Spin-2 Mesons in a Relativistic Hartree Description of Nuclei

TL;DR

This paper introduces a new class of spin-2 massive mesons into covariant density functional theory to improve modeling of nuclear spin-orbit interactions, addressing discrepancies with recent experimental data on neutron-rich nuclei.

Contribution

It proposes adding spin-2 massive mesons to the nuclear Lagrangian, enhancing the isovector spin-orbit sector and resolving issues in existing models.

Findings

Spin-2 mesons significantly contribute to the theory.

Enhanced models better match experimental neutron skin data.

Mitigates problems in previous spin-orbit enhancement approaches.

Abstract

The role of the isovector spin-orbit potential in nuclear modeling has recently been explored in greater detail due largely in part to the PREX and CREX parity violating electron scattering (PVES) experiments. The result of both experiments have shown that the neutron-rich skins of $^{48}$Ca and $^{208}$Pb, as determined by the experimental analysis, are largely incompatible with current nuclear model predictions. One way to address, and potentially solve, this incompatibility has been to enhance the isovector spin-orbit sector of density functional models. Here, we explore the range of possible enhancements in the context of covariant density functional theory by introducing a new class of spin-2 \textit{massive} mesons to the Lagrangian. In doing so, we find that these mesons not only meaningfully contribute to the theory, but also mitigate problems seen in other works which enhance spin-orbit effects in finite nuclei. We also discuss the implications and future regarding this ``dilemma'' as it pertains to studies of nuclear forces and future experiments such as the Mainz Radius Experiment (MREX).