Study of S = −2 Nuclear System by Emulsion and Scintillating
Fiber Hybrid Method (PS-E373)
For the unified description of the baryon-baryon interaction on the SU(3)f symmetry, it isimportant to study the singlet 1s sector of a doubly strange system, which is studied through only
S = −2 nuclear system. If the interaction between two hyperons is attractive based on thecalculation using the MIT bag model, the H dibaryon proposed by R.L.Jaffe [1] may exist. Thenuclear system with S = −2 is also interesting, because it gives us information about hyperonmixing, e.g. ΛΛ−ΞN−ΣΣ. Since the H state may appear in the above mixing, it will be
expected for various nuclear states. However, experimental information about ΛΛ interactionenergy (DBLL) was quite limited and the DBLL value has not been decided, yet. The main reason
was that the expected number of Ξ− hyperon stopping events in the past experiments was too
small to fix DBLL.
Therefore, the hybrid emulsion experiment E373 at KEK has been carried out to obtain Ξ−
hyperon stopping events with ten times more statistics than that of the previous E176, where thenumber of the events becomes 103. By the 10% data analysis of all, one twin single- Λhypernuclei event and two events of double-Λ hypernucleus have been successfully detected.
The twin single-Λ event was uniquely interpreted as Ξ−+14N → Λ5 He+Λ5 He+4He + n for the
first time [2]. In the first double-Λ hypernucleus event, ”Demachi-Y anagi, it was interpreted as1010Ξ−+12C → ΛΛBe(or ΛΛBe＊)+t, if two-body reaction was occurred at a Ξ−-12C system [3].
The second double-Λ hypernucleus event, ”NAGARA”, has been detected with a clearlyrecognized topology, as shown in Figure. The nuclide and its decay mode have been uniquely
66identified as; Ξ−+12C →LLHe+4He+t, LLHe → Λ5 He+π−+p, Λ5 He → p+d+2n etc. H. Bandoand his collaborators have indicated the importance of 6ΛΛHe as a multi-hypernuclear cluster
system, and proposed the name ”Lambpha” for it by analogy of the α particle [4]. By the6preliminary result of LLHe analyses, the attractive ΛΛ interaction has been established withDBLL ~1 MeV, where its error is less than 0.5 MeV. This value is considerably smaller than that
previously estimated experimentally. The violent disagreement between our result for DBLL andthat reported by D. J. Prowse [5] confirms the doubts on the authenticity of the previous event. In
addition, the lower mass limit of the H dibaryon has been obtained as ~2224 MeV at a 90%
confidence level using our value of DBLL.

References[1] R. L. Jaffe, Phys. Rev. Lett. 38 (1977) 195.
[2] A. Ichikawa et al., Phys. Lett. B500 (2001) 37.
[3] A. Ichikawa, Ph.D thesis, Kyoto University (2001), (unpublished); J. K. Ahn et al., in:Proceedings of Hadron and Nuclei, in Seoul 2001, AIP Press, to be published.
[4] H. Bando, K. Ikeda and T. Motoba, Prog. Theor. Phys. 66 (1981) 1344; ibid. 67 (1982)508.

[5] D. J. Prowse, Phys. Rev. Lett. 17 (1966) 782.

Figure. An emulsion image of Lambpha ( 6ΛΛHe double-Λ hypernucleus) event by E373. A Ξ−hyperon was captured by 12C in the emulsion at point A. The particle of track#1 produced withstable nuclei of #3 and #4 decayed at point B into track#2, #5 and #6. Track#2 decayed into #7,#8 and neutral particle(s). The end points of particles except for track#5 and #7 were clearlyrecognized in the emulsion. The particle of #6 was identified as π− by the topology at its end
point in the emulsion. Although track#7 was escaping from the emulsion, its stopping point wasdetected in Scintillating fiber block located downstream the emulsion, and then we could measure
its energy. The ranges of the double-Λ (track#1) and the single-Λ (track#2) hypernucleus are
8.3 and 9.2 μm, respectively. Since the event was found by Y. S. Iwata (graduate student ofGifu univ.), it was named ”NAGARA”, which is a river originating in Gifu.