The $K(1690)$ signal from COMPASS as a strange hybrid state

Abstract

A pseudoscalar resonance structure, denoted as the $K(1690)$, was recently discovered by the COMPASS Collaboration in the scattering reaction $K^-+p\to K^-\pi^-\pi^++p$. If the $K(1690)$ is a genuine state, there are three observed pseudoscalar strange mesons, namely the $K(1460)$, $K(1690)$, and $K(1830)$, in the 1.0$-$2.0 GeV region. However, within the conventional quark model, only the $2^1S_0$ and $3^1S_0$ strange meson states are expected in this energy region. Therefore, at least one of these states should be interpreted as an exotic candidate. In this work, we study the spectrum of strange mesons systematically, and find that the $K(1460)$ and $K(1830)$ are good candidates of $2^1S_0$ and $3^1S_0$ strange mesons, respectively. A further investigation of their strong decays also supports this assignment. Furthermore, we note that the production mechanism of the $K(1690)$ is similar to that of the $\pi_1(1600)$, which has been widely regarded as a hybrid meson candidate. We investigate the strong decays of the $K(1690)$ within a constituent gluon model by treating it as a $0^-$ strange hybrid meson and find that the results support the $K(1690)$ as a hybrid state. Finally, we identify several important decay modes of the $K(1690)$ that may be used in future experiments to test whether it contains the $n^1S_0$ ($n=$2 and 3) $s\bar{q}$ component. We also suggest BESIII to search for the $K(1690)$ state through the $J/\psi\to K K(1690) \to KK^{\ast}_0(1430)\pi\to KK\pi\pi$ process.