Evidence for a $\bar{K}NN$ quasi-bound state in the $\gamma d\to K^0\Lambda p$ reaction

Abstract

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 $Q$: $M=2.354\pm 0.011(stat.{)}_{-0.005}^{+0.009}(syst.)\mathrm{GeV}/c^2$, $\Gamma =0.055\pm 0.023(stat.{)}_{-0.009}^{+0.031}(syst.)\mathrm{GeV}/c^2$, and $Q=0.350\pm 0.041(stat.{)}_{-0.010}^{+0.035}(syst.)\mathrm{GeV}/c$. The obtained results support the existence of a $\bar{K}NN$ quasi-bound state in photoproduction.

9 pages, 3 figures; revised version accepted to Physics Letters B

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