Further search for the decay $K^+ \to \pi^+ \nu \bar \nu$ in the momentum region P < 195 MeV/c

TL;DR

This paper reports a search for the rare decay $K^+ o \pi^+ u ar u$ in a specific momentum range, setting new upper limits on its branching ratio and on related hypothetical particle decay modes.

Contribution

The study provides the most stringent upper limits to date on the branching ratio of $K^+ o \pi^+ u ar u$ in the specified momentum region, including improved constraints on decays involving hypothetical neutral particles.

Findings

No events observed in the search region.

Upper limit of $2.2 imes 10^{-9}$ on the branching ratio.

Improved limits on $K^+ o \pi^+ X^0$ and $K^+ o \pi^+ X^1 X^2$ decays.

Abstract

We report the results of a search for the decay $K^+ \to \pi^+ \nu \bar \nu$ in the kinematic region with $\pi^+$ momentum $140 < P < 195$ MeV/c using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of $1.22 \pm 0.24$ events results in a 90% C.L. upper limit of $2.2 \times 10^{-9}$ on the branching ratio of $K^+ \to \pi^+ \nu \bar \nu$. We also report improved limits on the rates of $K^+ \to \pi^+ X^0$ and $K^+ \to \pi^+ X^1 X^2$ where $X^0, X^1, X^2$ are hypothetical, massless, long-lived neutral particles.