  This paper presents the observation of the electroweak production of single
top quarks in the D0 detector at the Fermilab Tevatron Collider at a
center-of-mass energy of 1.96 TeV. Events containing an isolated electron or
muon and missing transverse energy, together with jets originating from the
fragmentation of b quarks are used to measure a cross section for single top
quark production of sigma(ppbar -> tb + X, tqb + X) = 3.94 +- 0.88 pb. The
probability to measure a cross section at this value or higher in the absence
of signal is 2.5X10^-7, corresponding to a 5.0 standard deviation significance.
