Low-density homogeneous symmetric nuclear matter: disclosing dinucleons in coexisting phases

The effect of in-medium dinucleon bound states on self-consistent single-particle fields in Brueckner, Bethe and Goldstone theory is investigated in symmetric nuclear matter at zero temperature. To this end, dinucleon bound state occurences in the $^1S_0$ and ${}^3SD_1$ channels are explicitly accounted for -within the continuous choice for the auxiliary fields- while imposing self-consistency in Brueckner-Hartree-Fock approximation calculations. Searches are carried out at Fermi momenta in the range $0<k_F\leq1.75$~fm$^{-1}$, using the Argonne $v_{18}$ bare nucleon-nucleon potential without resorting to the effective mass approximation. As a result, two distinct solutions meeting the self-consistency requirement are found with overlapping domains in the interval $0.130\;\textrm{fm}^{-1} \leq k_F \leq 0.285\;\textrm{fm}^{-1}$, corresponding to mass densities between $10^{11.4}$ and $10^{12.4}$ g cm$^{-3}$. Effective masses as high as three times the nucleon mass are found in the coexistence domain. The emergence of superfluidity in relationship with BCS pairing gap solutions is discussed.