Speaker
Description
The Second Random Phase Approximation (SRPA) is a natural extension of Random Phase Approximation obtained by introducing more general excitation operators where two particle-two hole configurations, in addition to the one particle-one hole ones, are considered. Only in the last years, large-scale SRPA calculations, without usually employed approximations have been performed [1,2].
The SRPA model corrected by a subtraction procedure [2] designed to cure double counting, instabilities, and ultraviolet divergences, is employed for the first time to analyze the dipole strength and polarizability in 48Ca [3]. All the terms of the residual interaction are included, leading to a fully self-consistent scheme. Results are illustrated with two Skyrme parametrizations, SGII and SLy4 and compared with the experimental data recently obtained at RCNP-Osaka, employing the (p,p’) reaction at forward angle [4].
The results obtained with the SGII interaction are particularly satisfactory. In this case, the low--lying strength below the neutron threshold is extremely well reproduced and the giant dipole resonance is described in a very satisfactory way especially in its spreading and fragmentation. Spreading and fragmentation are produced in a natural way within such a theoretical model by the coupling of 1 particle--1 hole and 2 particle--2 hole configurations. Owing to this feature, we may provide for the electric polarizability as a function of the excitation energy a curve with a similar slope around the centroid energy of the giant resonance compared to the corresponding experimental results.
This represents a considerable improvement with respect to previous theoretical predictions obtained with the random--phase approximation or with several ab--initio models. In such cases, the spreading width of the excitation cannot be reproduced and the polarizability as a function of the excitation energy displays a stiff increase around the predicted centroid energy of the giant resonance.
[1] D. Gambacurta, M. Grasso, and F. Catara, Phys. Rev. C 84, 034301 (2011).
[2] D. Gambacurta, M. Grasso and J.Engel,Phys. Rev. C 81, 054312 (2010); Phys. Rev. C 92 , 034303 (2015).
[3] D. Gambacurta , M. Grasso , O. Vasseur, Physics Letters B 777 163–168, (2018).
[4] A. Tamii et al., Phys. Rev. Lett. 107, 062502 (2011).
