Measurement of the mesonic decay branch of the $\bar{K}NN$ quasi-bound state

Abstract

We conducted measurements of $K^{-}+{}^3\mathrm{He}\to \pi YN+N^{\prime }$ reactions using a $1\mathrm{GeV}/c$ $K^{-}$-beam, with the objective of understanding the broad decay width of $\bar{K}NN$ (approximately twice as broad as that of $\Lambda (1405)$ considered to be the $\bar{K}N$ quasi-bound state). We successfully reproduced distributions of the $\pi YN$ invariant mass and momentum transfer for $\pi YN$ using model fitting functions for $\bar{K}NN$ formation and quasi-free $\bar{K}$ absorption (${\mathrm{QF}}_{\bar{K}-\mathrm{abs}}$) processes. The model can describe the experimental data quite well, and four $\bar{K}NN\to \pi YN$ cross-sections were obtained. The results indicate that mesonic decay is the dominant decay branch of $\bar{K}NN$. The results also suggest that ${\Gamma }_{\pi \Lambda N}\sim {\Gamma }_{\pi \Sigma N}$, which indicates that the $I_{\bar{K}N}=1$ absorption channel, in addition to the $I_{\bar{K}N}=0$ absorption channel, substantially contribute to the $\bar{K}NN$ decay, making the $\bar{K}NN$ state approximately twice as unstable as $\Lambda$(1405).