Impact of neutron-proton pairing on the nucleon high-momentum distribution in symmetric nuclear matter

TL;DR

This study investigates how neutron-proton pairing influences the high-momentum tail of nucleon momentum distributions in symmetric nuclear matter, revealing a notable contribution comparable to short-range correlations.

Contribution

It introduces a combined framework of extended Brueckner-Hartree-Fock and off-shell BCS theory to quantify the impact of neutron-proton pairing on nucleon momentum distributions.

Findings

The high-momentum tail ratio peaks at about 1.06 near 0.052 fm^{-3} density.

Neutron-proton pairing contributes approximately 6% relative to short-range correlations.

The pairing effect correlates with the squared relative pairing gap over the effective kinetic energy.

Abstract

The effect of neutron-proton ($np$) pairing on the high-momentum tail (HMT) of nucleon momentum distributions in symmetric nuclear matter is investigated within a combined framework of the extended Brueckner-Hartree-Fock approach with off-shell BCS theory. The HMT ratio, quantifying the high-momentum fraction in the BCS state relative to the normal state, reaches about $1.06$ around the density of $0.052\ \mathrm{fm}^{-3}$, indicating that the maximal contribution of the $np$ pairing, amounts to approximately 6\% that from short-range correlations (SRCs). This contribution exhibits a density dependence that closely follows the squared relative pairing gap $\widetilde{\Delta}_F=Z_F\Delta(k_F)$ with respect to the kinetic energy $E_{k_F}^*$ evaluated using the effective mass, suggesting that $\widetilde{\Delta}_F^2/E_{k_F}^{*2}$ provides a qualitative measure of the $np$ pairing effect on the HMT. These findings highlight the significant role of $np$ pairing and its interplay with SRCs in shaping nucleon momentum distributions in nuclear matter.