  Proton inelastic scattering off a neutron halo nucleus, 11Li, 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 11Li beam, a strongly populated excited state in 11Li was
observed at Ex=0.80 +/- 0.02 MeV with a width of 1.15 +/- 0.06 MeV. A DWBA
(distorted-wave Born approximation) analysis of the measured differential cross
section with isoscalar macroscopic form factors leads to conclude that this
observed state is excited in an electric dipole (E1) transition. Under the
assumption of isoscalar E1 transition, the strength is evaluated to be
extremely large amounting to 600 to 2000 Weisskopf units, exhausting 4% to 14%
of the isoscalar E1 energy-weighted sum rule (EWSR) value. The large observed
strength originates from the halo and is consistent with the simple di-neutron
model of 11Li halo.
