Double-$\Lambda$ hypernuclei observed in a hybrid emulsion experiment

A hybrid experiment with nuclear emulsion and scintillating-fiber detectors (KEK-E373) has been performed to search for double-strangeness systems. Among about 10${}^3$ events of ${\Xi }^{-}$ hyperons captured at rest by emulsion nuclei, we have observed four events which clearly show the topology of cascade weak decays of double-$\Lambda$ hypernuclei including the “Nagara” event. Regarding the Nagara event, values of the two-$\Lambda$ binding energy ($B_{\Lambda \Lambda }$) and the $\Lambda$-$\Lambda$ interaction energy ($\Delta B_{\Lambda \Lambda }$) of ${}_{\Lambda }^{}{}_{\Lambda }^6\mathrm{He}$ have been revised to 6.91 $\pm$ 0.16 and 0.67 $\pm$ 0.17 MeV, respectively, due to the recent change of the ${\Xi }^{-}$ mass value (Particle Data Group). For another three events, we have determined possible species of double-$\Lambda$ hypernuclei together with their binding energies.

Figure 1

Schematic view of the experimental setup around the $(K^{-},K^{+})$ reaction target.

Figure 2

Photograph and schematic drawing of the Nagara event.

Figure 3

Photograph and schematic drawing of the Mikage event.

Figure 4

Image of the scintillating-fiber detectors for the Mikage event. The solid lines are tracks of incoming $K^{-}$ and outgoing $K^{+}$ mesons measured with the spectrometer system. The dashed line is a predicted track of ${\Xi }^{-}$ hyperon reconstructed by Scifi-Bundle. The dashed-dotted line is an extrapolated one using the position and angle of track 6 (Tk#6 in the figure) in the emulsion.

Figure 5

Photograph and schematic drawing of the Demachiyanagi event. A photograph was obtained by summing up images taken at various focusing position of the microscope.

Figure 6

Photographs and schematic drawings of the event viewed from the vertical (a) and the horizontal (b) direction of the Demachiyanagi event. Each photo was made after swelling and slicing a tiny piece of the emulsion, as described in the text.

Figure 7

Image around the target region for the Hida event. The solid lines are tracks of incoming $K^{-}$ and outgoing $K^{+}$ mesons measured with the spectrometer system. The dashed line is a predicted track to be the reconstructed ${\Xi }^{-}$ hyperon scanned with the SciFi-Bundle. The dotted and dashed-dotted lines are extrapolated ones for the track of the ${\Xi }^{-}$ hyperon (associated with point A in Fig. 8) and tracks 6 and 7 using the position and angle of those tracks in the emulsion, respectively.

Figure 8

Photograph and schematic drawing of the Hida event.