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

TL;DR

This study used proton inelastic scattering to observe a large soft dipole resonance in 11Li, revealing significant halo-induced electric dipole strength that exhausts a notable portion of the EWSR.

Contribution

First direct measurement of a halo-induced soft dipole resonance in 11Li using proton scattering and DWBA analysis, demonstrating a large E1 strength associated with the halo structure.

Findings

Observed a state at 0.80 MeV with a width of 1.15 MeV in 11Li

The E1 strength is extremely large, 600-2000 Weisskopf units

The strength accounts for 4%-14% of the isoscalar EWSR

Abstract

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.