Observation of the top quark

The D0 Collaboration reports on a search for the Standard Model top quark in p (p) over bar collisions at root s = 1.8 TeV at the Fermilab Tevatron, with an integrated luminosity of approximately 50 pb(-1). We have searched for t (t) over bar production in the dilepton and single-lepton decay channels, with and without tagging of b quark jets. We observe 17 events with an expected background of 3.8 +/- 0.6 events. The probability for an upward fluctuation of the background to produce the observed signal is 2 . 10(-6) (equivalent to 4.6 standard deviations). The kinematic properties of the excess events are consistent with top-quark decay. We conclude that we have observed the top quark and measure its mass to be 199(-21)(+19)(stat) +/- 22(syst) GeV/c(2) and its production cross-section to be (6.4 +/- 2.2) pb.