Inclusive and differential cross-section measurements of t\bartZ production in pp collisions at √s=13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (tt¯Z) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at s√=13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb−1. The inclusive cross section is measured to be σtt¯Z=0.86±0.04 (stat.)±0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the tt¯Z system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in tt¯Z events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations

Observation of quantum entanglement in top-quark pairs using the ATLAS detector

We report the highest-energy observation of entanglement, in top−antitop quark events produced at the Large Hadron Collider, using a proton−proton collision data set with a center-of-mass energy of s√=13 TeV and an integrated luminosity of 140 fb−1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top−antitop quark production threshold, where the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D=−0.547±0.002 (stat.)±0.021 (syst.) for 340<mtt¯<380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes both the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement to date

Measurements of the associated production of a top quark pair and a Z boson with the ATLAS detector

This thesis presents measurements of the inclusive and differential cross-sections of a top quark pair produced in association with a Z boson with the ATLAS detector. The full LHC Run-2 dataset corresponding to an integrated luminosity of 139 fb−1 collected during proton-proton collisions at a centre-of-mass energy of √ s =13 TeV is used in the measurements. The thesis focuses on the three-lepton final states of the t ¯tZ system where only electrons or muons are considered. The measured inclusive cross-section is σ(pp → t ¯tZ) = 0.99 ± 0.05 (stat.) ± 0.08 (syst.) pb = 0.99 ± 0.10 pb. It is in good agreement with other experimental observations and the theoretical prediction of σt ¯tZ = 0.86+0.07 −0.08 (scale) ± 0.02 (PDF) pb. The measurements of the differential cross-sections are performed using profile likelihood unfolding. A comprehensive set of eight observables is used to probe different aspects of the t ¯tZ production. The differential cross-section in each observable is measured to be in good agreement with the theoretical prediction which uses the MADGRAPH5_AMC@NLO+PYTHIA8 Monte Carlo event generation. The measurements presented in this thesis achieve a significant improvement in precision with respect to the previous ATLAS measurements

Measurements of the inclusive and differential production cross sections of a top-quark-antiquark pair in association with a ZZ boson at s=13\sqrt{s} = 13 TeV with the ATLAS detector

Measurements of the inclusive and differential production cross sections of a top-quark-antiquark pair in association with a $Z$ boson (\ttZ) are presented in these proceedings. The measurements are performed using full Run 2 data that corresponds to 139 $\textrm{fb}^{-1}$ integrated luminosity and is collected with the ATLAS detector at the CERN Large Hadron Collider. The inclusive cross section, which is measured to be \sigma_{\ttZ} = 0.99 \pm 0.05 \, \textrm{(stat)} \pm 0.08 \, \textrm{(syst.)} \, \textrm{pb}, and the differential cross section measurements are in good agreement with the SM predicitons

Measurements of the inclusive production cross-section of a top-quark-antiquark pair in association with a Z boson at s=13\sqrt{s} = 13 TeV with the ATLAS detector.

The production of a top-quark-antiquark pair in association with a Z boson (ttZ) is a rare process that directly probes the neutral coupling of the top quark to the electroweak gauge bosons. A better understanding of this process also benefits precision measurements of other Standard Model processes and beyond the Standard Model searches where ttZ is an important background. The ttZ process became accessible only recently owing to the large centre-of-mass energy and luminosity of the Large Hadron Collider. This talk will present measurements of the inclusive ttZ production cross sections at a centre-of-mass energy of 13 TeV with the ATLAS detector. These measurements use the full set of data collected during Run 2 of the LHC from 2015 to 2018, corresponding to a total integrated luminosity of 139 fb^{-1}. The inclusive cross section is measured to be $\sigma_{t\bar{t}Z} = 0.99 \pm 0.05 (stat.) \pm 0.08 (syst.)$ pb, in good agreement with the theoretical predictions

Measurements of the associated production of a top quark pair and a Z boson with the ATLAS detector

This thesis presents measurements of the inclusive and differential cross-sections of a top quark pair produced in association with a $Z$ boson with the ATLAS detector. The full LHC Run-2 dataset corresponding to an integrated luminosity of $139 \, \text{fb}^{-1}$ collected during proton-proton collisions at a centre-of-mass energy of $13 \, \text{TeV}$ is used in the measurements. The thesis focuses on the three-lepton final states of the $t\bar{t}Z$ system where only electrons or muons are considered. The measured inclusive cross-section is $\sigma(pp \rightarrow t\bar{t}Z = 0.99 \pm 0.05 \, \text{(stat.)} \pm 0.08 \, \text{(syst.)} \, \text{pb} = 0.99 \pm 0.10 \, \text{pb}$. It is in good agreement with other experimental observations and the theoretical prediction of $\sigma_{t\bar{t}Z} = 0.86^{+0.07}_{-0.08} \, \text{(scale)} \pm 0.02 \, \text{(PDF)} \, \text{pb}$. The measurements of the differential cross-sections are performed using profile likelihood unfolding. A comprehensive set of eight observables is used to probe different aspects of the $t\bar{t}Z$ production. The differential cross-section in each observable is measured to be in good agreement with the theoretical prediction which uses the Madgraph5_aMC@NLO+Pythia8 Monte Carlo event generation. The measurements presented in this thesis achieve a significant improvement in precision with respect to the previous ATLAS measurements

Precision measurements of top-quark couplings and cross sections with the ATLAS detector at the LHC

The Standard Model of particle physics (SM) connects symmetries to the interactions of elementary particles. Effective field theories (EFT) extend the SM with higher-dimension operators that maintain the SM’s symmetries in the low-energy limit. The high center-of-mass energy of proton-proton collisions and the high integrated luminosities at the CERN Large Hadron Collider allows to use the large sample of top quark events to test SM theoretical predictions and constrain the effects of EFT operators with unprecedented sensitivity. Using data taken with the ATLAS detector at the LHC, recent measurements of the associated production of top quarks with Z bosons, photons and an additional top-quark pair (4-top production), a recent search for flavor-changing interactions of the top quark and the Z boson, as well as recent precision measurements of top-quark processes are presented that constrain EFT operators via observables related to the polarization of single-top-quark events, the transverse momentum of high-momentum top quarks in top-quark pair production and the energy asymmetry in top-quark pair production events with an additional jet