Observation of single top quark production in association with a Z boson in proton-proton collisions at $\sqrt{s} = 13~\mathrm{TeV}$

The observation of the production of a single top quark in association with a Z boson (tZq) is reported. Events from proton-proton (pp) collisions at a center-of-mass energy of $13~\text{TeV}$ containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data, collected with the CMS detector in 2016 and 2017 and corresponding to an integrated luminosity of $77.4~\text{fb}^{-1}$, are used. The observed significance of the tZq signal is well over five standard deviations, thus unequivocally excluding the background-only hypothesis. The measured tZq production cross section in the leptonic decay mode of the Z boson is $\sigma (\text{pp} \to \text{tZq} \to \text{t} \ell^{+} \ell^{-} \text{q} )=111~_{-13}^{+13}~\text{(stat)}~_{-9}^{+11}~\text{(syst)}~\text{fb}$, in agreement with the standard model expectation. The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous measurements of tZq production.