Papers by Leonidas Fountas
Physical Review Letters
A search is made for potential 𝑐𝑐 c c tetraquarks decaying into a pair of charmonium states in th... more A search is made for potential 𝑐𝑐 c c tetraquarks decaying into a pair of charmonium states in the four muon final state using proton-proton collision data at √ 𝑠 = 13 TeV, corresponding to an integrated luminosity of 140 fb -1 recorded by the ATLAS experiment at LHC. Two decay channels, 𝐽/𝜓 + 𝐽/𝜓 → 4𝜇 and 𝐽/𝜓 + 𝜓(2𝑆) → 4𝜇, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-𝐽/𝜓 channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the 𝐽/𝜓+𝜓(2S) channel. The fitted masses and decay widths of the structures are reported.
Anomaly detection search for new resonances decaying into a Higgs boson and a generic new particle $X$ in hadronic final states using $\sqrt{s} = 13$ TeV $pp$ collisions with the ATLAS detector
HAL (Le Centre pour la Communication Scientifique Directe), Jun 21, 2023
HAL (Le Centre pour la Communication Scientifique Directe), May 10, 2023
This paper describes a search for the single production of an up-type vector-like quark (𝑇) decay... more This paper describes a search for the single production of an up-type vector-like quark (𝑇) decaying as 𝑇 → 𝐻𝑡 or 𝑇 → 𝑍𝑡. The search utilises a dataset of 𝑝 𝑝 collisions at √ 𝑠 = 13 TeV collected with the ATLAS detector during the 2015-2018 data-taking period of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb -1 . Data are analysed in final states containing a single lepton with multiple jets and 𝑏-jets. The presence of boosted heavy resonances in the event is exploited to discriminate the signal from the Standard Model background. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross section of 𝑇 quarks in different decay channels. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (𝜅) of the vector-like quark. For singlet 𝑇 quarks, 𝜅 values above 0.53 are excluded for all masses below 2.3 TeV. At a mass of 1.6 TeV, 𝜅 values as low as 0.35 are excluded. For 𝑇 quarks in the doublet scenario, where the production cross section is much lower, 𝜅 values above 0.72 are excluded for all masses below 1.7 TeV, and this exclusion is extended to 𝜅 above 0.55 for low masses around 1.0 TeV.
Search for leptoquark pair production decaying into $te^- \bar{t}e^+$ or $t\mu^- \bar{t}\mu^+$ in multi-lepton final states in $pp$ collisions at 13 TeV with the ATLAS detector
HAL (Le Centre pour la Communication Scientifique Directe), Jul 5, 2023
Observation of $W\gamma\gamma$ triboson production in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector
HAL (Le Centre pour la Communication Scientifique Directe), Aug 16, 2023
arXiv (Cornell University), May 16, 2023
A search for supersymmetry targeting the direct production of winos and higgsinos is conducted in... more A search for supersymmetry targeting the direct production of winos and higgsinos is conducted in final states with either two leptons (𝑒 or 𝜇) with the same electric charge, or three leptons. The analysis uses 139 fb -1 of 𝑝 𝑝 collision data at √ 𝑠 = 13 TeV collected with the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess over the Standard Model expectation is observed. Simplified and complete models with and without 𝑅-parity conservation are considered. In topologies with intermediate states including either 𝑊 ℎ or 𝑊 𝑍 pairs, wino masses up to 525 GeV and 250 GeV are excluded, respectively, for a bino of vanishing mass. Higgsino masses smaller than 440 GeV are excluded in a natural 𝑅-parityviolating model with bilinear terms. Upper limits on the production cross section of generic events beyond the Standard Model as low as 40 ab are obtained in signal regions optimised for these models and also for an 𝑅-parity-violating scenario with baryon-number-violating higgsino decays into top quarks and jets. The analysis significantly improves sensitivity to supersymmetric models and other processes beyond the Standard Model that may contribute to the considered final states.
arXiv (Cornell University), Jul 14, 2023
arXiv (Cornell University), May 3, 2023
A search for new phenomena giving rise to pairs of opposite electrically charged muons with impac... more A search for new phenomena giving rise to pairs of opposite electrically charged muons with impact parameters in the millimetre range is presented, using 139 fb -1 of √ 𝑠 = 13 TeV 𝑝 𝑝 collision data from the ATLAS detector at the LHC. The search targets the gap in coverage between existing searches targeting final states with leptons with large displacement and prompt leptons. No significant excess over the background expectation is observed and exclusion limits are set on the mass of long-lived scalar supersymmetric muon-partners (smuons) with much lower lifetimes than previously targeted by displaced muon searches. Smuon lifetimes down to 1 ps are excluded for a smuon mass of 100 GeV, and smuon masses up to 520 GeV are excluded for a proper lifetime of 10 ps, at 95% confidence level. Finally, model-independent limits are set on the contribution from new phenomena to the signal-region yields.
The European Physical Journal C
This paper presents the muon momentum calibration and performance studies for the ATLAS detector ... more This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the pp collisions data sample produced at $$\sqrt{s}$$ s = 13 TeV at the LHC during Run 2 and corresponding to an integrated luminosity of 139 $${\textrm{fb}}^{-1}$$ fb - 1 . An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the $$Z\rightarrow \mu ^{+}\mu ^{-}$$ Z → μ + μ - resonance. The muon momentum scale and resolution are measured using samples of $$J/\psi \rightarrow \mu ^{+}\mu ^{-}$$ J / ψ → μ + μ - and $$Z\rightarrow \mu ^{+}\mu ^{-}$$ Z → μ + μ - events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent sample of $$\Upsilon \rightarrow \mu ^{+}\mu ^{-}$$ Υ → μ + μ - events. At the Z$$(J/\psi )$$ ( J / ψ ) peak, the momentum scale is measured with an uncertainty a...
Journal of High Energy Physics
A combination of measurements of the inclusive top-quark pair production cross-section performed ... more A combination of measurements of the inclusive top-quark pair production cross-section performed by ATLAS and CMS in proton–proton collisions at centre-of-mass energies of 7 and 8 TeV at the LHC is presented. The cross-sections are obtained using top-quark pair decays with an opposite-charge electron–muon pair in the final state and with data corresponding to an integrated luminosity of about 5 fb−1 at $$ \sqrt{s} $$ s = 7 TeV and about 20 fb−1 at $$ \sqrt{s} $$ s = 8 TeV for each experiment. The combined cross-sections are determined to be 178.5 ± 4.7 pb at $$ \sqrt{s} $$ s = 7 TeV and $$ {243.3}_{-5.9}^{+6.0} $$ 243.3 − 5.9 + 6.0 pb at $$ \sqrt{s} $$ s = 8 TeV with a correlation of 0.41, using a reference top-quark mass value of 172.5 GeV. The ratio of the combined cross-sections is determined to be R8/7 = 1.363 ± 0.032. The combined measured cross-sections and their ratio agree well with theory calculations using several parton distribution function (PDF) sets. The values of the ...
Journal of High Energy Physics
A search for diphoton resonances in the mass range between 10 and 70 GeV with the ATLAS experimen... more A search for diphoton resonances in the mass range between 10 and 70 GeV with the ATLAS experiment at the Large Hadron Collider (LHC) is presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 138 fb−1 at a centre-of-mass energy of 13 TeV recorded from 2015 to 2018. Previous searches for diphoton resonances at the LHC have explored masses down to 65 GeV, finding no evidence of new particles. This search exploits the particular kinematics of events with pairs of closely spaced photons reconstructed in the detector, allowing examination of invariant masses down to 10 GeV. The presented strategy covers a region previously unexplored at hadron colliders because of the experimental challenges of recording low-energy photons and estimating the backgrounds. No significant excess is observed and the reported limits provide the strongest bound on promptly decaying axion-like particles coupling to gluons and photons for masses between 10 and 70 GeV.
Journal of High Energy Physics
Differential and double-differential distributions of kinematic variables of leptons from decays ... more Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs ($$ t\overline{t} $$ t t ¯ ) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of $$ \sqrt{s} $$ s = 13 TeV and correspond to an integrated luminosity of 140 fb−1. The measurements use events containing an oppositely charged eμ pair and b-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton pT distributions is obtained by reweighting the $$ t\overline{t} $$ t t ¯ sample so as to reproduce the top-quark pT distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to be$$ {\sigma}...
Journal of High Energy Physics
This paper presents a search for a new Z′ vector gauge boson with the ATLAS experiment at the Lar... more This paper presents a search for a new Z′ vector gauge boson with the ATLAS experiment at the Large Hadron Collider using pp collision data collected at $$ \sqrt{s} $$ s = 13 TeV, corresponding to an integrated luminosity of 139 fb−1. The new gauge boson Z′ is predicted by Lμ − Lτ models to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4μ) final state, using a deep learning neural network classifier to separate the Z′ signal from the Standard Model background events. The di-muon invariant masses in the 4μ events are used to extract the Z′ resonance signature. No significant excess of events is observed over the predicted background. Upper limits at a 95% confidence level on the Z′ production cross-section times the decay branching fraction of pp → Z′μμ → 4μ are set from 0.31 to 4.3 fb for the Z′ mass ranging from 5 to 81 GeV. The corresponding common coupling strengths, gZ′, of the Z′ boson to the second and third gene...
Journal of High Energy Physics
A search for high-mass charged and neutral bosons decaying to Wγ and Zγ final states is presented... more A search for high-mass charged and neutral bosons decaying to Wγ and Zγ final states is presented in this paper. The analysis uses a data sample of $$ \sqrt{s} $$ s = 13 TeV proton-proton collisions with an integrated luminosity of 139 fb−1 collected by the ATLAS detector during LHC Run 2 operation. The sensitivity of the search is determined using models of the production and decay of spin-1 charged bosons and spin-0/2 neutral bosons. The range of resonance masses explored extends from 1.0 TeV to 6.8 TeV. At these high resonance masses, it is beneficial to target the hadronic decays of the W and Z bosons because of their large branching fractions. The decay products of the high-momentum W/Z bosons are strongly collimated and boosted-boson tagging techniques are employed to improve the sensitivity. No evidence of a signal above the Standard Model backgrounds is observed, and upper limits on the production cross-sections of these bosons times their branching fractions to Wγ and Zγ ar...
The European Physical Journal C
The exclusive production of pion pairs in the process $$pp\rightarrow pp\pi ^+\pi ^-$$ p p → p p ... more The exclusive production of pion pairs in the process $$pp\rightarrow pp\pi ^+\pi ^-$$ p p → p p π + π - has been measured at $$\sqrt{s}={7}\,\text {TeV}$$ s = 7 TeV with the ATLAS detector at the LHC, using $${80}\,{\upmu \textrm{b}}^{-1}$$ 80 μ b - 1 of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion–pion invariant mass. Cross-section values of $$4.8 \pm 1.0 \mathrm {\ (stat)} {~}^{+0.3}_{-0.2} \mathrm {\ (syst)}\ {\upmu \textrm{b}}$$ 4.8 ± 1.0 ( stat ) - 0.2 + 0.3 ( syst ) μ b and $$9 \pm 6 \mathrm {\ (stat)} {~}^{+2}_{-2} \mathrm {\ (syst)}\ {\upmu \textrm{b}} $$ 9 ± 6 ( stat ) - 2 + 2 ( syst ) μ b are obtained in ...
Journal of High Energy Physics
Measurements of Higgs boson production cross-sections are carried out in the diphoton decay chann... more Measurements of Higgs boson production cross-sections are carried out in the diphoton decay channel using 139 fb−1 of pp collision data at $$ \sqrt{s} $$ s = 13 TeV collected by the ATLAS experiment at the LHC. The analysis is based on the definition of 101 distinct signal regions using machine-learning techniques. The inclusive Higgs boson signal strength in the diphoton channel is measured to be $$ {1.04}_{-0.09}^{+0.10} $$ 1.04 − 0.09 + 0.10 . Cross-sections for gluon-gluon fusion, vector-boson fusion, associated production with a W or Z boson, and top associated production processes are reported. An upper limit of 10 times the Standard Model prediction is set for the associated production process of a Higgs boson with a single top quark, which has a unique sensitivity to the sign of the top quark Yukawa coupling. Higgs boson production is further characterized through measurements of Simplified Template Cross-Sections (STXS). In total, cross-sections of 28 STXS regions are measu...
Journal of High Energy Physics
Measurements of differential cross sections are presented for inclusive isolated-photon productio... more Measurements of differential cross sections are presented for inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13 TeV provided by the LHC and using 139 fb−1 of data recorded by the ATLAS experiment. The cross sections are measured as functions of the photon transverse energy in different regions of photon pseudorapidity. The photons are required to be isolated by means of a fixed-cone method with two different cone radii. The dependence of the inclusive-photon production on the photon isolation is investigated by measuring the fiducial cross sections as functions of the isolation-cone radius and the ratios of the differential cross sections with different radii in different regions of photon pseudorapidity. The results presented in this paper constitute an improvement with respect to those published by ATLAS earlier: the measurements are provided for different isolation radii and with a more granular segmentation in photon pseudorapidity that can b...
Journal of High Energy Physics
Differential cross-section measurements of Zγ production in association with hadronic jets are pr... more Differential cross-section measurements of Zγ production in association with hadronic jets are presented, using the full 139 fb−1 dataset of $$ \sqrt{s} $$ s = 13 TeV proton–proton collisions collected by the ATLAS detector during Run 2 of the LHC. Distributions are measured using events in which the Z boson decays leptonically and the photon is usually radiated from an initial-state quark. Measurements are made in both one and two observables, including those sensitive to the hard scattering in the event and others which probe additional soft and collinear radiation. Different Standard Model predictions, from both parton-shower Monte Carlo simulation and fixed-order QCD calculations, are compared with the measurements. In general, good agreement is observed between data and predictions from MATRIX and MiNNLOPS, as well as next-to-leading-order predictions from MadGraph5_aMC@NLO and Sherpa.
Journal of High Energy Physics
This paper presents a search for hypothetical massive, charged, long-lived particles with the ATL... more This paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb−1 of proton–proton collisions at $$ \sqrt{s} $$ s = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to $$ \mathcal{O} $$ O (1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.
The European Physical Journal C
Cross-sections for the production of a Z boson in association with two photons are measured in pr... more Cross-sections for the production of a Z boson in association with two photons are measured in proton–proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb$$^{-1}$$ - 1 recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated $$Z(\rightarrow \ell \ell )\gamma \gamma $$ Z ( → ℓ ℓ ) γ γ cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the $$Z\gamma \gamma $$ Z γ γ system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
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Papers by Leonidas Fountas