Measurements of jet substructure are key to probing the energy frontier at colliders, and many of them use track-based observables which take advantage of the angular precision of tracking detectors. Theoretical calculations of track-based observables require `track functions', which characterize the transverse momentum fraction $r_q$ carried by charged hadrons from a fragmenting quark or gluon. This letter presents a direct measurement of $r_q$ distributions in dijet events from the 140 fb$^{-1}$ of proton--proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector. The data are corrected for detector effects using machine-learning methods. The scale evolution of the moments of the $r_q$ distribution is sensitive to non-linear renormalization group evolution equations of QCD, and is compared with analytic predictions. When incorporated into future theoretical calculations, these results will enable a precision program of theory-data comparison for track-based jet substructure observables.
$r_{q}$, Gluon jets, $240\text{GeV} \leq p_T < 300~\text{GeV}$, Gluon $\eta$, Fig 5
$r_{q}$, Gluon jets, $300~\text{GeV} \leq p_T < 400~\text{GeV}$, Gluon $\eta$, Fig 5
$r_{q}$, Gluon jets, $400~\text{GeV} \leq p_T < 500~\text{GeV}$, Gluon $\eta$, Fig 5
The mass of the top quark is measured using top-antitop-quark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in proton--proton collisions at $\sqrt{s}=13$ TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb$^{-1}$. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be $m_t = 172.95 \pm 0.53$ GeV, which is the most precise ATLAS measurement from a single channel.
This Letter presents a search for highly ionizing magnetic monopoles in 262$~\mu$b$^{-1}$ of ultraperipheral Pb+Pb collision data at $\sqrt{s_{_\textrm{NN}}}=5.36$ TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross-section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20-150 GeV. The search significantly improves on the previous cross-section limits for production of low-mass monopoles in ultraperipheral Pb+Pb collisions.
Expected and observed cross-section upper limits computed using the CL$_{s}$ method for $|q_{m}| = 1 g_{\textrm{D}}$ and assuming FPA model
This paper presents the measurement of charged-hadron and identified-hadron ($K^\mathrm{0}_\mathrm{S}$, $Λ$, $Ξ^\mathrm{-}$) yields in photo-nuclear collisions using 1.7 $\mathrm{nb^{-1}}$ of $\sqrt{s_\mathrm{NN}} = 5.02$ TeV Pb+Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photo-nuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photo-nuclear collisions as a function of charged-particle multiplicity. These photo-nuclear results are compared with 0.1 $\mathrm{nb^{-1}}$ of $\sqrt{s_\mathrm{NN}} = 5.02$ TeV $p$+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photo-nuclear measurements shed light on potential quark-gluon plasma formation in photo-nuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo DPMJET-III generator and hydrodynamic calculations to test whether such photo-nuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.
The multiplicity distribution (#it{N}_{ch}^{rec}) from Pb+Pb photo-nuclear collisions.
The multiplicity distribution (#it{N}_{ch}^{rec}) from p+Pb collisions.
The Charged-hadron yields as a function of pT in different y selections in Pb+Pb photo-nuclear collisions.
Differential cross-section measurements of $Z\gamma$ production in association with hadronic jets are presented, using the full 139 fb$^{-1}$ dataset of $\sqrt{s}=13$ TeV proton-proton collisions collected by the ATLAS detector during Run 2 of the LHC. Distributions are measured using events in which the $Z$ boson decays leptonically and the photon is usually radiated from an initial-state quark. Measurements are made in both one and two observables, including those sensitive to the hard scattering in the event and others which probe additional soft and collinear radiation. Different Standard Model predictions, from both parton-shower Monte Carlo simulation and fixed-order QCD calculations, are compared with the measurements. In general, good agreement is observed between data and predictions from MATRIX and MiNNLO$_\text{PS}$, as well as next-to-leading-order predictions from MadGraph5_aMC@NLO and Sherpa.
Measured differential cross section as a function of observable $ p_{T}^{ll}$. Error on the measured cross-section include all the systematic uncertainties. SM predictions are produced with the event generators at particle level: Sherpa 2.2.4, Sherpa 2.2.11, MadGraph5_aMC@NLO, and MiNNLO$_{PS}$. Fixed order calculations results use MATRIX NNLO. Error represent statistical uncertainty and theoretical uncertainty (PDF and Scale variations).
Measured differential cross section as a function of observable $ p_{T}^{ll} - p_{T}^{\gamma}$. Error on the measured cross-section include all the systematic uncertainties. SM predictions are produced with the event generators at particle level: Sherpa 2.2.4, Sherpa 2.2.11, MadGraph5_aMC@NLO, and MiNNLO$_{PS}$. Fixed order calculations results use MATRIX NNLO. Error represent statistical uncertainty and theoretical uncertainty (PDF and Scale variations).
Measured differential cross section as a function of observable $ p_{T}^{ll} + p_{T}^{\gamma}$. Error on the measured cross-section include all the systematic uncertainties. SM predictions are produced with the event generators at particle level: Sherpa 2.2.4, Sherpa 2.2.11, MadGraph5_aMC@NLO, and MiNNLO$_{PS}$. Fixed order calculations results use MATRIX NNLO. Error represent statistical uncertainty and theoretical uncertainty (PDF and Scale variations).
Top-quark pair production is observed in lead-lead (Pb+Pb) collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of 1.9 nb$^{-1}$. Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross-section is $\sigma_{t\bar{t}} = 3.6\;^{+1.0}_{-0.9}\;\mathrm{(stat.)}\;^{+0.8}_{-0.5}\;\mathrm{(syst.)} ~\mathrm{\mu b}$, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the pre-equilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early universe.
The figure shows the post-fit distribution of events as a function of the dilepton invariant mass ($m_{e\mu}$), in lead-lead (Pb+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV, with an integrated luminosity of 1.9 nb$^{-1}$. The data correspond to the SR1 (Signal Region 1 (SR\(_1\)):} Events with exactly one muon and one oppositely charged electron, a dilepton invariant mass \( m_{e\mu} \geq 30 \, \mathrm{GeV} \), at least two jets with \( p_T \geq 35 \, \mathrm{GeV} \), and a dilepton transverse momentum \( p_T^{e\mu} > 40 \, \mathrm{GeV} \). This region is expected to be signal-dominated) channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield.
The figure shows the post-fit distribution of events as a function of the dilepton invariant mass ($m_{e\mu}$), in lead-lead (Pb+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV, with an integrated luminosity of 1.9 nb$^{-1}$. The data correspond to the SR2 (Signal Region 2 (SR\(_2\)):} Events meeting the same criteria as SR\(_1\), but with a dilepton transverse momentum \( p_T^{e\mu} \leq 40 \, \mathrm{GeV} \). This region includes events with a lower \( p_T^{e\mu} \) and has a larger background contribution) channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield.
The figure shows the measurement of the top quark pair production cross-section, $\sigma_{t\bar{t}}$, in lead-lead (Pb+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV, based on 1.9 nb$^{-1}$ of data.
This article presents a search for a heavy charged Higgs boson produced in association with a top quark and a bottom quark, and decaying into a $W$ boson and a $125$ GeV Higgs boson $h$. The search is performed in final states with one charged lepton, missing transverse momentum, and jets using proton-proton collision data at $\sqrt{s} = 13$ TeV recorded with the ATLAS detector during Run 2 of the LHC at CERN. This data set corresponds to a total integrated luminosity of 140 fb$^{-1}$. The search is conducted by examining the reconstructed invariant mass distribution of the $Wh$ candidates for evidence of a localised excess in the charged Higgs boson mass range from $250$ GeV to $3$ TeV. No significant excess is observed and 95% confidence-level upper limits between $2.8$ pb and $1.2$ fb are placed on the production cross-section times branching ratio for charged Higgs bosons decaying into $Wh$.
Upper limit at the 95% CL on the product of the cross-section for the $pp \rightarrow tb H^{\pm}$ process and the branching ratio $B(W^{\pm} \times B (h \rightarrow b \bar{b} ))$ from the combined fit to all signal and control regions of the resolved analysis.
Upper limit at the 95% CL on the product of the cross-section for the $pp \rightarrow tb H^{\pm}$ process and the branching ratio $B(W^{\pm} \times B (h \rightarrow b \bar{b} ))$ from the combined fit to all signal and control regions of the merged analysis.
Product of acceptance and efficiency for pp->tbH(->Wh) as function of the charged Higgs boson mass for the resolved qqbb low-purity signal region.
The paper presents a search for supersymmetric particles produced in proton-proton collisions at $\sqrt{s}=$ 13 TeV and decaying into final states with missing transverse momentum and jets originating from charm quarks. The data were taken with the ATLAS detector at the Large Hadron Collider at CERN from 2015 to 2018 and correspond to an integrated luminosity of 139 fb$^{-1}$. No significant excess of events over the expected Standard Model background expectation is observed in optimized signal regions, and limits are set on the production cross-sections of the supersymmetric particles. Pair production of charm squarks or top squarks, each decaying into a charm quark and the lightest supersymmetric particle $\tilde{\chi}^0_1$, is excluded at 95% confidence level for squarks with masses up to 900 GeV for scenarios where the mass of $\tilde{\chi}^0_1$ is below 50 GeV. Additionally, the production of leptoquarks with masses up to 900 GeV is excluded for the scenario where up-type leptoquarks decay into a charm quark and a neutrino. Model-independent limits on cross-sections and event yields for processes beyond the Standard Model are also reported.
Expected exclusion limit at 95% CL for pair production of top squarks decaying to charm quarks and neutralinos.
Expected exclusion limit $(-1\sigma)$ at 95% CL for pair production of top squarks decaying to charm quarks and neutralinos.
Expected exclusion limit $(+1\sigma)$ at 95% CL for pair production of top squarks decaying to charm quarks and neutralinos.
A search for exotic decays of the 125 GeV Higgs boson into a pair of new spin-0 particles, $H \to aa$, where one decays into a photon pair and the other into a $\tau$-lepton pair, is presented. Hadronic decays of the $\tau$-leptons are considered and reconstructed using a dedicated tagger for collimated $\tau$-lepton pairs. The search uses 140 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider. The search is performed in the mass range of the $a$ boson between 10 GeV and 60 GeV. No significant excess of events is observed above the Standard Model background expectation. Model-independent upper limits at 95$\% $ confidence level are set on the branching ratio of the Higgs boson to the $\gamma\gamma\tau\tau$ final state, $\mathcal{B}(H\to aa\to \gamma\gamma\tau\tau)$, ranging from 0.2$\% $ to 2$\% $, depending on the $a$-boson mass hypothesis.
Distribution of the diphoton invariant mass for all events satisfying the analysis selections in the full Run 2 dataset.
Scan of the observed $p$-value as a function of $m_{a}$ for the background-only hypothesis.
The observed and expected ($\pm1\sigma$) upper limits at 95% CL on the branching ratio for $H\rightarrow aa\rightarrow \gamma\gamma\tau\tau$ as a function of the resonance mass hypothesis $m_{a}$.
Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via $H^{\pm} \to \tau^{\pm}\nu_{\tau}$, are searched for in 140 $\text{fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with the $H^{\pm}$ decays hadronically or semi-leptonically, the search targets $\tau$+jets or $\tau$+lepton final states, in both cases with a $\tau$-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of $80 \leq m_{H^{\pm}} \leq 3000$ GeV, upper limits at 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction $\mathrm{\cal{B}}(H^{\pm} \to \tau^{\pm}\nu_{\tau})$ in the range 4.5 pb-0.4 fb. In the mass range 80-160 GeV, assuming the Standard Model cross-section for $t\bar{t}$ production, this corresponds to upper limits between 0.27% and 0.02% on $\mathrm{\cal{B}}(t\to bH^{\pm}) \times \mathrm{\cal{B}}(H^{\pm} \to \tau^{\pm}\nu_{\tau})$.
Observed and expected 95 % CL exclusion limits on $\sigma(pp\to tbH^+)\times \mathrm{\cal{B}}(H^+ \to \tau \nu)$ as a function of $m_{H^{\pm}}$, from a combined fit in the $\tau$+jets and $\tau$+lepton channels. The surrounding shaded bands correspond to the 1$\sigma$ and 2$\sigma$ confidence intervals around the expected limit.
Observed and expected 95 % CL exclusion limits on $\mathrm{\cal{B}}(t\to bH^+)\times \mathrm{\cal{B}}(H^+ \to \tau \nu)$ as a function of $m_{H^{\pm}}$, from a combined fit in the $\tau$+jets and $\tau$+lepton channels. The surrounding shaded bands correspond to the 1$\sigma$ and 2$\sigma$ confidence intervals around the expected limit.
Observed and expected 95 % CL exclusion limits on $\tan\beta$ as a function of $m_{H^{\pm}}$, shown in the context of the hMSSM scenario, for $m_{H^{\pm}}>150$ GeV and $(1 \leq \tan\beta \leq 60)$. The surrounding shaded bands correspond to the 1$\sigma$ and 2$\sigma$ confidence intervals around the expected limit.