production, limits are set for chargino masses up to $175$ GeV for the same mass splitting.

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arXiv2026

Search for quantum black holes in lepton+jet final states using proton-proton collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

A search for quantum black holes in electron+jet or muon+jet final states with high invariant mass is performed. The analysis uses data from $\sqrt{s}=13.6~\textrm{TeV}$ $pp$ collisions recorded by the ATLAS detector between 2022 and 2024 during Run~3 of the Large Hadron Collider, corresponding to an integrated luminosity of $164~\mathrm{fb}^{-1}$. This search is strongly motivated by a dramatic increase of the production cross-section by up to an order of magnitude for the highest masses considered, thanks to the small increase of $0.6~\textrm{TeV}$ in centre-of-mass energy between Run~2 and Run~3. No significant excess above the Standard Model background is observed, and 95\% CL upper limits are set on the production cross-section times branching ratio in several benchmark models, reaching a mass scale of $9.4~\textrm{TeV}$. These represent the strongest exclusion limits to date on quantum black hole production.

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arXiv2026

Search for a top-philic heavy resonance in association with top quarks in $pp$ collisions at $\sqrt{s} = $ 13 TeV and 13.6 TeV with the ATLAS detector

A search for a new top-philic vector boson ($Z'$) produced in association with a top-quark or a pair of top quarks, and decaying into a top quark pair is presented. The search uses $pp$ collision data collected by the ATLAS detector at the Large Hadron Collider at centre-of-mass energies of $\sqrt{s} =$ 13 TeV and 13.6 TeV, corresponding to integrated luminosities of 140 fb$^{-1}$ and 56 fb$^{-1}$, respectively. Events containing exactly two same-sign leptons or at least three leptons (electrons or muons) are selected. A transformer-based classifier parameterised as a function of the $Z'$ mass is used to optimise the signal-to-background discrimination. The results are interpreted in a simplified model with a colour-singlet vector boson coupled to top quarks with coupling strength $c_t$ and chirality angle $θ$. No significant excess is observed above the expected Standard Model background. A $Z'$ boson with mass below 1150 GeV is excluded at 95% confidence level for a benchmark scenario with $c_t = 1$ and $θ= 0$, with an expected limit of 1250 GeV.

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arXiv2026

Search for boosted vector boson scattering $W^{\pm}W^{\pm}H$ production in final states with same-sign leptons and $b$-jets in proton-proton collisions at $\sqrt{s}=$13 TeV with the ATLAS detector

A search for the electroweak production of a Higgs boson in association with a same-sign $W$ boson pair and two forward jets ($W^{\pm}W^{\pm}Hjj$) is presented. The full Run 2 proton-proton collision dataset recorded at a center-of-mass energy of $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider is used, corresponding to an integrated luminosity of 140 fb$^{-1}$. The search targets events where both $W$ bosons decay leptonically, yielding a final state with two same-sign charged leptons, missing transverse momentum, and a Higgs boson candidate. The high-momentum Higgs-boson candidate decaying into a $b\bar{b}$ pair is reconstructed as a single large-radius jet. This process provides direct sensitivity to the quartic $WWHH$ interaction. Possible deviations from the Standard Model prediction are parameterized through the coupling modifier $κ_{WW}$, where $κ_{WW}=1$ corresponds to the Standard Model. No significant excess over the Standard Model prediction is observed. Upper limits at the 95% confidence level are set on the $W^{\pm}W^{\pm}Hjj$ production cross-section as a function of $κ_{WW}$. Observed (expected) values of $κ_{WW}$ outside the interval $-2.1 < κ_{WW} < 4.1$ ($-1.9 < κ_{WW} < 3.9$) are excluded.

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arXiv2026

Differential cross-section measurements of coherent production of singly and doubly resonant top-quark in $WWbb$ events with one lepton at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This paper presents differential cross-section measurements of events containing a charged lepton, missing transverse momentum, two $b$-jets, and two light jets, consistent with the $W^{+}W^{-}b\bar{b}$ final state. The analysis is based on 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider (2015-2018). Differential cross-sections are unfolded at particle level for a range of kinematic observables and in three kinematic regions. Both regions and observables are optimised for sensitivity to the interference between $W^{+}W^{-}b\bar{b}$ produced with singly and doubly resonant top quarks. Results are compared with various theoretical predictions. Significant discrepancies are observed in regions sensitive to the interference between singly and doubly resonant top-quark production. The unfolded results provide new information about the modelling of interference and off-shell effects of the $W^{+}W^{-}b\bar{b}$ final state, improving the understanding of top-quark production mechanisms and providing new inputs for future Monte Carlo generator development.

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arXiv2026

Constraints on the Higgs boson total width from on-shell signal-background interference in the $H\toγγ$ decay channel with $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The total decay width of the Higgs boson, $Γ_H$, encodes all of its decay modes; any decay into particles beyond the Standard Model (SM) would increase $Γ_H$ above the SM prediction of 4.1 MeV. In this Letter, a direct, on-shell measurement of $Γ_H$ is reported, exploiting the interference between the resonant $H \to γγ$ amplitude and the continuum diphoton background, which distorts the diphoton invariant mass distribution in a way that depends on $Γ_H$ without requiring assumptions on the Higgs boson couplings. Using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded by the ATLAS detector during Run 2 of the LHC, an observed (expected) upper limit of $Γ_H < 307$ (258) MeV is set at 95% confidence level, approximately 75 (63) times the SM prediction. Unlike indirect off-shell constraints on $Γ_H$, this measurement does not rely on assumptions about the Higgs boson couplings, providing a novel complementary probe sensitive to potential Higgs boson decays into new particles.

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arXiv2026

Measurement of charged-particle production in $\sqrt{s_\text{NN}}=9.62$ TeV proton-oxygen collisions as a probe of cosmic-ray air showers with the ATLAS detector

This Letter presents a measurement of prompt charged-particle production in proton-oxygen interactions at $\sqrt{s_\text{NN}}=9.62$ TeV center-of-mass energy with the ATLAS detector, corresponding to 634 $μ$b$^{-1}$ of integrated luminosity. A total of 246 million selected events have at least one track with transverse momentum $p_\text{T}> 500$ MeV and pseudorapidity $|η|<2.5$. The measured fiducial proton-oxygen cross section is $σ_\text{fid.}^{p\text{O}}=396 \pm 6~(\text{exp.}) \pm 9~(\text{lumi.})~\text{mb}$ and the extrapolated inelastic proton-air cross section is $σ^{p+\text{air}}_\text{inel.} = 406 \pm 6~(\text{exp.}) \pm 9~(\text{lumi.}) \pm 28~(\text{th.})~\text{mb}$. Measurements of charged-particle multiplicity, $p_\text{T}$, and $η$ distributions are an order-of-magnitude more precise than differences between hadronic-interaction models. These results enable improved modeling of cosmic-ray air showers, which is important for astroparticle physics.

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arXiv2026

Evidence of $ZZγ$ Production with the ATLAS Detector

This Letter presents the first evidence of the simultaneous production of two $Z$ bosons and one photon with the ATLAS detector at the Large Hadron Collider. The measurement is performed using the full Run-2 dataset, recorded from 2015 to 2018, of proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 140 fb$^{-1}$. The fully leptonic final state with four leptons and one photon is analyzed, $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ with $\ell, \ell' = e$ or $μ$. This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of $p_{\mathrm{T}}^γ> 20$ GeV. Eight events are selected, with a background estimate of $0.92\pm0.15$. This results in an observed (expected) significance of the $ZZγ$ final state of $4.4σ$ ($4.0σ$). The measured cross-section for $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ in the fiducial region is $σ_{ZZγ} = 0.144 _{-0.051}^{+0.065} \text{ (stat.)} _{-0.004}^{+0.008} \text{ (syst.) fb}$, in agreement with the Standard Model prediction of $σ_{\textrm{fid.}}^{\textrm{SM}} = 0.130\pm0.006$ fb.

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arXiv2026

Search for charginos and neutralinos with $B-L$ $R$-parity violating decays in $\sqrt{s}=13$ TeV and $13.6$ TeV $pp$ collisions with the ATLAS detector

A search is performed for the electroweak pair production of charginos and associated production of a chargino and neutralino, each of which decays through an $R$-parity-violating coupling into a lepton and a $W$, $Z$, or Higgs boson. This search targets the Higgs boson decay channel of the charginos and neutralinos, using events with three or more $b$-tagged jets and one or two electrons or muons. The analyzed data corresponds to an integrated luminosity of 140 fb$^{-1}$ and 56 fb$^{-1}$ of proton-proton collision data produced by the Large Hadron Collider at center-of-mass energies of $\sqrt{s}=13$ TeV and $\sqrt{s}=13.6$ TeV respectively, collected by the ATLAS experiment between 2015 and 2023. The data are found to be consistent with predictions from the Standard Model. The results are interpreted as limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model. Limits are also set on the production of charginos and neutralinos for a Minimal Supersymmetric Standard Model with an additional $B-L$ gauge symmetry that is spontaneously broken. Charginos and neutralinos with masses between 150 GeV and 1100 GeV are excluded at 95% confidence level for a scenario in which they decay via Higgs bosons, assuming equal decay branching fractions to each lepton flavor. Additional limits on chargino and neutralino production are derived for flavor-specific decay scenarios where the charginos decay into a single lepton flavor.

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arXiv2026

Search for low-mass resonances decaying into two jets and produced in association with a photon or a jet at $\sqrt{s}=13$ TeV with the ATLAS detector

A search is performed for localized excesses in the low-mass dijet invariant mass distribution, targeting a hypothetical new particle decaying into two jets and produced in association with either a high transverse momentum photon or a jet. The search uses the full Run 2 data sample from LHC proton-proton collisions collected by the ATLAS experiment at a center-of-mass energy of 13 TeV during 2015-2018. Two variants of the search are presented for each type of initial-state radiation: one that makes no jet flavor requirements and one that requires both of the jets to have been identified as containing $b$-hadrons. No excess is observed relative to the Standard Model prediction, and the data are used to set upper limits on the production cross-section for a benchmark $Z'$ model and, separately, for generic, beyond the Standard Model scenarios which might produce a Gaussian-shaped contribution to dijet invariant mass distributions. The results extend the current constraints on dijet resonances to the mass range between 200 and 650 GeV.

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arXiv2026

Search for Higgs boson pair production in association with top-quark pairs using 196 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=$ 13 and 13.6 TeV with the ATLAS detector

This paper presents the first search for non-resonant Higgs boson pair production in association with a top-quark pair ($t\bar{t}HH$) using proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The data sample corresponds to an integrated luminosity of 196 fb$^{-1}$, comprising 140 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s}=13$ TeV and 56 fb$^{-1}$ at 13.6 TeV. The search targets three distinct final states expected from $t\Bar{t}HH$ decays: (i) one lepton (electron or muon) with at least five $b$-quarks, (ii) at least two $b$-quarks accompanied by two leptons with the same electric charges or multiple leptons, and (iii) at least three $b$-quarks with two photons. The $t\bar{t}HH$ production cross-section, relative to its Standard Model prediction, is measured to be $μ_{t\bar{t}HH}=-3^{+11}_{-12}$. This result corresponds to a 95\% confidence-level upper limit of 20 times the Standard Model prediction for the $t\bar{t}HH$ production cross-section, with an expected limit of 21. The Higgs effective field theory Wilson coefficient $c_{t\bar{t}HH}$ is also constrained at the same confidence level to the range of $-3.9<c_{t\bar{t}HH}<3.3$, compared with the expected range of $-4.0<c_{t\bar{t}HH}<3.5$.

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arXiv2026

Measurements of the Higgs boson production, fiducial and differential cross-sections in the four lepton decay channel using 164 fb$^{-1}$ of data collected at $\sqrt{s}$ = 13.6 TeV with the ATLAS detector

Inclusive, differential and production-mode cross-section measurements of the Higgs boson are performed in the $H \to ZZ^{*} \to 4\ell$ decay channel. The analysis uses proton-proton collision data produced at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13.6$ TeV and recorded with the ATLAS detector, corresponding to an integrated luminosity of 164 fb$^{-1}$. The inclusive fiducial cross-section for the $H \to ZZ^{*} \to 4\ell$ process is measured to be $σ_{\textrm{fid}} = 3.65^{+0.35}_{-0.33}~\textrm{fb}$, in agreement with the Standard Model prediction of $σ_{\textrm{fid}}^{\textrm{SM}} = 3.68 \pm 0.17~\textrm{fb}$. Differential cross-sections are measured as a function of key kinematic observables of the Higgs boson and the four-lepton final state. Cross-sections are measured for the main production-modes in several exclusive regions of the Higgs boson production phase space and combined to measure an overall Higgs boson signal strength, defined as the measured cross-section normalised to the SM prediction, of $μ= 0.99 \pm 0.13$. The results are interpreted in terms of modifications of Higgs boson couplings using the $κ$ framework, within the Standard Model Effective Field Theory, and in scenarios probing the Higgs boson self-coupling. All the results are consistent with Standard Model expectations.

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arXiv2026

Search for high-mass resonances in photon-jet final states using 140 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for new high-mass phenomena in events with a high-transverse-momentum photon and a jet is performed using data collected by the ATLAS experiment from proton-proton collisions at the LHC, with a centre-of-mass energy of 13 TeV. The analysis is based on a total integrated luminosity of 140 fb$^{-1}$. The $γ$$-$jet invariant mass spectrum measured in data is examined for localised excesses above the Standard Model background. No significant deviation from the Standard Model prediction is observed, allowing stringent limits to be set on the production cross sections of specific new physics scenarios. To investigate quark compositeness, excited quark models are studied for different couplings ($f$) and flavours: light $q_{\mathrm{L}}^{*}$ $(u^{*}/d^{*})$ and heavy ($b^{*}$, $c^{*}$). For $f=1$, masses up to 6.0 TeV ($q_{\mathrm{L}}^{*}$), 3.5 TeV ($c^{*}$), and 2.6 TeV ($b^{*}$) are excluded. These results provide the first direct constraints on $c^{*}$ production in the $γ$$-$ jet final state at the LHC. Additionally, the analysis excludes quantum black hole mass thresholds up to 5.3 (7.5) TeV in the Randall-Sundrum (Arkani-Hamed-Dimopoulos-Dvali) models at a 95% confidence level, further constraining these theoretical frameworks. Constraints on the visible cross-section are also obtained using generic Gaussian-shaped resonance production.

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