Ab-initio approach to effective single-particle energies in doubly closed shell nuclei

TL;DR

This paper investigates the concept of effective single-particle energies (ESPEs) in doubly closed shell nuclei using ab-initio coupled-cluster calculations, emphasizing their non-observability and the importance of consistent models for meaningful interpretation.

Contribution

It provides a detailed analysis of ESPEs in strongly correlated nuclei, highlighting the necessity of diagonalizing the centroid matrix and discussing their non-observable nature.

Findings

ESPEs can be modified without changing observables.

Correlations significantly impact one-nucleon separation energies.

Consistent models are essential for meaningful shell structure extraction.

Abstract

The present work discusses, from an ab-initio standpoint, the definition, the meaning, and the usefulness of effective single-particle energies (ESPEs) in doubly closed shell nuclei. We perform coupled-cluster calculations to quantify to what extent selected closed-shell nuclei in the oxygen and calcium isotopic chains can effectively be mapped onto an effective independent-particle picture. To do so, we revisit in detail the notion of ESPEs in the context of strongly correlated many-nucleon systems and illustrate the necessity to extract ESPEs through the diagonalization of the centroid {\it matrix}, as originally argued by Baranger. For the purpose of illustration, we analyse the impact of correlations on observable one-nucleon separation energies and non-observable ESPEs in selected closed-shell oxygen and calcium isotopes. We then state and illustrate the non-observability of ESPEs. Similarly to spectroscopic factors, ESPEs can indeed be modified by a redefinition of inaccessible quantities while leaving actual observables unchanged. This leads to the absolute necessity to employ consistent structure and reaction models based on the same nuclear Hamiltonian to extract the shell structure in a meaningful fashion from experimental data.