Connecting the pygmy dipole resonance to the neutron skin

Abstract

We study the correlation between the neutron skin development and the low-energy dipole response associated with the pygmy dipole resonance (PDR) in connection with the properties of symmetry energy. We perform our investigation within a microscopic transport model based on the Landau-Vlasov kinetic equation by employing three different equations of state in the isovector sector. Together with the giant dipole resonance (GDR) for all studied systems, we identify a PDR collective mode whose energy centroid is very well described by the parametrization EP DR = 41A −1/3 . A linear correlation between the energy weighted sum rule (EWSR) associated to PDR and the neutron skin thickness is evidenced. An increase of 15M eV f m 2 of EWSR, in correspondence to a change of 0.1f m of the neutron skin size, is obtained. We conjecture that different nuclei having close neutron skin sizes will exhaust the same EWSR in the pygmy region. This suggests that a precise experimental estimate of the total EWSR exhausted by the PDR allows the determination of the neutron skin size, constraining the slope parameter of the symmetry energy.

Mass and isospin dependence of the dipole response in a microscopic transport approach

“…Our results are in agreement with several experimental data, as is observed from the comparison presented in Fig. 4 (b), [22]. We shall also notice that other treatments based on Vlasov simulations indicate similar values for the PDR peaks even if a different interaction in the isovector sector was adopted [23].…”

“…We represent the mass dependence of this quantity for the three asy-EOS in Fig. 3 [8]) is 3.14 fm 3 , while we obtain values from 4.1 fm 3 to 5.7 fm 3 when we pass from asysoft to asysuperstiff EOS [22]. In the case of 208 Pb the experimental value of α D is around 20.1 fm 3 [4].…”

“…The solid line reffers to the best fit obtained in [22] Since one expects a strong correlation between the PDR mode and the development of the neutron skin [25,26,27,28], we report in Fig. 6 (b) the dependence of m 1y on the neutron skin thickness ∆R np = R n − R p , where:…”

Dipole response in neutron-rich nuclei with new Skyrme interactions

“…Then R n and R p are tuned in order to minimize the corresponding total energy, associated with the effective interaction adopted in the calculations. We note here that the test particle method is able to reproduce accurately the equation of state of nuclear matter and provide reliable results regarding the properties of nuclear surface [45] and ground state energy for finite nuclei [39,46].…”

“…In order to make a connection with previous studies, we shall also consider simplified Skyrme interactions where the momentum dependent terms are neglected (C ef f = D ef f = 0), corresponding to an incompressibility modulus equal to K = 200 MeV [39]. We will refer to these interactions as momentum-independent (MI) interactions.…”