Measurement of Branching Fractions in $B \to K \nu \bar{\nu}$ Decays

Flavor changing neutral-current transitions such as $b \to s \gamma$ are absent at tree level in the Standard Model and can only occur via loop diagrams. Several new physics models may enhance the rate of these transitions. We will present searches for the exclusive decays $B^{\pm} \to K^+ \nu \bar{\nu}$ and $B^0 \to K_s \nu \bar{\nu}$. Due to the presence of two neutrinos in the final state, we require the full reconstruction of one $B$-meson in the semileptonic decay channel $B^{\pm} \to D^{(*)0} \ell \nu$ and search in the recoil for our decay modes. This analysis uses approximately 430~fb$^{-1}$ or 470 million $B\bar{B}$ pairs collected with the {\sl B{\scriptsize A}B {\scriptsize AR}} detector at the PEP-II B-factory.