International Conference on Exotic Atoms and Related Topics and conference on Low Energy Antiprotons (EXA/LEAP 2024)

Prospect of hadronic-molecule with strangeness

29 Aug 2024, 09:15

30m

Festsaal (Vienna)

Speaker

Masahiko Iwasaki (RIKEN)

Description

Recently, we conducted a kaonic nuclear-bound state search experiment using a $K^{-}$ beam (1 GeV/c) bombarding a ${}^3$He target. We succeeded in observing a kaonic nuclear quasi-bound state, "$K^{-}pp$", via a nucleon knockout reaction, $K^{-}N\to \bar{K}{n}^{\prime }$, followed by the decay $\bar{K}NN\to \mathrm{\Lambda }p(2N_{\bar{K}A})$ in the two-nucleon $\bar{K}$ absorption process, resulting in the final state $\mathrm{\Lambda }p+n^{\prime }$. The result shows that the "$K^{-}pp$" binding energy is about 40 MeV below the binding threshold, with a decay width of about 100 MeV. From the $\mathrm{\Lambda }p$ decay, the isospin of the system is determined to be $I_{\bar{K}NN}=1/2$. The momentum transfer distribution of the $\mathrm{\Lambda }p$ system is very broad, implying that the size of the "$K^{-}pp$" system might be very compact [1, 2].

We extended our study on the kaonic nuclear-bound state in two ways: A) by studying the mesonic decay process of the $\bar{K}NN$ via one-nucleon $\bar{K}$ absorption ($1N_{\bar{K}A}:\bar{K}N\to \pi Y$ ), and B) by searching for the $\bar{K}NNN$ bound state through the $\mathrm{\Lambda }d$ invariant mass study of the $\mathrm{\Lambda }d+n^{\prime }$ final state with a $k^{-}$ beam (1 GeV/c) bombarding a ${}^4$He target. The aim of A) is to understand why the decay width of "$K^{-}pp$" is about twice as broad as that of $\mathrm{\Lambda }(1405)$ ($\approx 50$ MeV), which is assumed to be a molecule-like hadronic cluster composed of a $\bar{K}$ meson and a nucleon, i.e., $\mathrm{\Lambda }(1405)\equiv \bar{K}N$ , as introduced by R. H. Dalitz et. al. [3]. The result shows that the $\bar{K}NN\to \pi YN$ decay is dominant ($1N_{\bar{K}A}\gg 2N_{\bar{K}A}$) and that the $\pi \mathrm{\Sigma }N$ to $\pi \mathrm{\Lambda }N$ ratio is about 1:1, indicating that the $I_{\bar{K}N}=1$ absorption channel is approximately equal to the $I_{\bar{K}N}=0$ channel. The result also suggests that there is a hint of the "${\bar{K}}^{0}nn$" bound state, a charge mirror state of "$K^{-}pp$", existing in the ${\pi }^{-}\mathrm{\Lambda }p$ invariant mass spectrum of the ${\pi }^{-}\mathrm{\Lambda }p+p^{\prime }$ final state.

In the $\mathrm{\Lambda }d$ invariant mass study B), the two dimensional preliminary spectrum of the $\mathrm{\Lambda }d$ invariant mass and the momentum transfer to $\mathrm{\Lambda }d$ ($m_{\mathrm{\Lambda }d},q_{\mathrm{\Lambda }d}$) shows an almost identical distribution to ($m_{\mathrm{\Lambda }p},q_{\mathrm{\Lambda }p}$), indicating the presence of $\bar{K}NNN$, decaying to $\mathrm{\Lambda }d$. If this is another kaonic nuclear-bound state, then the isospin, spin parity is fixed to be $I(J^P)=0(1/2^{-})$.

In this talk, we'll describe these two new results on kaonic nuclear-bound states and discuss the prospects of studying the molecule-like hadronic cluster with strangeness.

References
[1] S. Ajimura et al., Phys. Lett. B 789, 620-625 (2019)
[2] T. Yamaga et al., Phys. Rev. C 102, 044002 (2020)
[3] R.H. Dalitz and S.F. Tuan, Ann. Phys., 3, 307 (1960)
[4] T. Yamaga et al., arXiv:2404.01773 (2024)

Presentation materials

EXA_2024.pdf

Sprited Away - Always with Me.mp3