Hypernuclear Fine Structure in ${}_{\Lambda }{}^{16}\mathrm{O}$ and the $\Lambda N$ Tensor Interaction

We have observed two $\gamma$-ray transitions in ${}_{\Lambda }{}^{16}\mathrm{O}$ from the 6.6 MeV excited $1_2^{-}$ state to both ground-state spin-doublet members ($1_1^{-},0^{-}$) by the ($K^{-},{\pi }^{-}\gamma$) reaction. We have obtained the ground-state doublet spacing to be $26.4\pm 1.6(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.5(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{k}\mathrm{e}\mathrm{V}$ and the excitation energy of the $1_2^{-}$ state to be $6561.7\pm 1.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 1.7(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{k}\mathrm{e}\mathrm{V}$. The ground-state doublet spacing provides a small but nonzero strength of the tensor interaction between a $\Lambda$ and a nucleon. This is the first experimental result on the $\Lambda N$ tensor interaction.

Figure 1

Mass spectra of ${}_{\Lambda }{}^{16}\mathrm{O}$ plotted for $\Lambda$ binding energy ($B_{\Lambda }$) in the ${}^{16}\mathrm{O}$ $(K^{-},{\pi }^{-})$ reaction, plotted for those events accompanying (a) $\gamma$ rays with 5–7 MeV energies and (b) $\gamma$ rays with Doppler-shift corrected energies from 6500 to 6600 keV corresponding to the peaks in Fig. 2.

Figure 2

Mass-gated $\gamma$-ray energy spectra measured in the ${}^{16}\mathrm{O}(K^{-},{\pi }^{-}){}_{\Lambda }{}^{16}\mathrm{O}$ reaction for (a) the highly unbound region ($-B_{\Lambda }>50\mathrm{M}\mathrm{e}\mathrm{V}$) and (b) the $1_2^{-}$ state region ($-17<-B_{\Lambda }<3\mathrm{M}\mathrm{e}\mathrm{V}$). (c) is the same spectrum as (b) but with event-by-event Doppler-shift correction applied. A structure around 6.5–6.6 MeV observed in (b) becomes two narrow peaks after Doppler-shift correction. The two peaks are assigned as $M1(1_2^{-}\to 1_1^{-},0^{-})$ transitions. Single escape (SE) peaks of the $M1$ transitions are also observed. (d) is the simulated peak shape for a $\gamma$ ray at 6.55 MeV, the dotted line for a $\gamma$ ray emitted at rest, the thin line for a $\gamma$ ray emitted before ${}_{\Lambda }{}^{16}\mathrm{O}$ slows down, and the thick line with Doppler-shift correction applied to the thin line. (e) shows the result of the fit with the simulated peak shape.

Figure 3

Level scheme of ${}_{\Lambda }{}^{16}\mathrm{O}$ determined in the present experiment. Corresponding level scheme of the core nucleus ${}^{15}\mathrm{O}$ is also shown. Both $1^{-}$ states of ${}_{\Lambda }{}^{16}\mathrm{O}$ are populated by the $0.93\mathrm{G}\mathrm{e}\mathrm{V}/c$ $(K^{-},{\pi }^{-})$ reaction. $\gamma$ transitions and excitation energies measured from the present experiment are given.