  We analyze the possibility of having new physics effects in the decay rate
difference, $\Delta \Gamma_d$, of neutral $B_d$ mesons. Three different sources
of enhancement are considered, CKM unitarity violations, beyond standard model
effects in the tree-level dimension six operators $(\bar{d}p)(\bar{p}'b)$ with
$p,p'=u,c$; and large enhancements of the almost unconstrained operators
$(\bar{d}b)(\bar{\tau}\tau)$. We find that deviations of several hundred per
cent from the standard model prediction of $\Delta \Gamma_d$ are not excluded
by current experimental data.
