Li nucleus is one of the flagship drip-line nuclei in the field of nuclear physics. A spatially extended structure of neutrons in Li, which is now widely known as “halo” structure, opened the very active field of research with unstable nuclear beams. Li have the nature of Borromean.[1] In many cases, Li is considered as a 3-body system of Li + 2 neutrons. However, recent theoretical studies pointed out that contribution of the excited Li core can be significant. According to the interpretation of [2], the ground state of 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 Li core excitation. Currently no experiment has succeeded in providing a direct information of the excited Li core in Li.
In this work, with the data of SAMURAI18 experiment, the quasi-free Li(p,pn)Li* reaction was employed to study the excited Li core. Because of spin-parity constraints, the first bound excited state of Li cannot contribute much and the 2nd state, which is unbound, can give the major contribution. Therefore, the Li excited core will decay into the Li + neutron. Using the invariant mass spectrum and dalitz plot of Li + 2 neutrons, we could get the direct information of the exited Li core in 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).
