Search for pair- and single-production of vector-like quarks in final states with at least one $Z $ boson decaying into a pair of electrons or muons in $pp $ collision data collected with the ATLAS detector at $\sqrt{s}=13\,$TeV

A search for pair- and single-production of vector-like quarks $T$ and $B$ with a leptonically decaying $Z$ boson is presented. The data were collected in $pp$ collisions at $\sqrt{s}=13\,\text{TeV}$ with the ATLAS detector at the LHC, corresponding to an integrated luminosity of $36.1\,\text{fb}^{-1}$. Final states are required to contain a $Z$ boson reconstructed from a same-flavor pair of leptons with opposite electric charge and a third generation quark. The analysis targets both pair- and single-production of vector-like quarks and no excess above the Standard model expectation is found. Lower mass limits on vector-like $T$ and $B$ quarks at 95% confidence level are derived and limits on the coupling to Standard model quarks are set for single-production.