Giant Resonances using Correlated Realistic Interactions: The Case for Second RPA

TL;DR

This paper demonstrates that Second RPA with correlated realistic interactions, specifically UCOM based on Argonne V18, significantly improves the description of nuclear giant resonances by including second-order configurations, addressing limitations of first-order RPA.

Contribution

The study introduces the use of UCOM-based SRPA for collective nuclear excitations, showing it effectively incorporates higher-order correlations and improves upon first-order RPA results.

Findings

Second RPA corrections are sizable and essential for realistic GR descriptions.

UCOM-SRPA captures more physics, such as fragmentation, than RPA.

Remaining discrepancies are due to missing three-body terms and self-consistency issues.

Abstract

Lately we have been tackling the problem of describing nuclear collective excitations starting from correlated realistic nucleon-nucleon (NN) interactions. The latter are constructed within the Unitary Correlation Operator Method (UCOM), starting from realistic NN potentials. It has been concluded that first-order RPA with a two-body UCOM interaction is not capable, in general, of reproducing quantitatively the properties of giant resonances (GRs), due to missing higher-order configurations and long-range correlations as well as neglected three-body terms in the Hamiltonian. Here we report results on GRs obtained by employing a UCOM interaction based on the Argonne V18 potential in Second RPA (SRPA) calculations. The same interaction is used to describe the Hartree-Fock (HF) ground state and the residual interactions. We find that the inclusion of second-order configurations -- which effectively dress the underlying HF single-particle states with self-energy insertions -- produces sizable corrections. The effect appears essential for a realistic description of GRs when using the UCOM. We argue that effects of higher than second order should be negligible. Therefore, the UCOM-SRPA emerges as a promising tool for consistent calculations of collective states in closed-shell nuclei. This is an interesting development, given that SRPA can accommodate more physics than RPA (e.g., fragmentation). Remaining discrepancies due to the missing three-body terms and self-consistency issues of the present SRPA model are pointed out.