Impact of isovector pairing fluctuations on neutrinoless double-β decay in multireference covariant density functional theory

We extend the multireference covariant density-functional theory (MR-CDFT) by including fluctuations in quadrupole deformations and average isovector pairing gaps simultaneously for the nuclear matrix elements (NMEs) of neutrinoless double-beta (0) decay in the candidate nuclei ^76Ge, ^82Se, ^100Mo, ^130Te, and ^136Xe assuming the exchange of either light or heavy neutrinos. The results indicate a linear correlation between the predicted NMEs and the isovector pairing strengths, as well as the excitation energies of 2₁^+ and 4₁^+ states. By adjusting the pairing strengths based on the excitation energies of the 2₁^+ states, we calculate the NMEs for 0 decay, which are reduced by approximately 12% to 62% compared with the results obtained in the previous studies by Song et al. Phys. Rev. C 95, 024305 (2017). Additionally, upon introducing the average isovector pairing gap as an additional generator coordinate in the calculation, the NMEs increase by a factor ranging from 56% to 218%.