Momentum Dependent Nucleon–Nucleon Contact Interactions and Their Effect on \(p-d\) Scattering Observables

Starting from a complete set of relativistic nucleon–nucleon contact operators up to order \(O(p^4)\) of the expansion in the soft (relative or nucleon) momentum p, we show that non-relativistic expansions of relativistic operators involve twenty-six independent combinations, two starting at \(O(p^0)\), seven at order \(O(p^2)\) and seventeen at order \(O(p^4)\). This demonstrates the existence of two low-energy free constants that parameterize interactions dependent on the total momentum of the pair of nucleons P. The latter, through the use of a unitary transformation, can be removed in the two-nucleon fourth-order contact interaction of the Chiral Effective Field Theory, generating a three-nucleon interaction at the same order. Within a hybrid approach in which this interaction is considered together with the phenomenological potential AV18, we show that the LECs involved can be used to fit very accurate data on the polarization observables of the low-energy \(p-d\) scattering, in particular the \(A_y\) asymmetry.