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Sub-barrier fusion enhancement due to positive Q-value four-neutron transfer

The influence of positive $Q$-value four-neutron transfer (PQ4NT) effects on the sub-barrier capture cross sections is systematically investigated using the empirical barrier distribution (EBD2) method. For 13 fusion reactions with $Q_{4n}>0$, sustained neutron-pair transfer is found to reduce barrier heights and enhance capture cross sections at sub-barrier energies. In contrast, reactions such as $^{18}$O+$^{58}$Ni, which have $Q_{2n}>0$ but $Q_{4n}<0$, exhibit no enhancement due to the stalling of subsequent neutron-pair transfer after the initial 2n transfer. By incorporating PQ4NT effects into EBD2 for systems with $Q_{4n}>0$, the average deviation between predicted and experimental capture cross sections (113 datasets) is significantly reduced by $20\%$. Additionally, comparing with the systems induced by $^{48}$Ca ($Q_{4n}<0$), much larger neutron pickup probabilities are observed in the quasi-elastic scattering of $^{40}$Ca-induced reactions ($Q_{4n}>0$) from the time-dependent Hartree-Fock (TDHF) calculations.