Measuring The Rare Decays $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and $K_L^0\to {\pi }^0\nu \bar{\nu }$

Abstract

In this paper I will describe the search for rare kaon decay processes $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and $K_L^0\to {\pi }^0\nu \bar{\nu }$. The small decay rate for these processes is considered a key prediction of the standard model. We searched for the charged kaon decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ using kaons decaying at rest in the E787 detector at Brookhaven National Laboratory in two different phase space regions: Region 1 with pion momentum above 205 MeV/c (the $K^{+}\to {\pi }^{+}{\pi }^0$ peak) and Region 2 with pion momentum below 205 MeV/c. We have found 2 events in Region 1 which is known to have small background ($0.15_{-0.032}^{+0.048}$). This observation leads to a branching ratio measurement of $(1.57_{-0.82}^{+1.75})\times 10^{-10}$. We found 1 event in Region 2 with an expectation of $0.73\pm 0.18$ background events. The observation from Region 2 is consistent with the branching ratio measured using Region 1. I will also describe new efforts underway to measure both the decay rates with much higher statistics.

23 pages 8 figures Talk given at Les Rencontres de Physique, La Thuile, Aosta Valley (Italy), March 3-9, 2002