  We investigate the effect of an extra fourth quark generation and FCNC
mediated $Z$ and $Z'$ bosons on the rare decay mode $B^- \to \phi \pi^-$. In
the standard model, this mode receives only $b \to d$ penguin contributions and
therefore, highly suppressed with branching ratio $\sim 5 \times 10^{-9}$. This
in turn makes this mode a very sensitive probe for new physics. We find that
due to the above mentioned new physics contributions there is a significant
enhancement in its branching ratio. Furthermore, the direct CP violation
parameter which is identically zero in the SM is found to be quite significant.
If this mode will be observed in the upcoming LHCb experiment, it will not only
provide a clear signal of new physics but also can be used to constrain the new
physics parameter space.
