Associated Production of a Top Quark and a Z Boson in <em>pp</em> Collisions at √s = 13 TeV Using the ATLAS Detector

The production of a top quark in association with an Z boson is studied using 36.1 fb-1 of proton-proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV. The search is performed in the trilepton channel, in which both the Z boson and the top quark decay to final states involving charged leptons. Events containing three identified leptons (electron and/or muon) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds come from diboson, top-pair production and Z+jets production. An artificial neural network is used to improve the background rejection and the tZq production cross-section is extracted. The result is in agreement with the Standard Model prediction and has an observed significance of 4.2 σ, thus yielding the first evidence for tZq associated production

Measurement of the production cross-section of a single top-quark in association with a ZZ boson at 13 TeV with the ATLAS detector

The production of a top quark in association with a $Z$ boson is studied in the trilepton channel. The data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $36.1\,\text{fb}^{-1}$. Events containing three identified leptons (electron and/or muon) and two jets, one of which is identified as a $b$-quark jet are selected. The major backgrounds come from diboson, $t\bar{t}$ and $Z+$jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance of the signal is $4.2\sigma$ in the data and the expected significance is $5.4\sigma$. The measured cross-section is $600 \pm 170\text{(stat)} \pm 140\text{(syst)}\,\text{fb}$

Measurements of ttˉt\bar{t} differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pppp collisions at s=13 \sqrt{s}=13\, TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \ \rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical prediction of $384 \pm 36$ fb

Measurements of ttˉt\bar{t} differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pppp collisions at s=13 \sqrt{s}=13\, TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \ \rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical prediction of $384 \pm 36$ fb