Physics of neutron drip line

There is an unsolved fundamental problem related to nuclear physics such that how many neutrons (protons) can be crammed into the nucleus. The limit where neutrons (protons) cannot be crammed any more is the 'drip line,' and it is undetermined. To explore its position leads to the understanding of the nuclear force and many-body correlation that works on the nucleon many-body system with excessive neutrons (protons). Halo nucleus, a typical example of the many-body correlation, is more stabilized by the development of halo, and there is a possibility that the drip line becomes farther. It is assumed that when two neutron systems have entered the nuclear surface, they are likely to behave as bounded two-neutron systems (dineutron). However, such dineutron has not been found yet. It is pointed out from the latest studies that there is the possibility of dineutron to directly appear on the neutron halo or neutron skin, and directly seeing dineutron has become one of the most important challenges in the unstable nuclear physics field. From the superconducting spectrometer SAMURAI, which was optimized for the invariant mass method research and installed at RIBF of RIKEN, the result that indicates the nature of halo structure has been obtained. (A.O.)