Observation of a new double-lambda hypernucleus in J-PARC E07 experiment
S.H. Hayakawa1), S. Ashikaga1,2), H. Ekawa3), M. Fujita4), T. Hashimoto1), S. Hasegawa1),
Y. Ichikawa1), K. Imai1), A. Kasagi1), K. Nakazawa5), T. Nanamura1,2), M. Naruki1,2),
H. Sako1), S. Sato1), T. Takahashi6), H. Tamura1,4), K. Tanida1), T.O. Yamamoto1), M. Yoshimoto5),
and the J-PARC E07 collaboration.
1) : Hadron Nuclear Physics Gr., JAEA 2): Kyoto Univ. 3): RIKEN4): Tohoku Univ. 5): Gifu Univ. 6): KEK

For understanding the baryon–baryon interaction as anextension of the nuclear force, it is essential to investigate on
octet baryons in the SU(3)flavor symmetry. While the nucleon–nucleon interaction has been well studied with rich experimental
data, further studies of hyperon–nucleon and hyperon–hyperon
interactions are necessary for the information of extended
baryon–baryon interaction. It is difficult to produce an intense
hyperon beam due to the very short lifetime of hyperons (τ~1010s, cτ~10-2 m). In particular, an experimental investigation
around the S = −2 sector has been awaited to explore suchextended baryon–baryon interaction. A double-Λ hypernucleus,a nucleus having two Λ hyperons, is a good probe to study theΛΛ interaction. The mass of double-Λ hypernucleus gives us theinformation of the ΛΛ interaction. One of the best ways tomeasure the mass of the double-Λ hypernucleus is a nuclear
emulsion. The emulsion detects tracks of sequential weak
decays of hypernuclei with the position resolution of sub-µm.
The J-PARC E07 experiment was performed to investigate S= -2 hypernuclei at J-PARC K1.8 beam line. Using a K－ meson
beam with the momentum of 1.8 GeV/c, Ξ － hyperons wereproduced in the quasi-free p(K－, K＋)Ξ－ reaction at a diamondtarget. A method using combined information between two
spectrometers and nuclear emulsion, called hybrid emulsion
method, was conducted by using silicon-strip detectors and one
hundred eighteen emulsion modules. The experimental setup
around the target is presented in Fig. 1. In total, 1.13 × 1011 K－
beam particles were irradiated to the target.
An impressive double-Λ hypernuclear event named MINO
[1] was observed with a newly developed emulsion scanningsystem. Figure 2 shows a photograph of the MINO event and its
schematic drawing. The Ξ− hyperon came to rest at vertex A,
where three charged particles of track #1, #3, and #4 wereemitted. The particle of track #1 decayed into three charged
particles of track #2, #5, and #6 at vertex B. The particle oftrack #2 decayed into three charged particles of track #7, #8, and#9 at vertex C. By kinematic analysis, the nuclide of the doubleΛ hypernucleus was uniquely identified as a new double
hypernucleus of beryllium, ΛΛBe. The event can be interpreted
as one of the following three candidates:
16O + Ξ − → 10 Be + 4He + t,ΛΛ
16O + Ξ − → 11 Be + 4He + d,ΛΛ
16O + Ξ − → 12 Be* + 4He + p.
ΛΛThe binding energies of two Λ hyperons in double- Λ
hypernuclei, BΛΛ, are obtained as 15.05 ± 0.11 MeV, 19.07 ±0.11 MeV, and 13.68 ± 0.11 MeV, respectively, by assuming Ξ−capture in the atomic 3D state with the binding energy of the Ξ−of 0.23 MeV. The most probable interpretation is the production
and decay of the 11ΛΛBe nucleus determined from the kinematic
fitting method.

The emulsion scanning of the E07 experiment is ongoing.
More than 10 events of double-Λ hypernuclei have beenobserved up to the present. Further events are expected to beobserved in the near future.

Fig. 1 Schematic view of the experimental setup around a
diamond target. The target size is 50 mm (W) × 30 mm (H) ×30 mm (T).

Fig. 2 A photograph of the MINO event and its schematicdrawing.
References[1] H. Ekawa et al., PTEP 2019, 021D02 (2019).