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

The ATLAS collaboration
Aad, Georges , Abbott, Braden Keim , Abbott, Dale , Abed Abud, Adam , Abeling, Kira , Abhayasinghe, Deshan Kavishka , Abidi, Haider , Abouzeid, Hass , Abouzeid, Ossama , Abraham, Nicola Louise
Eur.Phys.J.C 81 (2021) 737, 2021.

Abstract (data abstract)
CERN-LHC-ATLAS. Measurements of the inclusive and differential fiducial cross-sections for $t\bar{t}+Z$ production in proton-proton collisions at a centre-of-mass energy of 13 TeV with 139 fb−1 of data collected between 2015 and 2018. The fiducial regions for the trilepton channel are defined as: - Exactly 3 leptons ($e$ or $\mu$) with transverse momentum greater than 27, 20, 20 GeV for leading, subleading and third lepton, respectively. - The sum of the three lepton charges is required to be $\pm$1. - At least one opposite-sign-same-flavour (OSSF) lepton pair (considered to be lepton pair from $Z$ boson) with invaraint mass compatible with $Z$ boson ($|m_{\ell\ell}^{Z}-m_{Z}|<10$ GeV) and $m_{OSSF}>10$ GeV for each OSSF lepton pair. - Two orthogonal regions combined for inclusive measurement, different region for differential measurement. - $b$-jets requirements: pseudo-continuous $b$-tagging with different working points for inclusive measurement, fixed 85% $b$-tagging working point for differential measurement. - At least 3 (inclusive and differential region) or 4 (inclusive region) jets The fiducial regions for the tetralepton channel are defined as: - Exactly 4 leptons ($e$ or $\mu$) with transverse momentum greater than 27, 20, 10, 7 GeV for leading, subleading, third and fourth lepton, respectively. - The sum of the four lepton charges is required to be zero. - At least one opposite-sign-same-flavor (OSSF) lepton pair (considered to be lepton pair from $Z$ boson) with invaraint mass compatible with $Z$ boson ($|m_{\ell\ell}^{Z}-m_{Z}|<10$ GeV) and $m_{OSSF}>10$ GeV for each OSSF lepton pair. - Four regions separated based on $b$-jets multiplicity (1$b$ or 2$b$ jets at 85% $b$-tagging working point) and flavour composition of the non-$Z$ lepton pair (same flavour, different flavour). - At least 2 jets. - $E_{T}^{miss}$ requirements (depending on the region) for suppresion of $ZZ$+jets background.

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