Direct Measurement of the Top Quark Mass

We measure the top quark mass m{sub t} using t{ovr t} pairs produced in the D0 detector by {radical}(s)=1.8TeV p{ovr p} collisions in a 125pb{sup {minus}1} exposure at the Fermilab Tevatron. We make a two constraint fit to m{sub t} in t{ovr t}{r_arrow}bW{sup +}{ovr b}W{sup {minus}} final states with one {ital W} decaying to q{ovr q} and the other to e{nu} or {mu}{nu} . Events are binned in fit mass {ital versus} a measure of probability for events to be signal rather than background. Likelihood fits to the data yield m{sub t}=173.3{plus_minus}5.6(stat){plus_minus} 6.2(syst)GeV/c{sup 2} . {copyright} {ital 1997} {ital The American Physical Society}