Three-body correlation in Li studied via low-lying E1 excitation

We have studied the structure of the Borromean nucleus Li via the Coulomb breakup with a Pb target at 70 MeV/nucleon. Momentum vectors of the breakup particles, Li and two neutrons, were measured in coincidence to reconstruct the B(E1) energy spectrum. The strong E1 transitions were observed at Erel ∼ 0.3 MeV with the B(E1)=1.42(18)efm (=4.5(6)W.u.), which is the largest E1 strength ever observed in such low excitation energies. This result gives an evidence that the two valence neutrons have spatial dineutron-like correlation. The correlation of the two neutrons is supported by the non-energy weighted E1 cluster sum rule which provides the mean opening angle of 〈θ12〉= 48 −18 degrees for the two valence neutrons relative to the center of the core. It is suggested that this narrow angle can be related to the angular momentum mixing in the halo neutrons of Li.