Pygmy dipole resonance as a constraint on the neutron skin of heavy nuclei

The isotopic dependence of the isovector Pygmy dipole response in tin is studied within the framework of the relativistic random-phase approximation. Regarded as an oscillation of the neutron skin against the isospin-symmetric core, the pygmy dipole resonance may place important constraints on the neutron skin of heavy nuclei and, as a result, on the equation of state of neutron-rich matter. The present study centers around two questions. First, is there a strong correlation between the development of a neutron skin and the emergence of low-energy isovector dipole strength? Second, could one use the recently measured Pygmy dipole resonance in Sn-130 and Sn-132 to discriminate among theoretical models? For the first question we found that although a strong correlation between the neutron skin and the Pygmy dipole resonance exists, a mild anticorrelation develops beyond Sn-120. The answer to the second question suggests that models with overly large neutron skins-and thus stiff symmetry energies-are in conflict with experiment.