tetra neutron system studied by exothermic double-charge exchange reaction $^4$He($^8$He, $^8$Be)4n

A possible existence of the tetra-neutron system as a resonance state is still an open and fascinating question, while theoretical papers using ab-initio calculation suggests that the bound tetra-neutron does not exist. We have performed a missing-mass spectroscopy of the 4n system via the exothermic double-charge exchange reaction $^4$He($^8$He,$^8$Be)4n. The experiment was carried out at the RIBF at RIKEN using the SHARAQ spectrometer and the liquid He target system. Since the secondary beam, $^8$He at 190A MeV, has a large internal energy, it is possible to produce the 4n system in small momentum transfers of less than 20 MeV/c. In the present analysis, a new analytical framework to treat multi-particles under high beam rate condition (2 MHz) was developed for good statistics. At the SHARAQ spectrometer, $^8$Be can be identified by measuring the invariant mass of the coincident two-alpha particle with a good signal-to-noise ratio. About several tens of candidate events are obtained above the 4n threshold. We will show the preliminary result of missing-msss spectrum and discuss the shape of spectrum.