Speaker
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, $K^-N\rightarrow \overline{K}n'$, followed by the decay $\overline{K}NN\rightarrow \Lambda p(2N_{\overline{K}A})$ in the two-nucleon $\overline{K}$ absorption process, resulting in the final state $\Lambda p + n'$. 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 $\Lambda p$ decay, the isospin of the system is determined to be $I_{\overline{K}NN} = 1/2$. The momentum transfer distribution of the $\Lambda p$ 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 $\overline{K}NN$ via one-nucleon $\overline{K}$ absorption ($1N_{\overline{K}A}: \overline{K}N\rightarrow \pi Y$ ), and B) by searching for the $\overline{K}NNN$ bound state through the $\Lambda d$ invariant mass study of the $\Lambda d + n'$ 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 $\Lambda(1405)$ ($\approx 50$ MeV), which is assumed to be a molecule-like hadronic cluster composed of a $\overline{K}$ meson and a nucleon, i.e., $\Lambda(1405) \equiv \overline{K}N$ , as introduced by R. H. Dalitz et. al. [3]. The result shows that the $\overline{K}NN\rightarrow \pi YN$ decay is dominant ($1N_{\overline{K}A}\gg 2N_{\overline{K}A}$) and that the $\pi\Sigma N$ to $\pi\Lambda N$ ratio is about 1:1, indicating that the $I_{\overline{K}N} = 1$ absorption channel is approximately equal to the $I_{\overline{K}N} = 0$ channel. The result also suggests that there is a hint of the "$\overline{K}^0nn$" bound state, a charge mirror state of "$K^-pp$", existing in the $\pi^-\Lambda p$ invariant mass spectrum of the $\pi^-\Lambda p + p'$ final state.
In the $\Lambda d$ invariant mass study B), the two dimensional preliminary spectrum of the $\Lambda d$ invariant mass and the momentum transfer to $\Lambda d$ ($m_{\Lambda d}, q_{\Lambda d}$) shows an almost identical distribution to ($m_{\Lambda p}, q_{\Lambda p}$), indicating the presence of $\overline{K} NNN$, decaying to $\Lambda d$. 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)
