  We present a measurement of the differential cross section for $t\bar{t}$
events produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV as a function
of the transverse momentum ($p_T$) of the top quark. The selected events
contain a high-$p_T$ lepton ($\ell$), four or more jets, and a large imbalance
in $p_T$, and correspond to 1 fb${}^{-1}$ of integrated luminosity recorded
with the D0 detector. Each event must have at least one candidate for a $b$
jet. Objects in the event are associated through a constrained kinematic fit to
the $t\bar{t}\to WbW\bar{b} \to \ell\nu b q\bar{q}'\bar{b}$ process. Results
from next-to-leading-order perturbative QCD calculations agree with the
measured differential cross section. Comparisons are also provided to
predictions from Monte Carlo event generators using QCD calculations at
different levels of precision.
