Measurement of Rare Kaon Decay K^+ -> PI^+NUNU-BAR

TL;DR

This paper reports on a decade-long experimental search for the rare decay of a kaon into a pion and neutrinos, observing two events and measuring its branching ratio, which constrains the CKM matrix element Vtd.

Contribution

First measurement of the branching ratio for K^+ -> pi^+ nu nu-bar with observed events and CKM constraint from a long-term experiment.

Findings

Branching ratio Br(K^+ -> pi^+ nu nu-bar) = 1.57^{+1.75}_{-0.82} x 10^-10

Observed two signal events in a decade-long search

Set constraints on CKM matrix element 0.007<|Vtd|<0.030

Abstract

A decade long search for the rare kaon decay to pi^+nunu-bar has been pursued by E787. Two signal events are observed, giving a measurement of the branching ratio Br(K^+ -> PI^+NUNU-BAR)=1.57^+1.75_-0.82 x 10^-10 and a constraint on the Cabibbo-Kobayashi-Maskawa (CKM) matrix element 0.007<|Vtd|<0.030 (68% C.L.).