Universality of nucleon-nucleon short-range correlations: the   factorization property of the nuclear wave function, the relative and   center-of-mass momentum distributions, and the nuclear contacts
  The two-nucleon momentum distributions have been calculated for nuclei up to A=40 and various values of the relative and center-of-mass momenta and angle between them. For complex nuclei a parameter-free linked-cluster expansion, based upon a realistic local two-nucleon interaction of the Argonne family and variational wave function featuring central, tensor, spin and iso-spin correlations, has been used. The obtained results show that: 1) independently of the mass number A, at values of the relative momentum k_rel> 2 fm^{-1} the proton-neutron momentum distributions for back-to-back (BB) nucleons (K_cm=0) exhibit the factorization property n_A^{pn}(k_rel,K_cm=0)=C_A^{pn} n_D(k_rel) n_{cm}^{pn}(K_cm=0), where n_D is the deuteron momentum distribution, n_{cm}^{pn}(K_{cm}=0) the momentum distribution of the c.m. motion of the pair and C_A^{pn} the nuclear contact measuring the number of BB pn pairs with deuteron-like momenta; 2) the values of the proton-neutron nuclear contact C_A^{pn} are obtained in a model-independent way from the ratio n_A^{pn}(k_rel,K_cm=0)/n_D(k_rel) n_{cm}^{pn}(K_cm=0); 3) also the K_cm-integrated pn momentum distributions divided by the deuteron momentum distribution exhibits a constant behavior equal to C_A^{pn}, but only at very high values of k_{rel}> 3.5fm^{-1}, where the relative momentum distribution is entirely governed by BB short-range correlated nucleons; 4) the absolute value of the number of pn and pp short-range correlated pairs is calculated, illustrating that the high values (K_cm>1 fm^{-1}) of the pair c.m. momentum appreciably reduce the dominance of the pn over pp pairs produced by the tensor force when K_cm=0; 5) calculations are in good agreement with the VMC calculations for light nuclei and with available experimental on the processes A(e,e'pn)X and A(e,e'pp)X. 
Massimiliano Alvioli, Claudio Ciofi degli Atti, Hiko Morita
nucl-th nucl-ex