Direct Measurement of the Top Quark Mass

We measure the top quark mass ${m}_{t}$ using $t\overline{t}$ pairs produced in the D0 detector by $\sqrt{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\mathrm{TeV}p\overline{p}$ 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 decaying to $q\overline{q}$ and the other to $e\ensuremath{\nu}$ or $\ensuremath{\mu}\ensuremath{\nu}$. Events are binned in fit mass versus a measure of probability for events to be signal rather than background. Likelihood fits to the data yield ${m}_{t}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}173.3\ifmmode\pm\else\textpm\fi{}5.6(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}6.2(\mathrm{syst})\mathrm{GeV}{/c}^{2}$.