# A nucleon-pair and boson coexistent description of nuclei

## TL;DR

This paper investigates the coexistence and crossover between nucleon pairs and s-bosons in nuclei, demonstrating the robustness of BCS-BEC phenomena through realistic pf-shell calculations and B(E2) transition predictions.

## Contribution

It introduces a combined nucleon-pair and boson model with Pauli blocking effects, showing smooth BCS-BEC crossover behavior in nuclei with realistic interactions.

## Key findings

- Smooth BCS-BEC crossover near half-shell in pf-shell nuclei.
- B(E2) transition values are well reproduced for Sn isotopes.
- Robustness of coexistence phenomena confirmed with realistic single-particle energies.

## Abstract

We study a mixture of s-bosons and like-nucleon pairs with the standard pairing interaction outside a inert core. Competition between the nucleon-pairs and s-bosons is investigated in this scenario. The robustness of the BCS-BEC coexistence and crossover phenomena is examined through an analysis of pf-shell nuclei with realistic single-particle energies in which two configurations with Pauli blocking of nucleon-pair orbits due to the formation of the s-bosons is taken into account. When the nucleon-pair orbits are considered to be independent of the s-bosons, the BCS-BEC crossover becomes smooth with the number of the s-bosons noticeably more than that of the nucleonpairs near the half-shell point, a feature that is demonstrated in the pf-shell for several values of the standard pairing interaction strength. As a further test of the robustness of the BCS-BEC coexistence and crossover phenomena in nuclei, results are given for B(E2; 0^{+}_{g}->2^{+}_1) values of even-even 102-130Sn with 100Sn taken as a core and valence neutron pairs confined within the 1d5/2, 0g7/2, 1d3/2, 2s1/2, 1h11/2 orbits in the nucleon-pair orbit and the s-boson independent approximation. The results indicate that the B(E2) values are well reproduced.