Direct measurement of the top quark mass by the DØ Collaboration

We determine the top quark mass m{sub t} using t{bar t} pairs produced in the DO/ detector by {radical} (s) =1.8thinspTeV p{bar p} collisions in a 125thinsppb{sup {minus}1} exposure at the Fermilab Tevatron. We make a two constraint fit to m{sub t} in t{bar t}{r_arrow}bW{sup +}{bar b}W{sup {minus}} final states with one {ital W} boson decaying to q{bar q} and the other to e{nu} or {mu}{nu}. Likelihood fits to the data yield m{sub t}(l+jets)=173.3{plus_minus}5.6thinsp(stat)thinsp{plus_minus}thinsp5.5thinsp(s st) GeV/c{sup 2}. When this result is combined with an analysis of events in which both {ital W} bosons decay into leptons, we obtain m{sub t}=172.1{plus_minus}5.2thinsp(stat)thinsp{plus_minus}thinsp4.9thinsp(syst) GeV/c{sup 2}. An alternate analysis, using three constraint fits to fixed top quark masses, gives m{sub t}(l+jets)=176.0{plus_minus}7.9thinsp(stat){plus_minus}thinsp4.8thinsp(syst) GeV/c{sup 2}, consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. {copyright} {ital 1998} {ital The American Physical Society}