Observation of the Exotic 02+ Cluster State in He 8

We report here the first observation of the 02+ state of He8, which has been predicted to feature the condensatelike α+n2+n2 cluster structure. We show that this state is characterized by a spin parity of 0+, a large isoscalar monopole transition strength, and the emission of a strongly correlated neutron pair, in line with theoretical predictions. Our finding is further supported by the state-of-the-art microscopic α+4n model calculations. The present results may lead to new insights into clustering in neutron-rich nuclear systems and the pair correlation and condensation in quantum many-body systems under strong interactions.