Observation of Single Top Quark Production in Association with a Z Boson in Proton-Proton Collisions at √s =13 TeV - Sirunyan, AM; Tumasyan, A; Adam, W; Ambrogi, F; Asilar, E; Bergauer, T; Brandstetter, J; Dragicevic, M; Eroe, J; Del, Valle AE; Flechl, M; Fruehwirth, R; et al. (Physical Review Letters, 2019)