Direct measurement of the top quark mass by the DØ Collaboration

We determine the top quark mass ${m}_{t}$ using $t\overline{t}$ pairs produced in the D\O{} detector by $\sqrt{s}=1.8\mathrm{TeV}$ $p\overline{p}$ collisions in a $125{\mathrm{pb}}^{\ensuremath{-}1}$ exposure at the Fermilab Tevatron. We make a two constraint fit to ${m}_{t}$ in $t\overline{t}\ensuremath{\rightarrow}{\mathrm{bW}}^{+}\overline{b}{W}^{\ensuremath{-}}$ final states with one W boson decaying to $q\overline{q}$ and the other to $e\ensuremath{\nu}$ or $\ensuremath{\mu}\ensuremath{\nu}.$ Likelihood fits to the data yield ${m}_{t}(l+\mathrm{jets})=173.3\ifmmode\pm\else\textpm\fi{}5.6\mathrm{}(\mathrm{stat})\mathrm{}\ifmmode\pm\else\textpm\fi{}\mathrm{}5.5\mathrm{}(\mathrm{syst})\mathrm{ }\mathrm{GeV}{/c}^{2}.$ When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain ${m}_{t}=172.1\ifmmode\pm\else\textpm\fi{}5.2\mathrm{}(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}4.9\mathrm{}(\mathrm{syst})\mathrm{ }\mathrm{GeV}{/c}^{2}.$ An alternate analysis, using three constraint fits to fixed top quark masses, gives ${m}_{t}(l+\mathrm{jets})=176.0\ifmmode\pm\else\textpm\fi{}7.9\mathrm{}(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}4.8\mathrm{}(\mathrm{syst})\mathrm{ }\mathrm{GeV}{/c}^{2},$ 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