Evidence for a $\bar{K}NN$ quasi-bound state in the $\gamma d \to K^0\Lambda p$ reaction
The $\gamma d \to K^{0}\Lambda p$ reaction has been studied to search for a $\bar{K}NN$ quasi-bound state using the LEPS2 solenoid spectrometer at SPring-8. A localized enhancement concentrated at low $q$ is observed in the $(M_{\Lambda p},q)$ distribution below the $K^-pp$ mass threshold, where $M_{\Lambda p}$ is the $\Lambda p$ invariant mass and $q \equiv |\vec{p}_{\gamma}-\vec{p}_{K^{0}}| = |\vec{p}_{\Lambda p}|$ is the momentum transfer. A two-dimensional $(M_{\Lambda p},q)$ fit demonstrates that the enhancement near the threshold is statistically significant, yielding a local significance of $7.3,\sigma$. The enhancement is characterized by effective shape parameters including the Breit--Wigner mass $M$ and width $\Gamma$, and a Gaussian form-factor momentum scale $Q$: $M = 2.354 \pm 0.011(\mathrm{stat.})^{+0.009}_{-0.005}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, $\Gamma = 0.055 \pm 0.023(\mathrm{stat.})^{+0.031}_{-0.009}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, and $Q = 0.350 \pm 0.041(\mathrm{stat.})^{+0.035}_{-0.010}(\mathrm{syst.})~\mathrm{GeV}/c$. The obtained results support the existence of a $\bar{K}NN$ quasi-bound state in photoproduction.