  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 sqrt(s[NN]) = 8.16 TeV. The
measurement is performed using events with exactly one isolated electron or
muon and at least four jets. The data sample corresponds to an integrated
luminosity of 174 inverse nanobarns. The significance of the tt-bar signal
against the background-only hypothesis is above five standard deviations. The
measured cross section is sigma[tt-bar] = 45 +/- 8 nb, consistent with
predictions from perturbative quantum chromodynamics.
