Measurement of Coulomb-assisted nuclear state in “IBUKI” event at J-PARC E07M. Fujita1), H. Ekawa2), T. Hashimoto1), S. Hasegawa1), S.H. Hayakawa1), Y. Ichikawa1),
K. Imai1), A. Kasagi3), K. Nakazawa3), T. Nanamura1,4), M. Naruki1,4), H. Sako1), S. Sato1),
T. Takahashi5), H. Tamura1,6), K. Tanida1), T.O. Yamamoto1), J. Yoshida6), M. Yoshimoto3),
and the J-PARC E07 collaboration.
1): Hadron Nuclear Physics Gr., JAEA 2): RIKEN 3): Gifu Univ.
4): Kyoto Univ. 5): KEK 6): Tohoku Univ.

A baryon is a fermion consisting of three quarks. Nucleons (N),protons and neutrons, are baryons formed of only up and downquarks. The interaction between nucleons has investigated using
a lot of data obtained scattered experiments. On the other hand,baryons consisting of strange quarks, such as Λ, Σ, and Ξ, arecalled hyperons (Y). Strangeness nuclear physics whichinvestigates the YN and YY interactions is one of the important
topics for understanding the baryon-baryon interaction. Due tothe short lifetime of hyperons (τ~200 ps), it is difficult to perform
scattered experiment. Therefore, the YN and YY interactions havebeen investigated using hypernuclei which are nuclei containing
hyperons. As one of approaches to the ΞN interaction, the nuclear
emulsion experiments[1,2] and the mass spectroscopy ofhypernuclei[3,4] have been performed. It is that the interaction isattractive, however, no exact information was obtained on its
strength and potential form because of low statics and limited
resolution of their detectors.
To obtain more experimental data, we performed a nuclear
emulsion experiment (J-PARC E07[5]) using a newly developed
analysis method, called a counter-emulsion method. In thismethod, the Ξ− production reaction was identified by themagnetic spectrometers, and the position where the Ξ reached onemulsion was predicted by tracking detectors. It shortened thetime for the image analysis compared with the conventional
method searching by eyes. As a result, we have achieved 10 times
the statics of the previous experiment.
In the experiment, Ξ−s were produced via the (K-, K+) reaction
in a diamond target and they were stopped at a nuclear emulsion

located downstream of the target. Since the Ξ−s have a negative
charge, they are captured by atoms of C, N, O, Br, and Ag in theemulsion, then they form Ξ− atoms. The Ξ− potential and energy
states for the case of the 14N- Ξ system are shown in Fig. 1. Firstly,the Ξ− is in the outer orbit where the Coulomb interaction isdominant as shown in the dotted line and it forms atomic state.
Ξ− transits to inner orbits with X-ray emission, then it is boundand makes a nuclear state due to the strong ΞN interaction. Finally,the Ξ− is absorbed in a nucleus, namely, reacts as Ξ−p→ ΛΛ andit can produce twin Λ hypernuclei or a double Λ hypernucleus.
We reported on the double Λ hypernuclear event “MINO” in2018[6]. For the strong interaction potential U(r), an opticalpotential is employed, U(r) = V(r) + iW(r), and the real and theimaginary parts are written in V(r) and W(r), respectively. Theenergy levels depend on the strength of V(r). The strength of animaginary part W(r) shows the probability of Ξ− absorption intothe nucleus and effects to the energy width (Γ) of the states aswell. We also detected X rays from the Ξ atom using a germanium
detector array to measure the energy levels and widths.
Newly observed a twin hypernuclear event “IBUKI” isreported[7], and it was interpreted as the event in which the Ξ− isabsorbed in 14N nucleus, then decays to 10ΛBe and 5ΛHe hyper
nuclei. The BΞ, the binding energy of the Ξ−, was obtained to be
1.27 ± 0.21 MeV. It represents that this is observation of the Ξ−14N bound system. In addition to Coulomb interaction,
considering the strong interaction with reference to the Ehime
potential[8], the BΞ in Ξ−-14N system of nuclear 0s, 1p, and 2dstates are obtained as 5.93 MeV, 1.14 MeV and 0.174 MeV,
respectively. Therefore, IBUKI event, the BΞ of 1.27 MeV, isconsidered to be the 1p state. The contribution of the strong
interaction is almost zero at the 2d (atomic 3D) state, while bothinteractions are comparable at the 1p state, that is, the Coulomband strong force are resulting in the energy of 0.39 and 0.75 MeV.
The IBUKI is the first observation of the Coulomb-assisted
nuclear 1p state. According to the theoretical calculation whichassumed W(r) value based on the previous data, the probabilityof absorption from the s, p, and d states are estimated to be 0.000.07%, 0.2-5.6%, and 47.9-75.7%, respectively. The fact that a
Ξ− was not absorbed at the d state but reached to the p state in theIBUKI event suggests that the strength of the ΞN-ΛΛ coupling isweaker, in other words, W(r) is smaller than our expectation.
References

Fig. 1 Sketch of the ΞN potential and energy levels.

[1] S. Aoki et al., Nucl. Phy. A 828 191(2009).
[2] J. K. Ahn et al., Phys. Rev. C 88, 014003 (2013)[3] T. Fukuda et al., Phys. Rev. C 58 1306 (1998).
[4] P. Khaustov et al., Phys. Rev. C 61 054603 (2000).
[5] K. Imai et al., Proposal fpr J-PARC E07 experiment[6] S. H. Hayakawa et al., Annual Report of ASRC 2018[7] S. H. Hayakawa et al., Phys. Rev. Lett. 126, 062501 (2021)[8] M. Yamaguchi et al., Prog. Theor. Phys. 105, 627 (2001)