arXiv subjects

ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 19 recordsLinked to original sources

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 $\gamma$$-$jet invariant mass spectrum measured in data is examined for localised excesses above the Standard Model background. No significant deviation from the Standard Model prediction is observed, allowing stringent limits to be set on the production cross sections of specific new physics scenarios. To investigate quark compositeness, excited quark models are studied for different couplings ($f$) and flavours: light $q_{\mathrm{L}}^{*}$ $(u^{*}/d^{*})$ and heavy ($b^{*}$, $c^{*}$). For $f=1$, masses up to 6.0 TeV ($q_{\mathrm{L}}^{*}$), 3.5 TeV ($c^{*}$), and 2.6 TeV ($b^{*}$) are excluded. These results provide the first direct constraints on $c^{*}$ production in the $\gamma$$-$ 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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arXiv2026

Measurement of $tZq$ inclusive and differential cross-sections in $pp$ collisions at $\sqrt{s} = 13~\text{TeV}$ with the ATLAS detector

The $t$-channel production of a top quark in association with a $Z$ boson, $tZq$, is measured in the trilepton channel. The measured $pp \rightarrow t\ell^+\ell^-q$ process is defined for $m_{\ell\ell} > 30\,\text{GeV}$ and includes both resonant $Z$-boson production and non-resonant dilepton production. The data collected by the ATLAS experiment in 2015 to 2018 in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $140\,\text{fb}^{-1}$. Events containing three light leptons ($e$ or $\mu$) and two to four jets, one or two of which are identified as $b$-quark-initiated jets, are selected. The inclusive $pp \rightarrow t \ell\ell q$ cross-section with $m_{\ell\ell} > 30\,\text{GeV}$ is measured, along with the separate cross-sections for top-quark and top-antiquark production and their ratio. The inclusive cross-section is measured to be $95.2\;^{+8.5}_{-7.9}\;\text{(stat.)}\;^{+5.5}_{-5.3}\;\text{(syst.)}\,\text{fb}$ and found to be in agreement with the most advanced Standard Model predictions. Differential cross-section measurements as a function of many kinematic variables are also presented. Both absolute and normalised differential cross-section measurements are made at particle level and parton level in their respective phase spaces. The top-quark spin asymmetry is measured from the unfolded measurement of the top-quark polarisation angle as $A = 0.34 \pm 0.14 \;\text{(stat.)} \pm 0.06\;\text{(syst.)}$, in good agreement with the Standard Model prediction. The differential cross-section measurement results are interpreted in the framework of Standard Model effective field theory and limits are set on dimension-6 operators involving the top quark.

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arXiv2026

Combination of Higgs boson measurements at $\sqrt{s} =$ 13 TeV and their interpretations by the ATLAS experiment

This paper presents combined measurements of Higgs boson production and decay using up to 140 fb$^{-1}$ of proton--proton collision data collected by the ATLAS experiment at the LHC at $\sqrt{s} = 13$ TeV. The inclusive Higgs boson event rate relative to its Standard Model prediction is determined to be $0.990 ^{+0.053}_{-0.051} = 0.990 \pm 0.027 \text{ (stat.) } \pm 0.024 \text{ (exp.) } ^{+0.037}_{-0.035} \text{ (sig. theo.) } ^{+0.016}_{-0.015} \text{ (bkg. theo.)}$, separating statistical, experimental, signal-theory and background-theory uncertainty components. The signal theory uncertainty is the largest contribution, dominated by missing higher-order terms in QCD and the modelling of parton shower effects. Inclusive production cross-sections and decay branching ratios are measured, with sensitivities improved by up to 35% relative to the previous ATLAS publication. Production cross-sections for individual decay channels are also reported. The results are interpreted in terms of modifiers to Higgs boson couplings. The combination of single Higgs boson and Higgs boson pair production measurements yields $\kappa_\lambda = 1.3^{+3.1}_{-1.6}$ for the modifier to the Higgs boson self-coupling, with an interval of $[-1.5, 6.5]$ at 95% confidence level. The partial width in the $H\to b\bar{b}$ decay channel is used to obtain the first ATLAS determination of the running mass of the $b$-quark at the Higgs boson mass scale in the modified minimal subtraction scheme, found to be $m_b(m_H) = 2.73^{+0.27}_{-0.25}$ GeV. The total width of the Higgs boson is measured to be $\Gamma_H = 3.6^{+2.1}_{-1.6}$ MeV based on its on- and off-shell production rates. The kinematics of Higgs boson production are probed within the framework of Simplified Template Cross-sections in 44 kinematic regions, and the results are interpreted in the context of Standard Model Effective Field Theory models.

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arXiv2026

Measurement of forward jet suppression in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$$ TeV with the ATLAS detector

Inclusive jet production is measured with the ATLAS detector at the LHC at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV in Pb+Pb and $pp$ collisions, using data samples corresponding to integrated luminosities of 1.72 nb$^{-1}$ and 255 pb$^{-1}$, respectively. The jet nuclear modification factor, $R_{\mathrm{AA}}$, is reported for anti-$k_t$ jets with radius parameter $R=0.4$ in the forward-rapidity intervals $2.8<|y|<3.2$ and $3.2<|y|<3.6$, probing transverse momenta $p_\mathrm{T}>74$ GeV. The observed $R_{\mathrm{AA}}$ values show a significant suppression of jet yields in Pb+Pb collisions relative to those measured in $pp$ collisions. The suppression increases monotonically from peripheral to central collisions and exhibits a mild jet $p_\mathrm{T}$ dependence. A comparison with previous ATLAS measurements indicates stronger jet suppression in the $2.8<|y|<3.2$ region than in the $|y| < 2.8$ region. In the $3.2<|y|<3.6$ region, the precision of the measurement does not allow a conclusive comparison with the $|y|<2.8$ region. These results constitute the first measurement of inclusive jet suppression at forward rapidity in heavy-ion collisions and provide new constraints on flavor, geometric, and medium-density dependence of parton energy loss.

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arXiv2026

Measurement of the Higgs boson decay to a low-mass dilepton system and a photon in $pp$ collisions at $\sqrt{s} =$ 13 and 13.6 TeV with the ATLAS detector

A measurement is presented for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass $m_{\ell\ell} <$ 30 GeV, using $pp$ collision data at $\sqrt{s} =$ 13.6 TeV recorded by the ATLAS detector at the Large Hadron Collider during the years 2022-2024, corresponding to an integrated luminosity of 164 fb$^{-1}$. The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is measured to be $\mu = 0.64^{+0.44}_{-0.39}$ compared with the expected value of $\mu = 1.00^{+0.47}_{-0.41}$. The corresponding observed (expected) signal significance is 1.7 (2.5) standard deviations under the background-only hypothesis. The result is combined with a similar search performed using 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} =$ 13 TeV collected during the years 2015-2018. This combination gives an observed (expected) signal strength of $\mu = 1.03^{+0.35}_{-0.32}\,(\mu = 1.00^{+0.34}_{-0.32})$, corresponding to an observed (expected) significance of 3.4 (3.3) standard deviations.

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arXiv2026

Search for $\textit{tH}$ production in the $ H \to \tau\tau$ decay mode, and a combination with other searches, using $\textit{pp}$ collisions at 13 TeV and 13.6 TeV with the ATLAS detector

A search for the production of a Higgs boson in association with a single top quark in multi-lepton final states including $\tau$-leptons is presented, using an integrated luminosity of $140\,\text{fb}^{-1}$ of proton-proton collision data at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ collected by the ATLAS detector at the LHC. The analysis targets the $H \to \tau\tau$ decay, where at least one $\tau$-lepton decays hadronically, to indicate the presence of the Higgs boson. No significant excess over the expected background is observed, and an observed (expected) upper limit at the 95% confidence level on the signal strength is set at 8.6 (9.3) times the Standard Model prediction. This measurement is combined with previous publications of the ATLAS Collaboration targeting the same production process but in different final states, using $140\,\text{fb}^{-1}$ of proton-proton collision data at $\sqrt{s} = 13\,\text{TeV}$ and $164\,\text{fb}^{-1}$ at $13.6\,\text{TeV}$. The combined measurement of the signal strength yields $\mu_{tH} = 3.3^{+1.7}_{-1.5} = 3.3 \pm 1.2\,(\text{stat.})^{+1.1}_{-1.0}\,(\text{sys.})$ times the Standard Model expectation, corresponding to an observed (expected) $\textit{tH}$ significance of 2.3 (0.8) standard deviations. The combined observed (expected) upper limit at the 95% confidence level on the signal strength is set at 6.1 (2.7) times the Standard Model prediction. This combination represents the most precise measurement of $tH$ production to date.

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arXiv2026

Search for high-mass dilepton resonances in $pp$ collisions at $\sqrt{s}=13.6$ TeV combined with 13 TeV results using the ATLAS detector

A search for spin-1 $ee$ and $\mu\mu$ resonances in the high-mass range is presented. Two proton-proton ($pp$) collision datasets recorded by the ATLAS detector at CERN's Large Hadron Collider are utilised. A search is first performed using a dataset recorded at a center-of-mass energy of 13.6 TeV between 2022 and 2024, corresponding to an integrated luminosity of 165 fb$^{-1}$. The results of this search are subsequently statistically combined with results using 140 fb$^{-1}$ of data recorded between 2015 and 2018 at 13 TeV. The dilepton invariant mass spectrum is fitted with a functional form to model the contribution from Standard Model background processes. A generic spin-1 signal shape is used to determine the significance of observed deviations from the background expectation. No significant deviation is observed, therefore upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for a spin-1 resonance with various width hypotheses, individually for $ee$, $\mu\mu$ final states and for their combination. From the combined 13 TeV and 13.6 TeV results, mass limits are extracted for various benchmark model bosons, including a $Z^{\prime}_{\text{SSM}}$ (5.5 TeV), $Z^{\prime}_{\chi}$ (5.1 TeV) and $Z^{\prime}_{\psi}$ (4.8 TeV). These results represent the most stringent limits to date on high-mass dilepton resonances on the considered $Z'$ models.

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arXiv2026

Search for pair-production of vector-like $T$ quarks decaying into a top quark and a spin-0 particle in the diphoton final state in proton proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for pair-produced vector-like T quarks (VLQs) is presented. Each VLQ is assumed to decay into a top quark and an exotic (pseudo)scalar S. The search targets events with at least one resonant $S \to \gamma\gamma$ decay and large summed transverse momentum and missing transverse momentum from other final-state particles. The analysis uses data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector from 2015 to 2018 and corresponding to an integrated luminosity of 140 fb$^{-1}$. This is the first VLQ search performed in this final state. No significant excess over the Standard Model background is observed. The analysis is model-independent to a large extent but targets cascade decays into final states with electroweak bosons or $t\bar{t}$. Results are provided in the form of upper limits on the fiducial cross-section times branching ratio as a function of the masses of the VLQ and the spin-0 particle. The fiducial volume is defined by kinematic requirements on the photons from the spin-0 decay.

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arXiv2026

Search for Lorentz-boosted di-$\tau$ resonances produced in association with top quark pairs in $\sqrt{s}=13$ TeV pp collisions with the ATLAS detector

A search for a Lorentz-boosted pseudoscalar resonance $a$ in the mass range between 20 GeV and 85 GeV, produced in association with a top-antitop quark pair and decaying into a $\tau$-lepton pair, is presented. Both $\tau$-leptons are reconstructed in the hadronic decay mode. The boosted production of $a$ results in a small angular separation between its decay products, requiring the use of dedicated boosted di-$\tau$ objects with custom reconstruction, energy calibration, and identification procedures. The analysis uses proton-proton collision data at a centre-of-mass-energy of $\sqrt{s}=13$ TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{-1}$. No significant deviation is observed over the expected Standard Model background, and a model independent upper limit of 0.19 fb is placed at the 95% confidence level on the visible cross-section. Limits on $\sigma(t\bar{t}a) \times BR(a \to \tau\tau)$ are also obtained for a Two-Higgs-Doublet Model signal scenario, ranging from 0.4 pb at $m_a=20$ GeV to 0.05 pb at $m_a=85$ GeV.

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arXiv2026

Improved analysis of non-resonant Higgs boson pair production in the $b\bar{b}\tau^+\tau^-$ final state with $196$ fb$^{-1}$ of data collected at $\sqrt{s}$ = 13 TeV and 13.6 TeV with the ATLAS detector

A search for non-resonant Higgs boson pair production (HH) in the $b\bar{b}\tau^+\tau^-$ final state is performed using 140 $fb^{-1}$ and 56 $fb^{-1}$ of proton-proton collision data at centre-of-mass energies of $\sqrt{s}$ = 13 TeV and 13.6 TeV, respectively, recorded by the ATLAS detector during 2015-2023 at the CERN Large Hadron Collider. Relative to the previous ATLAS searches in the same final state, the analysis benefits from the additional dataset collected at 13.6 TeV and from improvements in both event reconstruction and analysis techniques. The Higgs boson pair production cross-section divided by the Standard Model (SM) prediction is found to be $\mu_{HH}= 2.6^{+1.4}_{-1.0}$, consistent with the SM expectation. This corresponds to an observed (expected) significance of 2.6 (1.2) standard deviations over the background-only hypothesis of no HH production and 1.65 standard deviations with respect to the SM prediction. Under the assumption that all other Higgs boson couplings take their SM values, the observed (expected) 95% confidence level interval for the Higgs boson self-coupling modifier is found to be $\kappa_{\lambda} \in [-3.4, 1.6] \cup [5.5, 10.1]$ ($\kappa_{\lambda} \in[-1.7, 8.5]$). As a validation of the analysis strategy, measurements of the ZH and ZZ processes are performed in the same final state using the same event selection, yielding observed (expected) significances of 3.5 (2.4) and 1.8 (2.7) standard deviations, respectively, relative to the background-only prediction.

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arXiv2026

Study of $t\bar{t}H$ and $tH$ production in the $H\to\tau\tau$ channel in $pp$ collisions at $\sqrt{s}=13$ TeV and 13.6 TeV with the ATLAS detector

A study of the production of the Higgs boson in association with either a top-quark pair ($t\bar{t}H$) or a single top quark ($tH$) in the $\tau$-lepton-pair decay channel is presented. The analysis relies on final states featuring fully hadronic decays of the top quarks and the $\tau$-leptons. It employs data samples of proton--proton collisions at $\sqrt{s}=13$ and 13.6 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to integrated luminosities of 140 fb$^{-1}$ and 161 fb$^{-1}$, respectively. The measured signal strength ($\mu$), defined as the measured cross-section normalised to the Standard Model prediction, is $\mu_{t\bar{t}H}=1.51^{+0.71}_{-0.62}$ for $t\bar{t}H$ and $\mu_{tH}=-0.4^{+5.6}_{-5.1}$ for $tH$. Additionally, the $t\bar{t}H$ cross-section is measured differentially in three bins of the Higgs boson transverse momentum in the simplified template cross-section framework.

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arXiv2026

Search for heavy Majorana neutrinos in vector boson scattering with $\tau$-lepton final states with the ATLAS detector

A search for heavy Majorana neutrinos coupling to third-generation leptons in vector boson scattering at the LHC is presented. The search is performed in the final state with a same-sign $\tau\tau$, $e\tau$ or $\mu\tau$ pair using data of proton--proton collisions at $\sqrt{s}$ = 13 TeV, recorded with the ATLAS detector and corresponding to an integrated luminosity of 140 $\text{fb}^{-1}$. No significant deviation from the Standard Model background is observed. Constraints on the squared mixing matrix element between the $\tau$-lepton and the heavy Majorana neutrino ($|V_{\tau N}|^2$) are derived for heavy Majorana neutrino masses between 92 GeV and 6.5 TeV. Additionally, two-dimensional constraints on ($|V_{eN}|^{2}, |V_{\tau N}|^{2}$) and ($|V_{\mu N}|^{2}, |V_{\tau N}|^{2}$) are set. These results improve the limits on heavy Majorana neutrinos coupling to third-generation leptons for heavy Majorana neutrino masses above 1.0 TeV, and extend the exclusion limits to an unexplored mass range above 1.5 TeV.

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arXiv2026

Observation of the electroweak production of $\gamma\gamma jj$ at $\sqrt{s}=13$ TeV in $140\ \text{fb}^{-1}$ of $pp$ collision data with the ATLAS detector

This Letter presents the observation of the electroweak production of two isolated photons in association with two jets together with measurements of the corresponding fiducial and differential cross sections. The analysis is performed using proton-proton collision data collected by ATLAS at the LHC at $\sqrt{s}$ = 13 TeV and with an integrated luminosity of 140 fb$^{-1}$. The electroweak production of two isolated photons in association with two jets is observed with a significance of $6.2 \sigma $ and with a fiducial cross-section of $\sigma^{\mathrm{EW}}_{\mathrm{obs}}= 13.8^{+3.0}_{-2.6}\, \mathrm{fb}$, in agreement with the Standard Model prediction of $\sigma^{\mathrm{EW}}_{\mathrm{obs}}= 17.1^{+2.4}_{-2.4}\, \mathrm{fb}$. Differential cross-sections are measured as a function of various kinematical variables probing photon and jet properties and compared with predictions. In addition, fiducial and differential cross-sections are reported for the combined QCD and electroweak production of two isolated photons and two jets. The measurement is also interpreted in the context of an effective field theory, and limits are set on the Wilson coefficients of several dimension-8 operators parameterising anomalous quartic gauge couplings.

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arXiv2026

The calibration of large-radius jets using the Run 2 dataset with the ATLAS detector

The ATLAS experiment has developed an extensive procedure based on Monte Carlo simulation and collision data to calibrate the response of large-radius jets. This paper describes the Run 2 calibration of large-radius jets reconstructed from Unified Flow Objects and groomed with the Soft Drop algorithm, using data produced in proton-proton collisions at $\sqrt{s}=13$ TeV. Results are presented for the Monte Carlo-based correction and for the in situ methods that are used to validate the jet energy and mass scales and their resolutions in collision data. The residual uncertainty on the jet energy and mass scales is close to 1% for transverse momenta up to 1 TeV and is 2-3% up to 2 TeV, enabling sensitive searches to beyond the Standard Model particles and precise Standard Model measurements with boosted objects.

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arXiv2026

Cross-section measurements of boosted Higgs boson production in final states with pairs of hadronically decaying $\tau$-leptons with the ATLAS experiment

Cross section measurements of high-Lorentz boosted Higgs boson production in final states with pairs of hadronically decaying $\tau$-leptons are presented. The analysis is based on the full Run 2 data sample of proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 140 $\mathrm{fb}^{-1}$, and a partial Run 3 data sample at $\sqrt{s} = 13.6$ TeV with an integrated luminosity of 162 $\mathrm{fb}^{-1}$ collected in 2022-2024 with the ATLAS experiment. The analysis extends the sensitivity of previous $H \to \tau\tau$ measurements to Higgs boson transverse momenta above 300 GeV by reconstructing $\tau$-lepton pairs produced with a small separation in large radius jets. The standard reconstruction of hadronic $\tau$-leptons has limited efficiency in this phase space, requiring dedicated boosted di-$\tau$ reconstruction and identification techniques, combined with multivariate analysis methods, to extract the signal. For Higgs boson transverse momenta above 300 GeV, results are presented for both inclusive and differential cross-sections. Within the simplified template cross-section framework, differential cross-section measurements in Higgs boson transverse momentum for gluon-gluon fusion production are provided, together with an inclusive cross-section measurement for vector-boson fusion production. For the inclusive measurement, the analysis yields an observed (expected) significance of 3.8 (3.3) standard deviations over the background-only hypothesis, providing evidence of Higgs boson production in the boosted regime of the di-$\tau$ final state.

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arXiv2026

Measurements of the electroweak production of a $W$ boson in association with two jets at $\sqrt{s}=13\,\mathrm{TeV}$ with the ATLAS detector

Measurements of the electroweak production of a $W$ boson in association with two jets at high dijet invariant mass are performed using a dataset from $140\,\mathrm{fb}^{-1}$ of $\sqrt{s}=13\,\mathrm{TeV}$ proton-proton collisions recorded by the ATLAS detector at the LHC. The measurements are made in a fiducial region where the $W$ boson's production via vector-boson fusion is enhanced. The fiducial region requires at least two jets, an electron or a muon, and a neutrino, assuming a leptonic $W$ decay. In addition to the integrated production cross-section, differential cross-sections unfolded to particle level are determined as functions of observables which characterise angular properties of the two highest-energy jets, the kinematics of the $W$ boson, and the transverse momenta of the highest-energy jet and the charged lepton. Results are interpreted in the context of an effective field theory, and the limits placed on anomalous triple-gauge-boson couplings are among the most stringent to date for linear terms of the extended Lagrangian.

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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 $\sigma_{WZjj\mathrm{-EW} \rightarrow \ell^{'} \nu \ell \ell jj} = 0.374 \; \pm 0.077 (\mathrm{stat.}) \; \pm 0.053 (\mathrm{syst.}) \; \pm 0.009 (\mathrm{lumi.}) \; \mathrm{fb}$, 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

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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