\section{Conclusion}
\cleqn
\clfn

In this paper we have
considered the distribution of $l^+$ from the semi-leptonic
decay of
top quark, where the parent top quark is
produced 
in $e^+e^- \to t\bar{t}$ near threshold.
Particularly, we have calculated the final-state interaction
corrections (rescattering corrections)
to the energy-angular distribution of lepton in the
top-quark decay.
Combining with previously known other
corrections, we have explicitly
written down the $l^+$ energy-angular distribution 
${d\sigma(\epem \! \to \ttbar \to bl^+\nu\bar{b}W^-)}/
{d^3\vc{p}_t dx_l d\Omega_l}$
including full ${\cal O}(\alpha_s)={\cal O}(\beta )$ corrections
near threshold.

Since the produced $t$ and $\bar{t}$ are slow and stays 
close to each other
near threshold, the QCD Coulombic interactions between 
$\bar{b}$ (from $\bar{t}$ decay) and $t$ or between
$b$ (from $t$ decay) and $\bar{t}$ are sizable and should be
properly taken into account.
In fact the $b$($\bar{b}$)-quark is attracted in the direction of 
$\bar{t}$($t$)-quark
due to the interaction, which in turn deforms not only the
momentum distribution of $bW^+$ or $\bar{b}W^-$ system but also
the lepton 
energy-angular distribution in the decay of top quark 
at the level of 10-20\%.

The effect of the Coulomb interaction between $\bar{b}$ and $t$,
when integrated over $\bar{b}W^-$ phase-space,
can be regarded as a correction to the top-quark production process.
The effect can be incorporated by modifying the top-quark production
cross section and the polarization vector of top quark.
The top-quark momentum distribution is shifted to take a smaller
average momentum due to the attraction by $\bar{b}$.
The Coulomb interaction between $b$ and $\bar{t}$ gives a 
non-factorizable correction with respect 
to the production and decay processes of the top quark.
The lepton distribution is deformed in favor of the kinematical 
configurations
`small $E_l$ in direction of $\bar{t}$' and
`large $E_l$ in direction of $t$',
which can be understood as originating from attraction of $b$ in the
direction of $\bar{t}$.
Exchange of gluon between $b$ and $\bar{b}$ turns out to vanish
when the top-quark energy is integrated out.

Without the non-factorizable effect $\xi$, the decay of top quark should
be perfectly symmetric around the polarization vector of parent top quark
in its rest frame.
The final-state interaction brings in anther direction to the problem,
the direction of $\bar{t}$.

Thus the final-state interaction 
effect is important and also allows for a simple
physical interpretation.

