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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 73 records · Page 4Linked to original sources

arXiv2026

Measurements of charged-particle pseudorapidity and transverse momentum distributions in O+O and Ne+Ne collisions at $\sqrt{s_{_\text{NN}}} = 5.36$ TeV with the ATLAS detector

Measurements of charged-particle transverse momentum spectra, multiplicity, and mean transverse momentum are presented as a function of pseudorapidity and collision centrality in O+O and Ne+Ne collisions at $\sqrt{s_{_\text{NN}}}= 5.36$ TeV using 27.7 $μ\text{b}^{-1}$ and 53.1 $μ\text{b}^{-1}$ data sets recorded by the ATLAS experiment at the LHC. The collision centrality is characterized by the total transverse energy measured in the ATLAS forward calorimeters. The kinematics of charged particles are reconstructed with the inner detector over the fiducial pseudorapidity and transverse momentum ranges $|η|<2.5$ and $0.27 < p_{\text{T}} < 5$ GeV using data from the ATLAS inner detector. The per-event charged-particle pseudorapidity density $dn/dη$ and mean transverse momentum $\langle p_{\text{T}}\rangle$ are measured over this fiducial range as a function of $η$. The results are reported in 5%-wide centrality intervals covering the 5-80% centrality range, and in 1%-wide intervals covering the 0-5% centrality range. Invariant per-event yields are evaluated as a function of $η$ and $p_{\text{T}}$. Their $p_{\text{T}}$ dependence is fitted to estimate extrapolated $dn/dη$ and $\langle p_{\text{T}}\rangle$ values over $0 < p_{\text{T}} < 5$ GeV. To evaluate the impact of using pseudorapidity instead of rapidity, measurements are also performed as a function of rapidity computed using a pion mass hypothesis. The fiducial and extrapolated results are compared with hydrodynamic calculations.

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arXiv2026

Observation of centrality-dependent dijet transverse momentum imbalance in O+O and Ne+Ne collisions at $\sqrt{s_{NN}}$ = 5.36 TeV with the ATLAS detector

The ATLAS experiment presents an observation of a centrality-dependent dijet transverse momentum imbalance in O+O and Ne+Ne collisions at a nucleon-nucleon center-of-mass energy of 5.36 TeV at the Large Hadron Collider. The measurement uses 8.0 nb$^{-1}$ of O+O and 1.0 nb$^{-1}$ of Ne+Ne data collected in 2025, together with 386 pb$^{-1}$ of \textit{pp} data at the same energy used as a reference. The dijet momentum balance is quantified using the ratio of the sub-leading jet transverse momentum to that of the leading jet, $x_J$. For dijets produced azimuthally back-to-back, the self-normalized $x_J$ distributions exhibit increasingly large deviations from the \textit{pp} reference as collisions become more central, corresponding to an increasing overlap of the colliding nuclei. The observed centrality dependence is consistent with medium-induced partonic energy loss in O+O and Ne+Ne collisions, demonstrating that such effects persist in collision systems considerably smaller than Pb+Pb and Xe+Xe. These results establish a new regime for investigating the path-length dependence of jet quenching and constrain the onset of quark-gluon plasma effects in small nuclear collision systems.

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arXiv2026

Measurement of isolated photon plus two-jet correlations in Pb+Pb and $pp$ collisions at 5.02 TeV with ATLAS

This paper presents a measurement of photon plus two-jet events in $pp$ and Pb+Pb collisions, i.e. events in which the transverse momentum of a single photon is balanced by two distinct jets. The measurement was performed using $pp$ data taken in 2017 with an integrated luminosity 260 pb$^{-1}$, and Pb+Pb data taken in 2018 with an integrated luminosity 1.72 nb$^{-1}$, both at $\sqrt{s_\mathrm{NN}}$ =5.02 TeV, as recorded by the ATLAS detector. Events with photons in the transverse momentum range 90-180 GeV and at least two anti-$k_t$ $R = 0.2$ jets with a $p_\mathrm{T}$ > 30 GeV are selected, and three observables are measured: ${\mathrm{x}}_{\mathrm{JJ}γ}$, $\mathrm{A}_{\mathrm{JJ}γ}$, and $ΔR_{\mathrm{JJ}}$. These observables characterise the overall energy loss of the multiparton system from medium interactions (${\mathrm{x}}_{\mathrm{JJ}γ}$), the relative energy loss between the two colour-charge carriers ($\mathrm{A}_{\mathrm{JJ}γ}$), and medium-induced modifications to their opening angle ($ΔR_{\mathrm{JJ}}$). The observables are corrected for uncorrelated combinatoric background contributions using a novel multijet mixing technique, for photon purity, and for detector resolution effects via iterative unfolding. Final results are presented per photon, and the ratio ($I_\mathrm{AA}$) is taken between measurements in Pb+Pb and $pp$ collisions, for Pb+Pb centrality intervals of 30-80%, 10-30%, and 0-10%. Significant suppression of per photon two-jet yields in all three observables, $I_\mathrm{AA} < 1$, is observed as a result of parton-medium interactions. The experimental measurements are compared to three different jet quenching models: JEWEL, JETSCAPE, and LBT.

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arXiv2026

Search for dark photons from Higgs boson decays in the gluon-gluon fusion channel in proton-proton collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

This paper presents a search for semi-visible Higgs boson decays in final states with a photon ($γ$) and missing transverse momentum $p_T^{miss}$. The analysis is optimised for the gluon-gluon fusion production channel and exploits a novel trigger deployed in 2023, with reduced thresholds down to 50 GeV on the photon transverse momentum ($p_T^γ$) and 70 GeV on the $p_T^{miss}$, in combination with a selection on the transverse mass of the $p_T^{miss}$-$γ$ system ($m_T$) above 80 GeV. The results are interpreted in terms of Higgs boson decays into a $γ$ and a dark photon $γ_d$ ($H \to γγ_d$). The analysis is performed using 135 fb$^{-1}$ of Run 3 data collected by ATLAS in 2023 and 2024. A boosted decision tree is used to identify events with misreconstructed primary vertices, which can affect the $p_T^{miss}$ calculation. Data-driven methods are employed to estimate the backgrounds from jets and electrons misidentified as photons, while the backgrounds with genuine photons are taken from Monte Carlo simulations, normalised to data in control regions with muons. A binned maximum-likelihood fit to the $m_T$ distribution is performed to evaluate the compatibility of the observed data with the Standard Model expectation. No significant excess is observed over the Standard Model prediction, and an observed (expected) upper limit on the branching ratio of $H \to γγ_d$ of 1.4% (1.2%) is obtained at 95% confidence level. Combining this result with previous Run 2 searches improves the observed (expected) upper limit to 0.9% (0.9%).

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arXiv2026

Effective field theory interpretation of ATLAS measurements involving the Higgs boson, electroweak bosons and the top quark

Wilson coefficients in dimension-six effective field theory are constrained in a combined fit to several ATLAS measurements. These inputs probe Higgs-boson processes across multiple production and decay modes, di-Higgs signatures in the $b\bar{b}γγ$ and $b\bar{b}ττ$ final states, $WW$ and $WZ$ diboson signatures, electroweak $Zjj$ final states, high-mass Drell-Yan interactions, and $t \bar t$ events in both resolved and boosted topologies. Precision electroweak observables from LEP, SLD, and ATLAS are also included. A total of 48 parameters, including individual Wilson coefficients in the Warsaw basis and linear combinations of Wilson coefficients, are constrained simultaneously. Constraints on two-Higgs-doublet models and heavy-vector-boson models are also obtained by matching a relevant sub-set of the results with their parameters. This combined fit provides the most comprehensive effective field theory interpretation of experimental data by the ATLAS Collaboration to date. No significant deviations from the Standard Model are observed.

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arXiv2026

Search for massive, long-lived particles in events with displaced vertices and displaced muons in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS experiment

A search is presented for massive long-lived particles in events featuring at least one displaced vertex and at least one displaced muon, using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider from 2022 to 2024 at a centre-of-mass energy of 13.6 TeV. The data sample corresponds to an integrated luminosity of 164 fb$^{-1}$. The analysis targets scenarios in which long-lived particles decay inside the ATLAS inner detector, resulting in a topology of at least one massive, displaced vertex (DV) with multiple associated tracks, and at least one muon with a large transverse impact parameter relative to the primary interaction point. The muon is not required to be associated with the DV. Two signal regions are defined by the transverse distance of the reconstructed DV from the interaction point. Background contributions are estimated by using fully data-driven techniques. No significant excess above the expected background is observed. Upper limits at 95% confidence level are set on the visible cross-section and on the production cross-sections of several benchmark models of $R$-parity-violating supersymmetry.

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arXiv2026

Measurement of differential $t$-channel single top (anti)quark production cross-sections at 13 TeV with the ATLAS detector

The production of single top quarks and top antiquarks via the $t$-channel exchange of a virtual $W$ boson is measured in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The full Run 2 data sample recorded with the ATLAS detector in the years 2015-2018 is used, corresponding to an integrated luminosity of 140 fb$^{-1}$. The absolute and normalised production cross-sections are measured differentially as a function of the transverse momentum and absolute rapidity of the top quark and top antiquark. In addition, the ratio of top quark to top antiquark production cross-sections is measured. The measured distributions are compared with next-to-leading-order quantum chromodynamics predictions obtained with different combinations of matrix-element generators, parton-shower programs and proton parton distribution functions, as well as to next-to-next-to-leading-order calculations. Overall, good agreement is observed between the measurements and the theoretical predictions. For most measured distributions, the sensitivity to differences between the predictions is limited by the systematic uncertainties in the measurement. The measured differential distributions are also interpreted in an effective field theory approach to constrain the Wilson-Coefficient $C_{Qq}^{3,1}$ associated with a four-quark operator. The interpretation accounts for the effect of the selection efficiency, which is altered significantly by non-zero contributions from $C_{Qq}^{3,1}$.

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arXiv2026

Probing the Higgs-top Yukawa interaction in the $t\bar{t}H$ and $tH$ processes using $H\rightarrowγγ$ with the ATLAS detector

A study of the structure of the coupling between the Higgs boson and the top quark is performed using events from $t\bar{t}H$ and $tH$ production in the $H\rightarrowγγ$ decay channel, with 164 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}$ = 13.6 TeV collected by the ATLAS detector at the LHC. The cross section of the $t\bar{t}H$ process times the Higgs to diphoton decay branching ratio is measured to be $1.46^{+0.40}_{-0.35} = 1.46^{+0.34}_{-0.32}\,\text{(stat.)}^{+0.22}_{-0.13}\,\text{(sys.)}$ fb, corresponding to $1.13^{+0.33}_{-0.28}$ times the Standard Model prediction. An observed 95% confidence level limit on the $tH$ production cross section times the Higgs to diphoton decay branching ratio is set at 6.2 times the Standard Model prediction, compared to an expected limit of 4.4 times, constituting the most stringent $tH$ upper limit achieved in a single measurement to date. The results are combined with 140 fb$^{-1}$ of proton-proton collision data collected at $\sqrt{s}$ = 13 TeV in the same production and decay channel, and a $CP$-mixing angle of $|α|>38^\circ$ is excluded at the 95% confidence level, with a purely $CP$-odd Higgs-top Yukawa coupling excluded at the level of 5.8 standard deviations, providing the most stringent direct constraints on the $CP$ structure of the Higgs-top Yukawa interaction to date.

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arXiv2026

Search for electroweak scale dijet resonances in pile-up collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for dijet resonances in the mass range of 100-250 GeV is presented using proton-proton collision data recorded by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Conventional searches for hadronic resonances in the sub-TeV regime are heavily constrained by high jet trigger thresholds required to manage the overwhelming rate of Standard Model multijet background processes described by quantum chromodynamics. To overcome this limitation, this analysis uses a novel strategy that separately reconstructs multiple proton-proton interactions per bunch crossing, known as pile-up collisions. The dataset was collected by the ATLAS experiment using single-electron and single-muon triggers in 2016-2018 corresponding to an integrated luminosity of 1.30 pb$^{-1}$, which represents the effective luminosity of pile-up collisions recorded alongside triggered events. This constitutes the first application of pile-up collisions to probe low dijet mass scales. No significant excess is observed above the background expectation and exclusion limits are set for a generic Gaussian model and a simplified dark matter model featuring an axial-vector mediator with coupling to quarks.

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arXiv2026

Search for a leptoquark in events with a hadronically decaying $τ$-lepton and missing transverse momentum using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for leptoquark signals is performed in final states with a hadronically decaying $τ$-lepton and missing transverse momentum, using 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 during Run 2 (2015--18), corresponding to an integrated luminosity of 140 fb$^{-1}$. The analysis is designed to probe both resonant production and non-resonant $t$-channel exchange of the leptoquark, covering a wide range of coupling scenarios. No excess above the Standard Model background prediction is observed. Limits are set on the couplings in the benchmark $U_1$ vector-leptoquark model at 95% confidence level for masses between 1.5 TeV and 3.0 TeV.

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arXiv2026

Measurement of the cross-section for the production of a $W$ boson in association with $b$-jets in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper presents a measurement of the production cross-section of a $W$ boson associated with one or two jets, where at least one jet originates from a $b$-quark. The data are collected with the ATLAS detector at the LHC in proton-proton collisions at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 140 fb$^{\text -1}$. Differential cross-sections are presented as a function of the transverse momentum of the $b$-jet for both the electron and muon decay modes of the $W$ boson. The results corrected for all detector effects are presented in a fiducial region defined with basic lepton and jet kinematics, and compared with theoretical predictions. The averaged fiducial cross-section of $W+b$-jets for the electron and muon channels is measured to be $16.6\pm1.9$ pb, which is consistent with the next-to-leading-order QCD prediction of $16.8\pm2.3$ pb. The relative precision of this measurement is improved by approximately a factor of two compared with the previous ATLAS measurement at $\sqrt{s}=7$ TeV.

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arXiv2026

Search for a resonance decaying into a scalar particle and a Higgs boson in the final state with two bottom quarks and two photons with 199 fb$^{-1}$ of data collected at $\sqrt{s}$=13 and 13.6 TeV with the ATLAS detector

A search for the resonant production of a heavy scalar $X$ decaying into a lighter scalar $S$ and a Higgs boson, through the process $X \rightarrow S (\rightarrow b \bar{b})H ( \rightarrow γγ)$, where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using integrated luminosities of 140 and 59 fb$^{-1}$ of proton-proton collision data at centre-of-mass energies of 13 and 13.6 TeV, respectively, recorded with the ATLAS detector at the LHC. The search is performed over the mass ranges of 170 $\leq$ $m_{X}$ $\leq$ 1000 GeV and 15 $\leq$ $m_{S}$ $\leq$ 500 GeV. No significant excess over the Standard Model background prediction is observed and limits at 95% confidence level are set on the product of cross section and branching fraction for the process $X \rightarrow S (\rightarrow b \bar{b})H ( \rightarrow γγ)$ at 13 TeV, ranging from 9 fb to 0.06 fb.

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arXiv2026

Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four-$τ$ final state at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into $τ$-leptons, $H\rightarrow aa\rightarrow τ^+τ^-τ^+τ^-$, is presented. In models with Yukawa-like couplings, the decay to $τ$-leptons is favoured for light $a$-bosons, with mass in the range of $2m_τ < m_a < 2m_{b}$. Results are presented in the range of $4 \, \mathrm{GeV} < m_a < 15 \, \mathrm{GeV} $ using the $140\,\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. This search focuses on the scenario where, for both di-$τ$ pairs, one of the $τ$-leptons decays to hadrons and neutrinos, while the other decays to a muon and neutrinos. In this mass range, the $a\rightarrow τ^+τ^-$ is Lorentz-boosted and a dedicated muon removal technique is used to reconstruct the di-$τ$ pairs. No significant excess above the Standard Model background prediction is observed. Upper limits on $(σ(H)/σ_{\mathrm{SM}}(H))\times \mathcal{B}(H\rightarrow aa\rightarrow 4τ)$ at $95\%$ confidence level are provided, ranging from $0.03$ to $0.10$ depending on the $a$-boson mass.

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arXiv2026

Search for soft unclustered energy patterns containing muons in the final state in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

Soft unclustered energy patterns (SUEPs) refer to high-multiplicity, isotropic distributions of low-momentum particles that could arise in strongly-coupled hidden sector theories. A search for SUEPs whose decay products contain muons in the final state is presented using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015-2018. The analysis targets SUEP signatures via gluon-gluon fusion production of a scalar mediator in Hidden Valley scenarios. No significant excess over the Standard Model expectation is observed. Exclusion limits are set on the product of the mediator production cross section and the branching fraction for its decay into a SUEP, down to 0.05 fb for a mediator mass of $m_S$ = 750 GeV, 0.4 fb for $m_S$ = 400 GeV, and 70 fb for $m_S$ = 125 GeV; the last value, when identifying the mediator with the Standard Model Higgs boson, translates to an upper limit on the branching fraction of around 0.2%.

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arXiv2026

Observation of a cross-section enhancement near the $t\bar{t}$ production threshold in $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

A measurement of $t\bar{t}$ production is presented in the invariant-mass region near the pair production threshold, $m_{t\bar{t}} \sim 345$ GeV, in final states with two charged leptons and multiple jets. The measurement is based on $140\,\mathrm{fb}^{-1}$ of proton-proton collision data collected at $\sqrt{s} = 13$ TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of $t\bar{t}$ production: a baseline model including only perturbative QCD predictions for the hard process at approximate next-to-next-to leading order accuracy in the strong coupling, and an extended model that, in addition, incorporates non-relativistic QCD simulations that also include the formation of colour-singlet quasi-bound-states near the $t\bar{t}$ threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed $m_{t\bar{t}}$ distributions, in bins of two angular observables sensitive to spin-correlations in the $t\bar{t}$ system. An excess of events is observed over the baseline perturbative QCD prediction, with an observed significance over $8$ standard deviations. This excess is consistent with the formation of colour-singlet and spin-singlet $S$-wave quasi-bound $t\bar{t}$ states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of $9.3^{+1.4}_{-1.3}$ pb.

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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 $\bar{b}c$ quarkonium system, the $B_c^{*+}$ meson.

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arXiv2026

Characterization of nuclear breakup as a function of hard-scattering kinematics using dijets measured by ATLAS in $p$+Pb collisions

This Letter analyzes the sensitivity of event geometry estimators to the initial-state kinematics of hard scattering in proton-lead collisions. This analysis uses dijets as a proxy for the parton-parton scattering configuration, correlating it with event geometry estimators, namely the energy deposited in the Zero-Degree Calorimeter and the transverse energy recorded in the Forward Calorimeter in the Pb-going direction. The analysis uses data recorded by the ATLAS detector at the Large Hadron Collider with a nucleon-nucleon center-of-mass energy of 8.16 TeV, corresponding to an integrated luminosity of 56 nb$^{-1}$. The jets are measured within the pseudorapidity interval $-$2.8 $<$ $η$ $<$ 4.5, where positive $η$ values correspond to the direction of the proton beam. Results are presented as a function of the Bjorken-$x$ of the parton originating from the proton, $x_{p}$. Both event geometry estimators are found to be dependent on $x_{p}$, with the energy deposited in the Zero-Degree Calorimeter about six times less sensitive to $x_{p}$ compared with the transverse energy deposited in the Forward Calorimeter.

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arXiv2026

Track and Vertex Reconstruction with the ATLAS Inner Detector

Charged-particle reconstruction is a fundamental part of the event reconstruction in modern multi-purpose high-energy physics detectors. This paper describes the algorithms used to reconstruct charged particles and primary vertices with the ATLAS Inner Detector. The most recent software configuration deployed for data-taking is described, and the performance obtained when this software is used to process Run 2 (2015-2018) data, a subset (from 2022) of Run 3 (2022-2026) data, and corresponding simulated data is presented. The ATLAS track and vertex reconstruction performance is shown for up to 80 simultaneous proton-proton interaction. It maintains a high efficiency, good resolution for key parameters, and low rates of mis-reconstructed candidates for Run 2 and Run 3 conditions.

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