New measurement of the K+ => pi+,nu,nubar branching ratio

Abstract

Three events for the decay K+ => pi+,nu,nubar have been observed in the pion momentum region below the K+ => pi+,pi0 peak, 140 < P_pi < 199 MeV/c, with an estimated background of 0.93+-0.17(stat.)+0.32-0.24(syst.) events. Combining this observation with previously reported results yields a branching ratio of B(K+ => pi+,nu,nubar) = (1.73+1.15-1.05)e-10 consistent with the standard model prediction.

Figures (1)

• Figure 1: Kinetic energy vs. range of all events passing all other cuts. The squares represent the events selected by this analysis. The circles and upward-pointing triangles represent the events selected by the E787 and E949 pnn1 analyses, respectively. The downward-pointing triangles represent the events selected by the E787 pnn2 analyses. The solid (dashed) lines represent the limits of the pnn1 and pnn2 signal regions for the E949 (E787) analyses. Despite the smaller signal region in $E_\pi$vs.$R_\pi$, the pnn1 analyses were 4.2 times more sensitive than the pnn2 analyses. The points near $E_\pi = 108$ MeV were $K_{\pi2}$ decays that survived the photon veto cuts and were predominantly from the pnn1 analyses due to the higher sensitivity and the less stringent photon veto cuts. The light gray points are simulated ${K}^+\to\pi^+\nu\bar{\nu}$ events that would be accepted by our trigger.