Halo-induced large enhancement of soft dipole excitation of 11 Li observed via proton inelastic scattering

Proton inelastic scattering off a neutron halo nucleus, (11) Li, has been studied in inverse kinematics at the IRIS facility at TRIUMF. The aim was to establish a soft dipole resonance and to obtain its dipole strength. Using a high quality 66 MeV (11) Li beam, a strongly populated excited state in (11) Li was observed at E-x= 0.80 +/- 0.02 MeV with a width of r = 1.15 +/- 0.06 MeV. A DWBA (distorted-wave Born approximation) analysis of the measured differential cross section with isoscalar macroscopic form factors leads us to conclude that this observed state is excited in an electric dipole (El) transition. Under the assumption of isoscalar El transitions, the strength is evaluated to be extremely large amounting to 30 similar to 296 Weisskopf units, exhausting 2.2% similar to 21% of the isoscalar El energy-weighted sum rule (EWSR) value. The large observed strength originates from the halo and is consistent with the simple di-neutron model of (11) Li halo. (c) 2017 The Authors. Published by Elsevier B.V.