Chiral interactions and superfluidity in the calcium isotopic chain

TL;DR

This study investigates nuclear superfluidity in calcium isotopes using ab initio calculations, highlighting the sensitivity of pairing gaps to interaction parameters and the need for refined models.

Contribution

It demonstrates that current chiral interactions underestimate pairing gaps and explores how interaction adjustments affect predictions, emphasizing the need for improved many-body theories.

Findings

Experimental pairing gaps can be matched by increasing attraction in models.

Adjusting interactions to fit gaps can lead to unphysical di-neutron bound states.

Pairing gaps are robust against Bayesian calibration of the nuclear interaction.

Abstract

We perform ab initio calculations of three-point mass differences in the odd- and even-mass $^{39-49}$Ca isotopes to probe nuclear superfluidity via empirical neutron pairing gaps. We also quantify the sensitivity of those gaps to the parameters of the interaction at mean-field level. Recent studies employing accurate chiral nuclear interactions have found these gaps to be too small. We show that experimental values can be reproduced at mean-field level by substantially increasing the attraction of the singlet $S$-wave two-nucleon contact interaction, but doing so induces an unphysical bound state of the di-neutron. The sensitivity of these predictions to the full calibration of the nuclear interaction is then studied by performing Bayesian posterior sampling in a delta-full chiral effective field theory at third chiral order. We find that pairing gaps remain largely unaffected, leaving the explanation of nuclear superfluidity as a future task for improved many-body modeling and refined interactions at higher chiral orders.