  We report a measurement of the top quark mass $M_t$ in the dilepton decay
channel $t\bar{t}\to b\ell'^{+}\nu'_\ell\bar{b}\ell^{-}\bar{\nu}_{\ell}$.
Events are selected with a neural network which has been directly optimized for
statistical precision in top quark mass using neuroevolution, a technique
modeled on biological evolution. The top quark mass is extracted from per-event
probability densities that are formed by the convolution of leading order
matrix elements and detector resolution functions. The joint probability is the
product of the probability densities from 344 candidate events in 2.0 fb$^{-1}$
of $p\bar{p}$ collisions collected with the CDF II detector, yielding a
measurement of $M_t= 171.2\pm 2.7(\textrm{stat.})\pm
2.9(\textrm{syst.})\mathrm{GeV}/c^2$.
