Toshio Numao
TRIUMF Laboratory

Status of rare kaon decay experiments at BNL

In the decays $K \to \pi \nu \bar\nu$, the
dominant second-order loop-diagrams with a top quark make
these flavor-changing neutral current modes very sensitive to
$V_{td}$.
The E787 group at Brookhaven National Laboratory (BNL)
recently reported two events in the $K^+$ mode
at a branching ratio of
$1.57^{+1.75}_{-0.82} \times 10^{-10}$,
which is consistent with a fit-based prediction of
$(0.82 \pm 0.32) \times 10^{-10}$.
The goal of the subsequent
experiment E949 is to improve the event statistics by a factor
of five (a 15 \% measurement of $|V_{td}|$).
The KOPIO experiment at BNL
plans to reach a sensitivity of $10^{-13}$ or
to measure about 50 events from the CP-violating
$K_L \rightarrow \pi^0 \nu \overline{\nu}$ decay,
suppressing backgrounds by hermetic photon detection and complete
reconstruction of decay kinematics using
a low-energy micro-bunched  $K^0_L$ beam to allow
determination of the incident kaon momentum.
The status of both kaon decay experiments will be presented.