First Direct Limit on the Top Quark Lifetime from CDF

The massive top quark is predicted to have a very short lifetime of
$10^{-25}$ s such that it decays before hadronizing since the CKM
matrix element $V_{tb}$ is expected to be $~0.9991 \pm 0.0002$. We
are testing this experimentally by measuring the lifetime of the top quark
in $t\bar{t}$ events where the $W$ from one of the top decays decays leptonically.
The lifetime is measuried using the displacement of the lepton track from the
primary interaction vertex. About 350 pb$^{-1}$ of data taken by the CDF experiment
at the Tevatron $p\bar{p}$ collider are used. 
The measured lifetime is consistent with zero, and
an upper limit of $c\tau <52.5 \mu m$ at 95\% CL is set.