Observation of strong low-lying E1 strength in the two-neutron halo nucleus 11Li

An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus Li-11 at 70 MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about E-x=0.6 MeV with B(E1)=1.42(18) e(2) fm(2) for E-rel <= 3 MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule.