Electric Dipole Polarizability of Ca 48 and Implications for the Neutron Skin

The electric dipole strength distribution in Ca48 between 5 and 25 MeV has been determined at RCNP, Osaka from proton inelastic scattering experiments at forward angles. Combined with photoabsorption data at higher excitation energy, this enables the first extraction of the electric dipole polarizability αD(Ca48)=2.07(22) fm3. Remarkably, the dipole response of Ca48 is found to be very similar to that of Ca40, consistent with a small neutron skin in Ca48. The experimental results are in good agreement with ab initio calculations based on chiral effective field theory interactions and with state-of-the-art density-functional calculations, implying a neutron skin in Ca48 of 0.14-0.20 fm.