Measurement of the mesonic decay branch of the $\overline{K}NN$ quasibound state

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 $\overline{K}NN$ (approximately twice as broad as that of $\Lambda \left(1405\right)$ considered to be the $\overline{K}N$ quasibound state). We successfully reproduced distributions of the $\pi YN$ invariant mass and momentum transfer for $\pi YN$ using model-fitting functions for $\overline{K}NN$ formation and quasifree $\overline{K}$ absorption (${\mathrm{QF}}_{\overline{K}-\mathrm{abs}}$) processes. The model can describe the experimental data quite well, and four $\overline{K}NN\to \pi YN$ cross sections were obtained. The results indicate that mesonic decay is the dominant decay branch of $\overline{K}NN$. The results also suggest that ${\Gamma }_{\pi \Lambda N}\approx {\Gamma }_{\pi \Sigma N}$, which indicates that the $I_{\overline{K}N}=1$ absorption channel, in addition to the $I_{\overline{K}N}=0$ absorption channel, substantially contributes to the $\overline{K}NN$ decay, making the $\overline{K}NN$ state approximately twice as unstable as $\Lambda$(1405).