Search for the Decay $K^+ \to \pi^+ \nu \overline{\nu}$

TL;DR

This paper reports experimental upper limits on the rare decay $K^+ o \pi^+ u ar{ u}$ and related processes, constraining new physics scenarios involving weakly interacting neutral particles.

Contribution

It provides the most stringent upper limits to date on the branching ratios of these rare kaon decays, including scenarios with massless or massive neutral particles.

Findings

Upper limit of $2.4 imes 10^{-9}$ on $K^+ o \pi^+ u ar{ u}$

Upper limit of $5.2 imes 10^{-10}$ on $K^+ o \pi^+ X^0$

Constraints on new physics involving weakly interacting neutral particles

Abstract

An upper limit on the branching ratio for the decay $K^+ \! \rightarrow \! \pi^+ \nu \overline{\nu}$ is set at $2.4 \times 10^{-9}$ at the 90\% C.L. using pions in the kinematic region $214~{\rm MeV}/c < P_\pi < 231~{\rm MeV}/c$. An upper limit of $5.2 \times 10^{-10}$ is found on the branching ratio for decays $K^+ \! \rightarrow \! \pi^+ X^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0$.