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 $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}p\overline{p}$ collisions in a $125{\mathrm{pb}}^{-1\mathrm{}}$ exposure at the Fermilab Tevatron. We make a two constraint fit to $m_t$ in $t\overline{t}\to {\mathrm{bW}}^{+}\overline{b}{W}^{-}$ final states with one W decaying to $q\overline{q}$ and the other to $e\nu$ or $\mu \nu$. 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 $m_t=173.3\mathrm{}\pm 5.6\left(\mathrm{stat}\right)\pm 6.2\left(\mathrm{syst}\right)\mathrm{GeV}{/c}^2$.