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$^{11}$Li nucleus is one of the flagship drip-line nuclei in the field of nuclear physics. A spatially extended structure of neutrons in $^{11}$Li, which is now widely known as “halo” structure, opened the very active field of research with unstable nuclear beams. $^{11}$Li have the nature of Borromean.[1] In many cases, $^{11}$Li is considered as a 3-body system of $^9$Li + 2 neutrons. However, recent theoretical studies pointed out that contribution of the excited $^9$Li core can be significant. According to the interpretation of [2], the ground state of $^{11}$Li has components which contain excited state of the core. In Ref [3], they showed that the E1 cluster sum rule value should be reduced by about 15% due to the $^9$Li core excitation. Currently no experiment has succeeded in providing a direct information of the excited $^9$Li core in $^{11}$Li.
In this work, with the data of SAMURAI18 experiment, the quasi-free $^{11}$Li(p,pn)$^9$Li* reaction was employed to study the excited $^9$Li core. Because of spin-parity constraints, the first bound excited state of $^9$Li cannot contribute much and the 2nd state, which is unbound, can give the major contribution. Therefore, the $^9$Li excited core will decay into the $^8$Li + neutron. Using the invariant mass spectrum and dalitz plot of $^8$Li + 2 neutrons, we could get the direct information of the exited $^9$Li core in $^{11}$Li.
[1] M. V. Zhukov, et al., Phys. Rep. 231, 151 (1993).
[2] G. Potel, F. Barranco, E. Vigezzi, and R. A. Broglia, Phys. Rev. Lett. 105, 172502 (2010)
[3] Y. Kikuchi, et al., Phys. Rev. C 87, 034606 (2013).
