  We present the first measurement of the mass of the top quark in a sample of
$t\bar{t}\to\ell\bar{\nu} b\bar{b} q\bar{q}$ events (where $\ell = e$, $\mu$)
selected by identifying jets containing a muon candidate from the semileptonic
decay of heavy-flavor hadrons (soft muon $b$-tagging). The $p\bar{p}$ collision
data used corresponds to an integrated luminosity of 2 fb$^{-1}$ and was
collected by the CDF II detector at the Fermilab Tevatron. The measurement is
based on a novel technique exploiting the invariant mass of a subset of the
decay particles, specifically the lepton from the $W$ boson of the $t\to Wb$
decay, and the muon from a semileptonic $b$ decay. We fit template histograms,
derived from simulation of $t\bar{t}$ events and a modeling of the background,
to the mass distribution observed in the data and measure a top quark mass of
$180.5\pm12.0({\rm stat.})\pm3.6({\rm syst.}) {\rm GeV}/c^2$, consistent with
the current world average.
