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 $pp \to W^{\pm} (\to μ^{\pm} ν_μ) X$ cross-sections are measured at a proton-proton centre-of-mass energy $\sqrt{s} = 5.02$ TeV using a dataset corresponding to an integrated luminosity of 100 pb$^{-1}$ recorded by the LHCb experiment. Considering muons in the pseudorapidity range $2.2 < η< 4.4$, the cross-sections are measured differentially in twelve intervals of muon transverse momentum between $28 < p_\mathrm{T} < 52$ GeV. Integrated over $p_\mathrm{T}$, the measured cross-sections are \begin{align*} σ_{W^+ \to μ^+ ν_μ} &= 300.9 \pm 2.4 \pm 3.8 \pm 6.0~\text{pb}, \\ σ_{W^- \to μ^- \barν_μ} &= 236.9 \pm 2.1 \pm 2.7 \pm 4.7~\text{pb}, \end{align*} where the first uncertainties are statistical, the second are systematic, and the third are associated with the luminosity calibration. These integrated results are consistent with theoretical predictions. This analysis introduces a new method to determine the $W$-boson mass using the measured differential cross-sections corrected for detector effects. The measurement is performed on this statistically limited dataset as a proof of principle and yields \begin{align*} m_W = 80369 \pm 130 \pm 33~\text{MeV}, \end{align*} where the first uncertainty is experimental and the second is theoretical.
The measured differential cross sections ($d\sigma/dp_T$) for $W^+$. The first systematic uncertainty is statistical and the second is systematic.
The measured differential cross sections ($d\sigma/dp_T$) for $W^-$. The first systematic uncertainty is statistical and the second is systematic.
The correlation matrix corresponding to the statistical uncertainties on the differential cross-section ($d\sigma/dp_T$) fit results for $W^+$. To combine with $W^-$, use the rows and columns ordered as $W^+$ and then $W^-$. Assume no correlation in the statistical uncertainties between $W^+$ and $W^-$ (zero entries in the off-diagonal blocks).
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.
The measurement of $\Sigma^{+}$ production in pp collisions at $\sqrt{s} = 13$ TeV is presented. The measurement is performed at midrapidity in both minimum-bias and high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV. The $\Sigma^{+}$ is reconstructed via its weak-decay topology in the decay channel $\Sigma^{+} \rightarrow {p} + \pi^{0}$ with $\pi^{0} \rightarrow \gamma + \gamma$. In a novel approach, the neutral pion is reconstructed by combining photons that convert in the detector material with photons measured in the calorimeters. The transverse-momentum ($p_{T}$) distributions of the $\Sigma^{+}$ and its rapidity densities d$N/$dy in both event classes are reported. The $p_{T}$ spectrum in minimum-bias collisions is compared to QCD-inspired event generators. The ratio of $\Sigma^{+}$ to previously measured $\Lambda$ baryons is in good agreement with calculations from the Statistical Hadronization Model. The high efficiency and purity of the novel reconstruction method for $\Sigma^{+}$ presented here will enable future studies of the interaction of $\Sigma^{+}$ with protons in the context of femtoscopic measurements, which could be crucial for understanding the equation of state of neutron stars.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the high-multiplicity triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the minimum-bias triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
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.
The first measurement at midrapidity ($|y| < 0.5$) of the production yield of the strange-charm baryons $Ξ_c^+$ and $Ξ_c^0$ as a function of charged-particle multiplicity in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ALICE experiment at the LHC is reported. The $Ξ_c^+$ baryon is reconstructed via the $Ξ_c^+ \rightarrow Ξ^-π^+π^+$ decay channel in the range $4 < p_{\rm T} < 12$ GeV/$c$, while the $Ξ_c^0$ baryon is reconstructed via both the $Ξ_c^0 \rightarrow Ξ^-π^+$ and $Ξ_c^0 \rightarrow Ξ^-e^+ν_e$ decay channels in the range $2 < p_{\rm T} < 12$ GeV/$c$. The baryon-to-meson ($Ξ_c^{0,+}/D^0$) and the baryon-to-baryon ($Ξ_c^{0,+}/Λ_c^+$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at $e^+e^-$ and $e^-$p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed. Moreover, the branching-fraction ratio of BR($Ξ_c^0 \rightarrow Ξ^-e^+ν_e$)/BR($Ξ_c^0 \rightarrow Ξ^-π^+$) is measured as 0.825 $\pm$ 0.094 (stat.) $\pm$ 0.081 (syst.). This value supersedes the previous ALICE measurement, improving the statistical precision by a factor of 1.6.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^0$ baryons measured in the different multiplicity classes.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^+$ baryons measured in the different multiplicity classes.
Ratio between the prompt $\Xi_c^0$ baryons in a multiplicity class to the multiplicity-integrated (INEL $>$ 0) class.
The dependence of f$_0$(980) production on the final-state charged-particle multiplicity is reported for proton-proton (pp) collisions at the centre-of-mass energy, $\sqrt{s}= 13$ TeV. The production of f$_0$(980) is measured with the ALICE detector via the f$_0(980) \rightarrow π^{+}π^{-}$ decay channel in a midrapidity region of $|y| < 0.5$. The evolution of the integrated yields and mean transverse momentum of f$_{0}$(980) as a function of charged-particle multiplicity measured in pp at $\sqrt{s} = 13$ TeV follows the trends observed in pp at $\sqrt{s} = 5.02$ TeV and in proton-lead (p-Pb) collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV. Particle yield ratios of f$_{0}$(980) to $π^{\pm}$ and K$^{*}$(892)$^{0}$ are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f$_{0}$(980) is assumed, which suggests that the tetraquark interpretation of the f$_{0}$(980) is disfavored.
Transverse momentum spectra in different multiplicity classes. Each spectrum is corrected for the branching ratio of (46 $\pm$ 6)% based on [Phys. Rev. Lett. 111 no. 6, (2013) 062001].
The ratio of transverse momentum spectrum to the INEL > 0 spectrum
Transverse momentum integrated f0(980) yield in pp collisions
The transverse momentum spectra and integrated yields of $\overlineΣ^{\pm}$ have been measured in pp and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel $\overlineΣ^{\pm} \rightarrow {\rm\overline{n}}π^{\pm}$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The $p_{\rm T}$ spectra of $\overlineΣ^{\pm}$ are measured in the range $0.5 < p_{\rm T} < 3$ GeV/$c$ and compared to predictions of the PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4 models. The EPOS LHC and EPOS4 models provide the best descriptions of the measured spectra both in pp and p-Pb collisions, while models which do not account for multiparton interactions provide a considerably worse description at high $p_{\rm T}$. The total yields of $\overlineΣ^{\pm}$ in both pp and p-Pb collisions are compared to predictions of the Thermal-FIST model and dynamical models PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4. All models reproduce the total yields in both colliding systems within uncertainties. The nuclear modification factors $R_{\rm pPb}$ for both $\overlineΣ^{+}$ and $\overlineΣ^{-}$ are evaluated and compared to those of protons, $Λ$ and $Ξ$ hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of $R_{\rm pPb}$ for $\overlineΣ^{\pm}$ from the model predictions or measurements for other hadrons are found within uncertainties.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{-}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in NSD p-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y_\mathrm{CMS}|<0.5$.
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.
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