This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks-a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: $e$+light jet, $μ$+light jet, $e$+$b$-jet, and $μ$+$b$-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additional same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s} = 13$ TeV, and from a part of Run 3 (2022-2023), corresponding to 55 fb$^{-1}$ at $\sqrt{s} = 13.6$ TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark ($\tilde{S}_1$) production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded $\tilde{S}_1$ mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings $y_{de} = 1.0$, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for $y_{sμ}$, $y_{be}$, and $y_{bμ}$ couplings set to 3.5, respectively.
Data (dots) and post-fit SM distribution (histograms) of m<sub>ℓj</sub> in (a, b) SR-1L-ej and (c, d) SR-2L-ej of the e+light-jet channel obtained by a CR+SR background-only fit for Run 2 and Run 3, respectively. The lower panel shows the ratio of observed data to the total post- and pre-fit SM prediction. The last bin includes the overflow. Uncertainties in the background estimates include both the statistical and systematic uncertainties, with correlations between uncertainties taken into account. The dashed lines show the predicted yields for two benchmark signal models corresponding to S̃<sub>1</sub> (m, y<sub>de</sub>) = (2.0 TeV, 1.0) and S̃<sub>1</sub> (m, y<sub>de</sub>) = (3.0 TeV, 1.0), respectively. Note: the values in the table are normalized by the width of corresponding bin
Data (dots) and post-fit SM distribution (histograms) of m<sub>ℓj</sub> in (a, b) SR-1L-ej and (c, d) SR-2L-ej of the e+light-jet channel obtained by a CR+SR background-only fit for Run 2 and Run 3, respectively. The lower panel shows the ratio of observed data to the total post- and pre-fit SM prediction. The last bin includes the overflow. Uncertainties in the background estimates include both the statistical and systematic uncertainties, with correlations between uncertainties taken into account. The dashed lines show the predicted yields for two benchmark signal models corresponding to S̃<sub>1</sub> (m, y<sub>de</sub>) = (2.0 TeV, 1.0) and S̃<sub>1</sub> (m, y<sub>de</sub>) = (3.0 TeV, 1.0), respectively. Note: the values in the table are normalized by the width of corresponding bin
Data (dots) and post-fit SM distribution (histograms) of m<sub>ℓj</sub> in (a, b) SR-1L-ej and (c, d) SR-2L-ej of the e+light-jet channel obtained by a CR+SR background-only fit for Run 2 and Run 3, respectively. The lower panel shows the ratio of observed data to the total post- and pre-fit SM prediction. The last bin includes the overflow. Uncertainties in the background estimates include both the statistical and systematic uncertainties, with correlations between uncertainties taken into account. The dashed lines show the predicted yields for two benchmark signal models corresponding to S̃<sub>1</sub> (m, y<sub>de</sub>) = (2.0 TeV, 1.0) and S̃<sub>1</sub> (m, y<sub>de</sub>) = (3.0 TeV, 1.0), respectively. Note: the values in the table are normalized by the width of corresponding bin
A measurement of the top-quark pole mass $m_{t}^\text{pole}$ is presented in $t\bar{t}$ events with an additional jet, $t\bar{t}+1\text{-jet}$, produced in $pp$ collisions at $\sqrt{s}=13$ TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of $140~\text{fb}^{-1}$. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $t\bar{t}+1\text{-jet}$ differential cross-section as a function of the inverse of the invariant mass of the $t\bar{t}+1\text{-jet}$ system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process $pp \to t\bar{t}j$ ($2 \rightarrow 3$), where top quarks are taken as stable particles, and the process $pp \to b\bar{b}l^+νl^- \barν j$ ($2 \to 7$), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a $χ^2$ fit of the unfolded normalized differential cross-section distribution. The results obtained with the $2 \to 3$ and $2 \to 7$ calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the $2 \to 3 $ measurement: $m_{t}^\text{pole}=170.7\pm0.3~(\text{stat.})\pm1.4~(\text{syst.})~\pm 0.3~(\text{scale})~\pm 0.2~(\text{PDF}\oplusα_\text{S})~\text{GeV},$ which is in good agreement with other top-quark mass results.
Unfolded number of events in the 2-to-3measurement (not normalized). The parton level is defined with two stable top-quarks and a jet with $p_{T}>50$ GeV and $|\eta|<2.5$.
Covariance matrix for statistical effects of the measured number of events after unfolding, for the 2-to-3 measurement (not normalized)
Covariance matrix for statistical and systematic effects of the measured number of events after unfolding, for the 2-to-3 measurement (not normalized)
A search for single production of a vector-like quark $Q$, which could be either a singlet $T$, with charge $\tfrac23$, or a $Y$ from a $(T,B,Y)$ triplet, with charge $-\tfrac43$, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb$^{-1}$ recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis targets $Q \to Wb$ decays where the $W$ boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the $Q$ to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet $T$, the 95% confidence level limit on the coupling strength $κ$ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the $(T,B,Y)$ triplet, the limits on $κ$ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.
Distributions of the VLQ-candidate mass, m<sub>VLQ</sub>, in the (a–c) SRs, (d–f) W+jets CRs and (g–i) tt̄ CRs after the fit to the background-only hypothesis. The columns correspond from left to right to the low-, middle-, and high-p<sub>T</sub><sup>W</sup> bins in each region. Other includes remaining backgrounds from top quarks or that contain two W/Z bosons. The last bin includes overflow. Note: the 'Data' values in the table are normalized by the width of the bin to correspond to the number of events per 100 GeV
Distributions of the VLQ-candidate mass, m<sub>VLQ</sub>, in the (a–c) SRs, (d–f) W+jets CRs and (g–i) tt̄ CRs after the fit to the background-only hypothesis. The columns correspond from left to right to the low-, middle-, and high-p<sub>T</sub><sup>W</sup> bins in each region. Other includes remaining backgrounds from top quarks or that contain two W/Z bosons. The last bin includes overflow. Note: the 'Data' values in the table are normalized by the width of the bin to correspond to the number of events per 100 GeV
Distributions of the VLQ-candidate mass, m<sub>VLQ</sub>, in the (a–c) SRs, (d–f) W+jets CRs and (g–i) tt̄ CRs after the fit to the background-only hypothesis. The columns correspond from left to right to the low-, middle-, and high-p<sub>T</sub><sup>W</sup> bins in each region. Other includes remaining backgrounds from top quarks or that contain two W/Z bosons. The last bin includes overflow. Note: the 'Data' values in the table are normalized by the width of the bin to correspond to the number of events per 100 GeV
Ultrarelativistic heavy-ion collisions create a quark-gluon plasma (QGP), a hot and dense state of strongly interacting QCD matter. In ultracentral collisions, the QGP volume remains nearly constant event-by-event, while its total entropy can fluctuate due to quantum effects, leading to temperature variations. These features allow the correlation between the mean transverse momentum $(\langle p_{\mathrm{T}} \rangle)$ of charged hadrons and their multiplicity to serve as a probe of the QGP's speed of sound, $c_{s}$. This study extracts $c_{s}$ by analyzing the relative increase in $\langle p_{\mathrm{T}} \rangle$ with respect to the charged-particle density $(\langle \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta \rangle)$ at midrapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$, using data from the ALICE detector. Centrality is determined with estimators based on multiplicity and transverse energy, applying a pseudorapidity gap to reduce selection biases. The extracted value of $c_{s}^{2}$ is found to strongly depend on the employed centrality estimator and ranges between $0.1146 \pm 0.0028 \,\mathrm{(stat.)} \pm 0.0065 \,\mathrm{(syst.)}$ and $0.4374 \pm 0.0006 \mathrm{(stat.)} \pm 0.0184 \mathrm{(syst.)}$ in natural units. Additionally, the event-by-event $[p_{\mathrm{T}}]$ distribution is studied through its variance, skewness, and kurtosis. A pronounced decrease in the self-normalized variance and a peak followed by a drop in skewness suggest the suppression of impact-parameter fluctuations in ultracentral collisions. These observations provide new insights into the thermodynamic properties and initial-state fluctuations of the QGP.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimators based on $N_{\mathrm{ch}}$, ${N_{\mathrm{tracklets}}}$, and $E_{\mathrm{T}}$ within $|\eta|\leq 0.8$.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $-3.7<\eta<-1.7$ and $2.8 < \eta <5.1$.
Normalized $p_{\mathrm{T}}$-spectrum ratio as a function as a function of centrality in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $0.5 \leq |\eta|\leq 0.8$.
This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{t}\gamma\gamma$). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb$^{-1}$ recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{t}\gamma\gamma$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be $2.42^{+0.58}_{-0.53}\, \text{fb}$, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{t}\gamma\gamma$ to top-quark pair production in association with one photon is determined, yielding $(3.30^{+0.70}_{-0.65})\times 10^{-3}$.
Measured $t\bar{t}\gamma\gamma$ production fiducial inclusive cross-section in single-lepton decay channel.
Measured ratio of production cross sections of $t\bar{t}\gamma\gamma$ to $t\bar{t}\gamma$ in single-lepton decay channel.
Summary of the relative impact of all the systematic uncertainties, in percentage, on the $t\bar{t}\gamma\gamma$ fiducial inclusive cross section and $R_{t\bar{t}\gamma\gamma/t\bar{t}\gamma}$ grouped into different categories. The category ‘Jet’ corresponds to the effect of JES, jet resolution and JVT uncertainties, ‘Photon’ and ‘Leptons’ include all experimental uncertainties related to photons and leptons (including trigger uncertainties), respectively.
Measurements of $W^+W^-\rightarrow e^\pm νμ^\mp ν$ production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics and the electroweak theory. The measurements are based on data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015-2018, corresponding to an integrated luminosity of 140 fb$^{-1}$. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with $b$-hadron decays. An improved methodology for estimating the remaining top-quark background enables a precise measurement of $W^+W^-$ cross-sections with no additional requirements on jets. The fiducial $W^+W^-$ cross-section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total $W^+W^-$ cross-section of $127\pm4$ pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of $W^+W^-$ events. The measurements are compared with state-of-the-art theory calculations and excellent agreement with predictions is observed. A charge asymmetry in the lepton rapidity is observed as a function of the dilepton invariant mass, in agreement with the Standard Model expectation. A CP-odd observable is measured to be consistent with no CP violation. Limits on Standard Model effective field theory Wilson coefficients in the Warsaw basis are obtained from the differential cross-sections.
Measured fiducial cross-section compared with theoretical predictions from MiNNLO+Pythia8, Geneva+Pythia8, Sherpa2.2.12, and MATRIX2.1. The predictions are based on the NNPDF3.0 (red squares) and NNPDF3.1 luxQED (blue dots) PDF sets. The nNNLO predictions include photon-induced contributions (always using NNPDF3.1 luxQED) and NLO QCD corrections to the gluon-gluon initial state. The $q\bar{q}\rightarrow WW$ predictions from MiNNLO, Geneva, and Sherpa2.2.12 are combined with a Sherpa2.2.2 prediction of gluon-induced production, scaled by an inclusive NLO K-factor of 1.7. Inner (outer) error bars on the theory predictions correspond to PDF (the combination of scale and PDF) uncertainties. The MATRIX nNNLO QCD $\otimes$ NLO EW prediction using NNPDF3.1 luxQED, the best available prediction of the integrated fiducial cross-section, is in good agreement with the measurement.
Fiducial differential cross-sections as a function of $p_{\mathrm{T}}^{\mathrm{lead.\,lep.}}$. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The right-hand-side axis indicates the integrated cross-section of the rightmost bin. The results are compared to fixed-order nNNLO QCD + NLO EW predictions of Matrix 2.1, with the NNLO + PS predictions from Powheg MiNNLO + Pythia8 and Geneva + Pythia8, as well as Sherpa2.2.12 NLO + PS predictions. The last three predictions are combined with Sherpa 2.2.2 for the $gg$ initial state and Sherpa 2.2.12 for electroweak $WWjj$ production. These contributions are modelled at LO but a NLO QCD $k$-factor of 1.7 is applied for gluon induced production. Theoretical predictions are indicated as markers with vertical lines denoting PDF, scale and parton shower uncertainties. Markers are staggered for better visibility.
Correlation matrix of the statistical uncertainties in the measured fiducial cross section for the observable $p_{\mathrm{T}}^{\mathrm{lead.\,lep.}}$.
A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with an emphasis on signatures of a strongly-coupled hidden dark sector accessed via resonant production of a $Z'$ mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 is used, consisting of proton-proton collisions at $\sqrt{s}$ = 13 TeV and corresponding to an integrated luminosity of 140 fb$^{-1}$. The $Z'$ mediator is considered to decay to two dark quarks, which each hadronize and decay to showers containing both dark and Standard Model particles, producing a topology of interacting and non-interacting particles within a jet known as ``semi-visible". Machine learning methods are used to select these dark showers and reject the dominant background of mismeasured multijet events, including an anomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonance search is performed by fitting the transverse mass spectrum based on a functional form background estimation. No significant excess over the expected background is observed. Results are presented as limits on the production cross section of semi-visible jet signals, parameterized by the fraction of invisible particles in the decay and the $Z'$ mass, and by quantifying the significance of any generic Gaussian-shaped mass peak in the anomaly region.
Acceptance times efficiency weighted yields across the signal grid.
The 95% CL limits on the cross-section $\sigma(pp \rightarrow Z' \rightarrow \chi \chi$) times branching ratio B in fb with all statistical and systematic uncertainties, for the $R_{\text{inv}}=$0.2 signal points.
The 95% CL limits on the cross-section $\sigma(pp \rightarrow Z' \rightarrow \chi \chi$) times branching ratio B in fb with all statistical and systematic uncertainties, for the $R_{\text{inv}}=$0.4 signal points.
This work reports femtoscopic correlations of p$-$p ($\bar{\rm p}-\bar{\rm p}$) and p$-$d ($\bar{\rm p}-\bar{\rm d}$) pairs measured in Pb$-$Pb collisions at center-of-mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV by the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass ($m_\text{T}$) and on the average charge particle multiplicity $\langle\text{dN}_\text{ch}/\text{d}\eta\rangle^{1/3}$ for three centrality intervals (0$-$10$\%$, 10$-$30$\%$, 30$-$50$\%$). In both cases, the expected power-law and linear scalings are observed, respectively. The measured p$-$d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p$-$(p$-$n) system, due to the large inter-particle distances in Pb$-$Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at $2.73^{+0.05}_{-0.05}$ and $3.10^{+1.04}_{-0.86}$ fm, respectively, for the 30$-$50$\%$ centrality collisions. Moreover, the $m_{\rm{T}}$-scaling obtained for the p$-$p and p$-$d systems is compatible within 1$\sigma$ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.
proton-proton (same charge) correlation function for centrality 0-10% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 10-30% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 30-50% from Pb-Pb collisions at 5020 GeV
Higgs boson production cross-sections via gluon-gluon fusion and vector-boson fusion in proton-proton collisions are measured in the $H\rightarrow WW^\ast \rightarrow \ellν\ellν$ decay channel. The Large Hadron Collider delivered proton-proton collisions at a centre-of-mass energy of $13\,\textrm{TeV}$ between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $140\,\textrm{fb}^{-1}$. The total cross-sections for Higgs boson production by gluon-gluon fusion and vector-boson fusion times the $H\rightarrow WW^\ast$ branching ratio are measured to be $12.4^{+1.3}_{-1.2}\,\textrm{pb}$ and $0.79^{+0.18}_{-0.16}\,\textrm{pb}$, respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterised through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory.
Expected values and uncertainties for the $H \to WW^{\ast}$ cross-sections measured in each of the STXS categories, normalised to the corresponding SM predictions.
Best-fit values and uncertainties for the $H \to WW^{\ast}$ cross-sections measured in each of the STXS categories, normalised to the corresponding SM predictions.
Expected correlations between the production cross-sections multiplied by the $H \to WW^{\ast}$ branching ratio for each of the STXS categories.
This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_{0}(p_\mathrm{T})$, which serves as a probe of radial flow and medium properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_\mathrm{T}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_\mathrm{T}$ ($> 3$ GeV/$c$), protons exhibit larger $v_{0}(p_\mathrm{T})$ than pions and kaons, in agreement with expectations from quark-recombination models. These results are sensitive to the bulk viscosity and the equation of state of the QCD medium formed in heavy-ion collisions.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 10$–$20% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 30$–$40% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 60$–$70% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
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