Extraction of anti-analog giant dipole resonance and neutron skin thickness for 208Pb

The anti-analog giant dipole resonance (AGDR) was separated from other excitations such as the spin-dipole resonance by multipole decomposition analysis of the Pb-208((p) over right arrow, (n) over right arrow) reaction at a bombarding energy of T-p = 296 MeV. The polarization transfer observables were found to be useful for carrying out this separation. The energy difference between the AGDR and the isobaric analog state (IAS) was determined to be Delta E = 8.69 +/- 0.36 MeV, where the uncertainty includes both statistical and systematic contributions. Theoretical calculations using the proton-neutron relativistic quasi-particle random phase approximation predicted a strong correlation between Delta E and the neutron skin thickness Delta R-pn. Under the assumption that the correlation predicted in this model is correct, the present Delta E value corresponds to a neutron skin thickness of Delta R-pn = 0.216 +/- 0.046 +/- 0.015 fm, where the first and second uncertainties are the experimental and theoretical uncertainties, respectively.