The first observation of top quark production in proton-nucleus collisions is reported using proton-lead data collected by the CMS experiment at the CERN LHC at a nucleon-nucleon center-of-mass energy of √sNN=8.16 TeV. The measurement is performed using events with exactly one isolated electron or muon candidate and at least four jets. The data sample corresponds to an integrated luminosity of 174 nb−1. The significance of the t ̄t signal against the background-only hypothesis is above 5 standard deviations. The measured cross section is σt ̄t=45±8 nb, consistent with predictions from perturbative quantum chromodynamics.
