Nuclear mass predictions of the relativistic continuum Hartree-Bogoliubov theory with the kernel ridge regression. II. Odd-even effects

Background: Masses of neutron-rich exotic nuclei are important for both nuclear physics and nuclear astrophysics, yet most of them remain beyond the current experimental capabilities. The relativistic continuum Hartree-Bogoliubov (RCHB) theory has achieved great success in the study of exotic nuclei. In the first paper of this series Phys. Rev. C 109, 024310 (2024), the kernel ridge regression (KRR) approach has been employed to refine the RCHB mass model, and the predictive power has been validated through comparison with the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) for even-even nuclei. Recently, the mass data for even-odd nuclei have been released by the DRHBc Mass Table Collaboration At. Data Nucl. Data Tables 158, 101661 (2024). The KRR approach has been extended to incorporate odd-even effects Phys. Lett. B 819, 136387 (2021), thereby enhancing its capability in nuclear mass predictions.Purpose: The aim of this work is to combine the RCHB mass model and the extended KRR approach with odd-even effects (KRRoe) to improve the predictions of masses and separation energies for neutron-rich exotic nuclei.Method: In the KRRoe approach, the odd-even effect is incorporated by remodulating the kernel function in the KRR framework. The KRRoe network is trained with the RCHB mass residuals, i.e., deviations between experimental and calculated masses. Four hyperparameters contained in the KRRoe approach are optimized through the leave-one-out cross validation. The mass data in different AME series are employed to validate the generalization ability of the KRRoe approach. The DRHBc mass model for even-even and even-odd nuclei is employed to help pin down the capacity of the KRRoe extrapolations.Results: The refined RCHB mass model with KRRoe corrections can achieve a mass accuracy with a root-mean-square deviation of 177 keV from the experimental data. Especially, it improves the accuracy of separation energy from approximately 500 keV, given by the KRR approach, to the level of 200 keV. The generalization and extrapolation of the KRRoe approach in refining the RCHB predictions are found reliable and extensive.Conclusions: A mass model incorporating the RCHB theory with properly treated continuum effects and the KRRoe approach with properly included odd-even effects has been developed. This model demonstrates accuracy in the reproduction of experimentally known masses and separation energies, as well as reliability in generalization and extrapolation to experimentally unknown neutron-rich regions.