A search is presented for emerging jets using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the ATLAS experiment between 2015 and 2018. The search looks for the existence of a dark sector with symmetries similar to those in quantum chromodynamics. This dark sector is populated with dark quarks, which undergo showering similar to quarks in the Standard Model, leading to a high multiplicity of long-lived dark hadrons within a dark jet. These dark hadrons subsequently decay to Standard Model particles via a new heavy scalar mediating particle $ϕ$. This results in jets which contain multiple displaced vertices, known as emerging jets. This analysis targets four-jet topologies, with two emerging jets and two Standard Model jets, resulting from the decay of pair-produced scalar mediators. No significant excess above the Standard Model background is observed. For dark pion proper decay lengths of 20 mm, mediator masses are excluded between 1 TeV and 2 TeV assuming a dark pion mass of 20 GeV.
Comparison of the data with N<sub>DV</sub> > 1 and the estimated background in the SR using the modified ABCD method. The division between the SR and CR is shown by the vertical dashed line. In the final fit, the bins with R > 0.4 are combined into a single bin. Three selected signal samples are included for comparison.
95% CL upper limits as a function of (left) cτ<sub>π<sub>d</sub></sub> and (right) M<sub>φ</sub>. The upper plots show the expected and observed limits on σ(pp →φ<sup>†</sup>φ) for m<sub>π<sub>d</sub></sub> = 20 GeV: (a) using M<sub>φ</sub> = 1.6 TeV and (b) using cτ<sub>π<sub>d</sub></sub> = 20 mm. The lower plots show a comparison of the observed limits for all three dark pion masses: (c) using M<sub>φ</sub> = 1.4 TeV, and (d) using cτ<sub>π<sub>d</sub></sub> = 1 mm. The mediator mass-dependent theoretical cross-section is given with the band corresponding to the uncertainty from NNLL-Fast.
95% CL upper limits as a function of (left) cτ<sub>π<sub>d</sub></sub> and (right) M<sub>φ</sub>. The upper plots show the expected and observed limits on σ(pp →φ<sup>†</sup>φ) for m<sub>π<sub>d</sub></sub> = 20 GeV: (a) using M<sub>φ</sub> = 1.6 TeV and (b) using cτ<sub>π<sub>d</sub></sub> = 20 mm. The lower plots show a comparison of the observed limits for all three dark pion masses: (c) using M<sub>φ</sub> = 1.4 TeV, and (d) using cτ<sub>π<sub>d</sub></sub> = 1 mm. The mediator mass-dependent theoretical cross-section is given with the band corresponding to the uncertainty from NNLL-Fast.
The angular distributions of Drell-Yan lepton pairs provide sensitive probes of the underlying dynamics of quantum chromodynamics (QCD) effects in vector-boson production. This paper presents for the first time the measurement of the full set of angular coefficients together with the differential cross-section as a function of the transverse momentum of the $W$ boson, in the full phase space of the decay leptons. The measurements are performed separately for the $W^-$ and $W^+$ channels. The analysis uses proton-proton collision data recorded by the ATLAS experiment at the Large Hadron Collider in 2017 and 2018, during special low-luminosity runs with a reduced number of interactions per bunch crossings (pile-up). The data correspond to an integrated luminosity of $338$ pb$^{-1}$ at a centre-of-mass energy of $\sqrt{s} = 13$ TeV. The low pile-up conditions enable an optimised reconstruction of the $W$ boson transverse momentum. All results agree with theory predictions incorporating finite-order QCD corrections up to next-to-next-to-leading-order in the strong coupling constant, $α_S$.
The measured angular coefficients for $W^-$ in bins of the $p_T$ of the W.
The measured angular coefficients for $W^+$ in bins of the $p_T$ of the W.
The measured differential cross-section for $W^-$ in bins of the $p_T$ of the $W$.
This article reports on a search for dijet resonances using $132$ fb$^{-1}$ of $pp$ collision data recorded at $\sqrt{s} = 13$ TeV by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of $p_{\textrm{T}} \ge 85$ GeV and jet rapidity separation of $|y^{*}|<0.6$ are analysed for dijet resonances with invariant masses from $375$ to $1800$ GeV. A data-driven background estimate is used to model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at $95\%$ confidence level on coupling values for a benchmark leptophobic axial-vector $Z^{\prime}$ model and on the production cross-section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.
Observed $m_{jj}$ distribution for the J50 signal region, using variable-width bins and the analysis selections. The background estimate corresponds to the ansatz fit, integrated over each bin.
Observed $m_{jj}$ distribution for the J100 signal region, using variable-width bins and the analysis selections. The background estimate corresponds to the ansatz fit, integrated over each bin.
Observed 95% $\text{CL}_\text{S}$ upper limits on the production cross-section times acceptance times branching ratio to jets, $\sigma \cdot A \cdot \text{BR}$, of Gaussian-shaped signals of 5%, 10%, and 15% width relative to their peak mass, $m_G$. Also included are the corresponding expected upper limits predicted for the case the $m_{jj}$ distribution is observed to be identical to the background prediction in each bin and the $1\sigma$ and $2\sigma$ envelopes of outcomes expected for Poisson fluctuations around the background expectation. Limits are derived from the J50 signal region.
The existence of right-handed neutrinos with Majorana masses below the electroweak scale could help address the origins of neutrino masses, the matter-antimatter asymmetry, and dark matter. In this paper, leptonic decays of W bosons from 140 fb$^{-1}$ of 13 TeV proton-proton collisions at the LHC, reconstructed in the ATLAS experiment, are used to search for heavy neutral leptons produced through their mixing with muon or electron neutrinos in a scenario with lepton number violation. The search is conducted using prompt leptonic decay signatures. The considered final states require two same-charge leptons or three leptons, while vetoing three-lepton same-flavour topologies. No significant excess over the expected Standard Model backgrounds is found, leading to constraints on the heavy neutral lepton's mixing with muon and electron neutrinos for heavy-neutral-lepton masses. The analysis excludes $|U_{e}|^2$ values above $8\times 10^{-5}$ and $|U_μ|^2$ values above $5.0 \times 10^{-5}$ in the full mass range of 8-65 GeV. The strongest constraints are placed on heavy-neutral-lepton masses in the range 15--30 GeV of $|U_{e}|^2 < 1.1 \times 10^{-5}$ and $|U_μ|^2 < 5 \times 10^{-6}$.
Comparison between the data and estimated background at preselection level. Events entering the SRs defined in Section 5 are vetoed. The events are classified in terms of the number of leptons and their flavours, as well as the number of b-jets. The ℓ<sup>±</sup>ℓ<sup>±</sup> bins have a ≥2 signal leptons selection, with no requirement on the number of baseline leptons; the ℓ<sup>±</sup>ℓ<sup>±</sup>ℓ'<sup>∓</sup> bins have a =3 signal leptons selection. The uncertainties shown with hashed bands, include only the statistical uncertainties and the full uncertainties associated with the data-driven background estimates. The bottom panel shows the ratio of the observed data yields to the predicted background yields.
Comparison between the data and estimated background in the validation regions. The hatched band represents the total uncertainty in the estimated background.
Observed 95% confidence level (CL) exclusion limits for the (a) |U<sub>e</sub>|<sup>2</sup> and (b) |U<sub>μ</sub>|<sup>2</sup> mixing parameters versus the HNL mass. The expected (dashed line) exclusion limits are also shown. The 1σ and 2σ uncertainty bands around the expected exclusion limit reflect uncertainties in signal and background yields.
A model-agnostic search for Beyond the Standard Model physics is presented, targeting final states with at least four light leptons (electrons or muons). The search regions are separated by event topology and unsupervised machine learning is used to identify anomalous events in the full 140 fb$^{-1}$ of proton-proton collision data collected with the ATLAS detector during Run 2. No significant excess above the Standard Model background expectation is observed. Model-agnostic limits are presented in each topology, along with limits on several benchmark models including vector-like leptons, wino-like charginos and neutralinos, or smuons. Limits are set on the flavourful vector-like lepton model for the first time.
Comparison between data and the background prediction for the (a) m<sub>T</sub>(4ℓ, E<sub>T</sub><sup>miss</sup>), (b) m<sup>high</sup>(3ℓ), (c) m(Z), (d) E<sub>T</sub><sup>miss</sup>, (e) p<sub>T</sub>(Z), and (f) N<sub>jets</sub> distribution in the (a, d) 2Z 0b, (b, e) 1Z 1b 2SFOS, and (c, f) 0Z 2SFOS region, after requiring the anomaly score to be below the 90% background rejection point. The background contributions after the likelihood fit to data ('post-fit') for the background-only hypothesis are shown as filled histograms. The 'tt+X' background component includes the tt̄Z, and tt̄H processes. The 'HF ℓ' ('LF ℓ') background component refers to processes containing one non-prompt light lepton from heavy-flavour (light-flavour) hadron decays. The ratio of the data to the background prediction ('Bkg.') is shown in the lower panel. The 'Other' contribution is dominated by the tWZ production. The size of the combined statistical and systematic uncertainty in the background prediction is indicated by the blue hatched band. The upward-pointing blue arrows indicate points for which the data-to-background ('Data/Bkg.’) ratio exceeds the vertical range of the figure. The last bin contains the overflow.
Comparison between data and the background prediction for the (a) m<sub>T</sub>(4ℓ, E<sub>T</sub><sup>miss</sup>), (b) m<sup>high</sup>(3ℓ), (c) m(Z), (d) E<sub>T</sub><sup>miss</sup>, (e) p<sub>T</sub>(Z), and (f) N<sub>jets</sub> distribution in the (a, d) 2Z 0b, (b, e) 1Z 1b 2SFOS, and (c, f) 0Z 2SFOS region, after requiring the anomaly score to be below the 90% background rejection point. The background contributions after the likelihood fit to data ('post-fit') for the background-only hypothesis are shown as filled histograms. The 'tt+X' background component includes the tt̄Z, and tt̄H processes. The 'HF ℓ' ('LF ℓ') background component refers to processes containing one non-prompt light lepton from heavy-flavour (light-flavour) hadron decays. The ratio of the data to the background prediction ('Bkg.') is shown in the lower panel. The 'Other' contribution is dominated by the tWZ production. The size of the combined statistical and systematic uncertainty in the background prediction is indicated by the blue hatched band. The upward-pointing blue arrows indicate points for which the data-to-background ('Data/Bkg.’) ratio exceeds the vertical range of the figure. The last bin contains the overflow.
Comparison between data and the background prediction for the (a) m<sub>T</sub>(4ℓ, E<sub>T</sub><sup>miss</sup>), (b) m<sup>high</sup>(3ℓ), (c) m(Z), (d) E<sub>T</sub><sup>miss</sup>, (e) p<sub>T</sub>(Z), and (f) N<sub>jets</sub> distribution in the (a, d) 2Z 0b, (b, e) 1Z 1b 2SFOS, and (c, f) 0Z 2SFOS region, after requiring the anomaly score to be below the 90% background rejection point. The background contributions after the likelihood fit to data ('post-fit') for the background-only hypothesis are shown as filled histograms. The 'tt+X' background component includes the tt̄Z, and tt̄H processes. The 'HF ℓ' ('LF ℓ') background component refers to processes containing one non-prompt light lepton from heavy-flavour (light-flavour) hadron decays. The ratio of the data to the background prediction ('Bkg.') is shown in the lower panel. The 'Other' contribution is dominated by the tWZ production. The size of the combined statistical and systematic uncertainty in the background prediction is indicated by the blue hatched band. The upward-pointing blue arrows indicate points for which the data-to-background ('Data/Bkg.’) ratio exceeds the vertical range of the figure. The last bin contains the overflow.
A search for Higgs boson pair production in the $b \overline{b} γγ$ final state is performed. The proton-proton collision dataset in this analysis corresponds to an integrated luminosity of 308 fb$^{-1}$, consisting of two samples, 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV and 168 fb$^{-1}$ at 13.6 TeV, recorded between 2015 and 2024 by the ATLAS detector at the CERN Large Hadron Collider. In addition to a larger dataset, this analysis improves upon the previous search in the same final state through several methodological and technical developments. The Higgs boson pair production cross section divided by the Standard Model prediction is found to be $μ_{HH} = 0.9^{+1.4}_{-1.1}$ ($μ_{HH} = 1^{+1.3}_{-1.0}$ expected), which translates into a 95% confidence-level upper limit of $μ_{HH}<3.8$. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range $-1.7 < κ_λ< 6.6$ ($-1.8 < κ_λ< 6.9$ expected).
Weighted di-photon invariant mass distribution summed over all categories and the two data-taking periods. The events in each category are weighted by $log(1+S_{SM}/B)$. $S_{SM}$ is the expected signal yield assuming $\mu_{HH}$=1, while B is the continuum background yield obtained from a fit to the sidebands plus the single Higgs boson background obtained from simulation, all in a ± 5 GeV window around the Higgs boson mass. The lines show the fit results for the continuum background only (light dotted), adding single Higgs boson backgrounds (black dotted) and the full fit (solid).
Weighted di-photon invariant mass distribution summed over all categories and the two data-taking periods. The events in each category are weighted by $log(1+S_{SM}/B)$. $S_{SM}$ is the expected signal yield assuming $\mu_{HH}$=1, while B is the continuum background yield obtained from a fit to the sidebands plus the single Higgs boson background obtained from simulation, all in a ± 5 GeV window around the Higgs boson mass. The lines show the fit results for the continuum background only (light dotted), adding single Higgs boson backgrounds (black dotted) and the full fit (solid).
The 95% CL upper limits on the signal strength, obtained with separate fits to Run-2 and Run-3 data as well as their combination. When computing the significance or upper limit for one data-taking period only, $\mu_{HH}$ of the other period is left free to vary. All other parameters of interest are fixed to their SM expectation.
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.
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}}=$0.4 signal points.
Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system's radial expansion, has received less attention. Notably, experimental evidence for the global and collective nature of radial flow has been lacking. This Letter presents the first measurement of transverse momentum ($p_{\mathrm{T}}$) dependence of radial flow fluctuations ($v_0(p_{\mathrm{T}})$) over $0.5<p_{\mathrm{T}}<10$ GeV, using a two-particle correlation method in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in $p_{\mathrm{T}}$, and centrality-independent shape in $p_{\mathrm{T}}$. The comparison with a hydrodynamic model demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.
Data from Figure 2, panel a, $v_{0}$
Data from Figure 2, panel c, upper panel, Normalized Covariance $\times 10^{3}$ in 0-5% Centrality
Data from Figure 2, panel c, lower panel, Normalized Covariance $\times 10^{3}$ in 50-60% Centrality
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.
Summary of the one-DV limits for the H/ϕ arrow ss model. Comparison between observed and expected 95% CL limits on (σ/σggH)×B for an SM-like Higgs boson portal mediator and ms=35 GeV. The observed limits are consistent with the expected ones within the uncertainties.
Observed 95% CL limits on (σ/σggH)×B for all Higgs boson portal mediator samples where the cross-section is normalized to the SM Higgs boson gluon–gluon fusion production cross-section, σggH = 48.61 pb [97]. The observed limits are consistent with the expected ones within the uncertainties.
Observed 95% CL limits on σ×B for mϕ≠ 125 GeV. The observed limits are consistent with the expected ones within the uncertainties.