Observation of the Top Quark.

The D0 collaboration reports on a search for the Standard Model top quark in p{bar p} collisions at {radical}s = 1.8 TeV at the Fermilab Tevatron, with an integrated luminosity of approximately 50 pb{sup {minus}1}. We have searched for t{bar t} 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 {plus_minus} 0.6 events. The probability for an upward fluctuation of the background to produce the observed signal is 2 {times} 10{sup {minus}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{sub {minus}21}{sup +19} (stat.) {plus_minus}22 (syst.) GeV/c{sup 2} and its production cross section to be 6.4 {plus_minus} 2.2 pb.