A Rare Charged Kaon Experiment at Fermilab

The measurement of the branching ratio for K+→π+νν¯ is considered to be one of the best ways of measuring the CKM matrix element Vtd. The BNL experiments E787 and E949 have seen 3 instances of this decay, using a stopped kaon beam. To improve the statistics enough to make a good measurement will require a decay-in-flight experiment. Fermilab has approved such an experiment-E921. The status of this experiment and a new proposed experiment-P940-is discussed here.