  We study the critical behavior of the conductivity $\sigma(\omega)$ at the
zero temperature superconductor-Mott insulator transition in $d$ space-time
dimensions for a model of bosons with short-range interaction and no disorder.
We obtain $\sigma(\omega_n ) = (4e^2/\hbar) \sigma_{\epsilon}
\omega_n^{1-\epsilon}$, as predicted by the scaling theory, and the prefactor
$\sigma_{\epsilon}$ is calculated in the $\epsilon$-expansion, to order
$\epsilon ^2$ ($\epsilon = 4-d$). In two spatial dimensions, ($d=3$), we find a
value of the universal conductance $\sigma^\star =0.315 (4e^2/h)$, in good
agreement with the known Monte Carlo results.
