Experimental Status of K -> pi nu nu_bar

Abstract

The experimental program for the study of the rare kaon decays, K -> pnn, is summarized. A review of recent results is provided along with a discussion of prospects for the future of this program. The primary focus of the world-wide kaon program is the two golden modes: K+ -> pnn and KL -> pnn. The first step in an ambitious program to precisely measure both branching ratios has been successfully completed with the observation of two K+ -> pnn events by E787. The E949 experiment is poised to reach an order of magnitude further in sensitivity and to observe ~10 Standard Model events, and the CKM experiment should observe ~100 SM events by the end of this decade. Limits on the neutral analog KL -> pnn have been set by KTeV and within the next couple of years will be pushed by E391a. Measurements of the branching ratio should be made within the next 10 years by KOPIO, with a goal of ~50 events, and at the JHF, with a goal of up to 1000 events.