Direct measurement of the top quark mass by the DØ Collaboration

We determine the top quark mass mₜ using tt̄ pairs produced in the D{} detector by √s=1.8TeV pp̄ collisions in a 125pb^-1 exposure at the Fermilab Tevatron. We make a two constraint fit to mₜ in tt̄→bW⁺b̄W^- final states with one W boson decaying to qq̄ and the other to eν or μν. Likelihood fits to the data yield mₜ(l+jets)=173.3±5.6(stat)±5.5(syst) GeV/c². When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain mₜ=172.1±5.2(stat)±4.9(syst) GeV/c². An alternate analysis, using three constraint fits to fixed top quark masses, gives mₜ(l+jets)=176.0±7.9(stat)±4.8(syst) GeV/c², consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. 14.65.Ha, 13.85.Ni, 13.85.Qk