  In the Minimal Supersymmetric Standard Model (MSSM), large chiral symmetry
breaking term $A_t$, which plays an important role in Higgs mass, may
significantly contribute in flavor changing neutral current (FCNC) processes
$B\to X_{s}\gamma$ and $B_{s}\to \mu^{+}\mu^{-}$. Though the above processes
can both be categorized as $b\to s$ transitions, the two rare decays behave
completely different in MSSM. With an on-shell photon in the final state,
helicity of initial state $b$-quark and final state $s$-quark must be flipped
in $B\to X_{s}\gamma$, which corresponds to the simultaneous breaking of chiral
symmetry and electroweak symmetry. The common feature is shared by fermion mass
generation. Same as radiative mass generation in MSSM, Peccei-Quinn and $R$
symmetry breaking contributions, for example from a Higgsino-stop loop when
$\mu A_t<0$, may significantly cancel the contribution from charged Higgs and
reduce the prediction of $B\to X_{s}\gamma$. For the latter process, including
Babu-Kolda FCNC proportional to $\mu A_t$, $B_{s}\to \mu^{+}\mu^{-}$ is
mediated by a scalar $H_d$ boson which corresponds to chiral symmetry breaking.
In addition, as a result of interference among the Higgs extension sector and
$Z$ contributions, in the region of $\mu A_t <0$ which is favored by $B\to
X_{s}\gamma$, there may simultaneously exist large enhancement in $B_{s}\to
\mu^{+}\mu^{-}$. However, we still find viable parameter region with light
Higgsino of a few hundreds GeV when charged Higgs contribution is not
negligible with $M_A$ of 400 GeV.
