  We present a measurement of the mass of the top quark using data
corresponding to an integrated luminosity of 1.9fb^-1 of ppbar collisions
collected at sqrt{s}=1.96 TeV with the CDF II detector at Fermilab's Tevatron.
This is the first measurement of the top quark mass using top-antitop pair
candidate events in the lepton + jets and dilepton decay channels
simultaneously. We reconstruct two observables in each channel and use a
non-parametric kernel density estimation technique to derive two-dimensional
probability density functions from simulated signal and background samples. The
observables are the top quark mass and the invariant mass of two jets from the
W decay in the lepton + jets channel, and the top quark mass and the scalar sum
of transverse energy of the event in the dilepton channel. We perform a
simultaneous fit for the top quark mass and the jet energy scale, which is
constrained in situ by the hadronic W boson mass. Using 332 lepton + jets
candidate events and 144 dilepton candidate events, we measure the top quark
mass to be mtop=171.9 +/- 1.7 (stat. + JES) +/- 1.1 (syst.) GeV/c^2 = 171.9 +/-
2.0 GeV/c^2.
