We present a search for the rare flavour-changing neutral-current decay $B^{ -}\to K^{ -} \nu \bar{ \nu}$ based on a sample of $(86.9 \pm 1.0) \times 10^{ 6}$ $\Upsilon (4S) \to $B\bar{ B}$ events collected by the BABAR experiment at the SLAC B-factory. Signal candidate events are selected by fully reconstructing a $B^+ \to \bar{ D}^{ 0} X^+$ decay, where $X^+$ represents a combination of up to three charged pions or kaons and up to two $\pi^0$ candidates. The charged tracks and calorimeter clusters not used in the $B^+$ reconstruction are required to be compatible with a $B^{ -}\to K^{ -} \nu \bar{ \nu}$ decay. We observe a total of three signal candidate events with an expected background of $2.7 \pm 0.8$, resulting in a preliminary limit of $\mathcal{ B}(B^{ -}\to K^{ -} \nu \bar{ \nu}) < 1.05 \times 10^{ -4}$ at the 90% confidence level. This search is combined with the results of a previous and statistically independent preliminary BABAR search for $B^{ -}\to K^{ -} \nu \bar{ \nu}$ to give a limit of $\mathcal{ B}(B^{ -}\to K^{ -} \nu \bar{ \nu}) < 7.0 \times 10^{ -5}$ at the 90% confidence level.
Categories: hep-ex