  Recently we have developed a theory of Keldysh formalism for mesoscopic
systems. The resulting nonequilibrium Kubo formula for differential conductance
makes it possible to propose the new formula of shot noise $S_h$, and thus to
study shot noise in correlated systems at any temperature and any bias voltage.
Employing this new approach, we analyze shot noise in the Kondo regime through
a quantum dot for the symmetric case at zero temperature. Using the
renormalized perturbation theory, we prove that in the leading order of bias
voltage $S_h$ equal to noise power at zero temperature conventionally used as
shot noise. With $S_h$, we calculate the Fano factor for a backscattering
current $I_b$: $F_b=S_h/2eI_b$. It is shown that the Fano factor takes the
universal form of $F_b=1+4(R-1)^2/(1+5(R-1)^2)$ determined by the Wilson ratio
$R$ for arbitrary strength of the Coulomb interaction. Using the Wilson ratio
R=2, our result coincides with the fractional value of $F_b=5/3$ already
derived in the Kondo regime.
