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% of that from short-range correlations (SRCs). This contribution exhibits a density dependence that closely follows the squared relative pairing gap ${\tilde{\mathrm{\Delta }}}_F=Z_F\mathrm{\Delta }\left(k_F\right)$ with respect to the kinetic energy $E_{k_F}^{*}$ evaluated using the effective mass, suggesting that ${\tilde{\mathrm{\Delta }}}_F^2/E_{k_F}^{*2}$ 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.

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• Nuclear matter
• Nucleon-nucleon interactions
• Ab initio calculations
• Nuclear many-body theory