Showing papers by "Eric Lancon published in 2023"

Measurements of the suppression and correlations of dijets in Pb+Pb collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>5.02<

Abstract: Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and pp collisions at $\sqrt{s_{_\text{NN}}}$ = 5.02 TeV, measurements of the leading dijet transverse momentum ($p_{\mathrm{T}}$) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet $p_{\mathrm{T}}$ are made. The measurements are performed using the ATLAS detector at the LHC with 260 pb$^{-1}$ of pp data collected in 2017 and 2.2 nb$^{-1}$ of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet $p_{\mathrm{T}}$ distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet $p_{\mathrm{T}}$ greater than 100 GeV. Measurements of the dijet-yield-normalized $x_{\mathrm{J}}$ distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to pp collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 TeV in the overlapping kinematic range. Measurements of the absolutely-normalized dijet rate in Pb+Pb and pp collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with $p_{\mathrm{T}}$ between 100 and 316 GeV for all centralities in Pb+Pb collisions.

Search for excited τ-leptons and leptoquarks in the final state with τ-leptons and jets in pp collisions at $$ \sqrt{s} $$ = 13 TeV with the ATLAS detector

Abstract: A bstract A search is reported for excited τ -leptons and leptoquarks in events with two hadronically decaying τ -leptons and two or more jets. The search uses proton-proton ( pp ) collision data at $$ \sqrt{s} $$ s = 13 TeV recorded by the ATLAS experiment during the Run 2 of the Large Hadron Collider in 2015–2018. The total integrated luminosity is 139 fb − 1 . The excited τ -lepton is assumed to be produced and to decay via a four-fermion contact interaction into an ordinary τ -lepton and a quark-antiquark pair. The leptoquarks are assumed to be produced in pairs via the strong interaction, and each leptoquark is assumed to couple to a charm or lighter quark and a τ -lepton. No excess over the background prediction is observed. Excited τ -leptons with masses below 2.8 TeV are excluded at 95% CL in scenarios with the contact interaction scale Λ set to 10 TeV. At the extreme limit of model validity where Λ is set equal to the excited τ -lepton mass, excited τ -leptons with masses below 4.6 TeV are excluded. Leptoquarks with masses below 1.3 TeV are excluded at 95% CL if their branching ratio to a charm quark and a τ -lepton equals 1. The analysis does not exploit flavour-tagging in the signal region.

Production of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">Υ</mml:mi><mml:mo>(</mml:mo><mml:mi>n</mml:mi><mml:mi>S</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> mesons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:

Abstract: A measurement of the production of vector bottomonium states, $\varUpsilon(\textrm{1S})$, $\varUpsilon(\textrm{2S})$, and $\varUpsilon(\textrm{3S})$, in Pb+Pb and $pp$ collisions at a center-of-mass energy per nucleon pair of 5.02 TeV is presented. The data correspond to integrated luminosities of 1.38~$\textrm{nb}^{-1}$ of Pb+Pb data collected in 2018, 0.44 $\textrm{nb}^{-1}$ of Pb+Pb data collected in 2015, and 0.26 $\textrm{fb}^{-1}$ of $pp$ data collected in 2017 by the ATLAS detector at the Large Hadron Collider. The measurements are performed in the dimuon decay channel for transverse momentum $p_\textrm{T}^{\mu\mu} < 30$~ GeV, absolute rapidity $|y^{\mu\mu}|<1.5$, and Pb+Pb event centrality 0-80%. The production rates of the three bottomonium states in Pb+Pb collisions are compared with those in $pp$ collisions to extract the nuclear modification factors as functions of event centrality, $p_\textrm{T}^{\mu\mu}$, and $|y^{\mu\mu}|$. In addition, the suppression of the excited states relative to the ground state is studied. The results are compared with theoretical model calculations.

Observation of four-top-quark production in the multilepton final state with the ATLAS detector

Abstract: This paper presents the observation of four-top-quark ($t\bar{t}t\bar{t}$) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured $t\bar{t}t\bar{t}$ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The $t\bar{t}t\bar{t}$ production cross section is measured to be $22.5^{+6.6}_{-5.5}$ fb, consistent with the SM prediction of $12.0 \pm 2.4$ fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect $t\bar{t}t\bar{t}$ production.

Measurement of single top-quark production in the s-channel in proton–proton collisions at $$ \sqrt{s} $$ = 13 TeV with the ATLAS detector

Abstract: A measurement of single top-quark production in the s-channel is performed in proton$-$proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb$^{-1}$. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two $b$-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and $W$-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is $\sigma=8.2^{+3.5}_{-2.9}$ pb, consistent with the Standard Model prediction of $\sigma^{\mathrm{SM}}=10.32^{+0.40}_{-0.36}$ pb.

Measurement of substructure-dependent jet suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

Abstract: The ATLAS detector at the Large Hadron Collider has been used to measure jet substructure modification and suppression in Pb+Pb collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_\mathrm{NN}}}=5.02~\mathrm{TeV}$ in comparison with $pp$ collisions at $\sqrt{s}=5.02~\mathrm{TeV}$. The Pb+Pb data, collected in 2018, have an integrated luminosity of $1.72~\mathrm{nb^{-1}}$, while the $pp$ data, collected in 2017, have an integrated luminosity of $260~\mathrm{pb}^{-1}$. Jets used in this analysis are clustered using the anti-$k_{t}$ algorithm with a radius parameter $R=0.4$. The jet constituents, defined by both tracking and calorimeter information, are used to determine the angular scale $r_\mathrm{g}$ of the first hard splitting inside the jet by reclustering them using the Cambridge-Aachen algorithm and employing the soft-drop grooming technique. The nuclear modification factor, $R_\mathrm{AA}$, used to characterize jet suppression in Pb+Pb collisions, is presented differentially in $r_\mathrm{g}$, jet transverse momentum, and in intervals of collision centrality. The $R_\mathrm{AA}$ value is observed to depend significantly on jet $r_\mathrm{g}$. Jets produced with the largest measured $r_\mathrm{g}$ are found to be twice as suppressed as those with the smallest $r_\mathrm{g}$ in central Pb+Pb collisions. The $R_\mathrm{AA}$ values do not exhibit a strong variation with jet $p_\mathrm{T}$ in any of the $r_\mathrm{g}$ intervals. The $r_\mathrm{g}$ and $p_\mathrm{T}$ dependence of jet $R_\mathrm{AA}$ is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.

Measurement of Zγγ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Z\gamma \gamma $$\end{document} production in pp col

Abstract: Abstract 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.

Measurement of the total cross section and $$\rho $$-parameter from elastic scattering in pp collisions at $$\sqrt{s}=13$$ TeV with the ATLAS detector

Abstract: In a special run of the LHC with $\beta^\star = 2.5~$km, proton-proton elastic-scattering events were recorded at $\sqrt{s} = 13~$TeV with an integrated luminosity of $340~\mu \textrm{b}^{-1}$ using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam $t$ variable in the range from $-t = 2.5 \cdot 10^{-4}~$GeV$^{2}$ to $-t = 0.46~$GeV$^{2}$ using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section $\sigma_{\textrm{tot}}$, parameters of the nuclear slope, and the $\rho$-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit $t \rightarrow 0$. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the $t$-dependence. The results for $\sigma_{\textrm{tot}}$ and $\rho$ are \begin{equation*} \sigma_{\textrm{tot}}(pp\rightarrow X) = \mbox{104.7} \pm 1.1 \; \mbox{mb} , \; \; \; \rho = \mbox{0.098} \pm 0.011 . \end{equation*} The uncertainty in $\sigma_{\textrm{tot}}$ is dominated by the luminosity measurement, and in $\rho$ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.

Correlations between flow and transverse momentum in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Xe</mml:mi><mml:mo>+</mml:mo><mml:mi>Xe</mml:mi></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Pb</mml:mi><mml:mo>

Abstract: The correlations between flow harmonics $v_n$ for $n=2$, 3 and 4 and mean transverse momentum $[p_\mathrm{T}]$ in $^{129}$Xe+$^{129}$Xe and $^{208}$Pb+$^{208}$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV and 5.02 TeV, respectively, are measured using charged particles with the ATLAS detector. The correlations are sensitive to the shape and size of the initial geometry, nuclear deformation, and initial momentum anisotropy. The effects from non-flow and centrality fluctuations are minimized, respectively, via a subevent cumulant method and event activity selection based on particle production in the very forward rapidity. The results show strong dependences on centrality, harmonic number $n$, $p_{\mathrm{T}}$ and pseudorapidity range. Current models describe qualitatively the overall centrality- and system-dependent trends but fail to quantitatively reproduce all the data. In the central collisions, where models generally show good agreement, the $v_2$-$[p_\mathrm{T}]$ correlations are sensitive to the triaxiality of the quadruple deformation. The comparison of model to the Pb+Pb and Xe+Xe data suggests that the $^{129}$Xe nucleus is a highly deformed triaxial ellipsoid that is neither a prolate nor an oblate shape. This provides strong evidence for a triaxial deformation of $^{129}$Xe nucleus using high-energy heavy-ion collision.

Exclusive dielectron production in ultraperipheral Pb+Pb collisions at $$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV with ATLAS

Abstract: Exclusive production of dielectron pairs, $\gamma\gamma\rightarrow e^+e^-$, is studied using $\mathcal{L}_\mathrm{int}=1.72\; \mathrm{nb^{-1}}$ of data from ultraperipheral collisions of lead nuclei at $\sqrt{s_{_{\text{NN}}}} = 5.02$ TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon-photon interaction in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum, $p_{\textrm{T}}^{e} > 2.5$ GeV, absolute electron pseudorapidity, $|\eta^{e}| < 2.5$, invariant mass of the dielectron system, $m_{ee} > 5$ GeV, and transverse momentum of the dielecton pair, $p_{\textrm{T}}^{ee} < 2$ GeV. Differential cross-sections are measured as a function of $m_{ee}$, average $p_{\textrm{T}}^{e}$, absolute rapidity of the dielectron system, $|y_{ee}|$, and scattering angle in the dielectron rest frame, $|\cos\theta^*|$ in the inclusive sample, and also under the requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be $215 \pm 1 \text{(stat.)} ^{+23}_{-20} \text{(syst.)} \pm 4 \text{(lumi.)}\; \mu$b. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences with these predictions which are more pronounced at high $|y_{ee}|$ and $|\cos\theta^*|$ values.

Search for Higgs boson pair production in association with a vector boson in pp collisions at s=13TeV\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidema

Abstract: This paper reports a search for Higgs boson pair ($hh$) production in association with a vector boson ($W$ or $Z$) using 139 $fb^{-1}$ of proton-proton collision data at $\sqrt{s}=$ 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed in final states in which the vector boson decays leptonically ($W\to\ell u, Z\to\ell\ell, u u$ with $\ell=e, \mu$) and the Higgs bosons each decay into a pair of $b$-quarks. It targets $Vhh$ signals from both non-resonant $hh$ production, present in the Standard Model (SM), and resonant $hh$ production, as predicted in some SM extensions. A 95% confidence-level upper limit of 183 (87) times the SM cross-section is observed (expected) for non-resonant $Vhh$ production when assuming the kinematics are as expected in the SM. Constraints are also placed on Higgs boson coupling modifiers. For the resonant search, upper limits on the production cross-sections are derived for two specific models: one is the production of a vector boson along with a neutral heavy scalar resonance $H$, in the mass range 260-1000 GeV, that decays into $hh$, and the other is the production of a heavier neutral pseudoscalar resonance $A$ that decays into a $Z$ boson and $H$ boson, where the $A$ boson mass is 360-800 GeV and the $H$ boson mass is 260-400 GeV. Constraints are also derived in the parameter space of two-Higgs-doublet models.

Measurement of the CP properties of Higgs boson interactions with $$\tau $$-leptons with the ATLAS detector

Abstract: A study of the charge conjugation and parity ($CP$) properties of the interaction between the Higgs boson and $\tau$-leptons is presented. The study is based on a measurement of $CP$-sensitive angular observables defined by the visible decay products of $\tau$-lepton decays, where at least one hadronic decay is required. The analysis uses 139 fb$^{-1}$ of proton$-$proton collision data recorded at a centre-of-mass energy of $\sqrt{s}= 13$ TeV with the ATLAS detector at the Large Hadron Collider. Contributions from $CP$-violating interactions between the Higgs boson and $\tau$-leptons are described by a single mixing angle parameter $\phi_{\tau}$ in the generalised Yukawa interaction. Without assuming the Standard Model hypothesis for the $H\rightarrow\tau\tau$ signal strength, the mixing angle $\phi_{\tau}$ is measured to be $9^{\circ} \pm 16^{\circ}$, with an expected value of $0^{\circ} \pm 28^{\circ}$ at the 68% confidence level. The pure $CP$-odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.

Search for leptonic charge asymmetry in $$ t\overline{t}W $$ production in final states with three leptons at $$ \sqrt{s} $$ = 13 TeV

Abstract: A bstract A search for the leptonic charge asymmetry ( $$ {A}_{\textrm{c}}^{\ell } $$ A c ℓ ) of top-quark-antiquark pair production in association with a W boson ( $$ t\overline{t}W $$ t t ¯ W ) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on $$ \sqrt{s} $$ s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015–2018, corresponding to an integrated luminosity of 139 fb − 1 . A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be −0 . 12 ± 0 . 14 (stat.) ± 0 . 05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of −0 . 11 ± 0 . 17 (stat.) ± 0 . 05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are $$ -{0.084}_{-0.003}^{+0.005} $$ − 0.084 − 0.003 + 0.005 (scale) ± 0 . 006 (MC stat.) and $$ -{0.063}_{-0.004}^{+0.007} $$ − 0.063 − 0.004 + 0.007 (scale) ± 0 . 004 (MC stat.) respectively, and in agreement with the measurements.