New results for K ⁺ → π⁺ vv¯ at low π⁺ Momentum from BNL E949

Abstract

The BNL E949 experiment was a successor to BNL E787 and was aimed to measure the branching ratio of the K ⁺ → π⁺ vv¯ decay. After detector upgrades were made, data equivalent to 1.7 × 10¹² K ⁺ decays at rest were collected in a 2002 physics run. The data were analyzed via a blind analysis with a technique called the “Bifurcation Method”. In this report we present the results of a search for K ⁺ → π⁺ vv¯ in the pion momentum region below K ⁺ → π⁺ → π⁰ peak. Three new candidate events were observed with an estimated background of $0.93 \pm 0.17_{ - 0.24}^{ + 0.32}$ events. Combining these observation with previously reported results yields abranching ratio of $BR(K^ + \to \pi ^ + v\bar v) = (1.73_{ - 1.05}^{ + 1.15} ) \times 10^{ - 10}$ consistent with the standard model prediction.