Perturbative computations of neutron-proton scattering observables using renormalization-group invariant $\chi$EFT up to N$^3$LO

Understand

We predict neutron-proton scattering cross-sections and polarization observables up to next-to-next-to-next-to leading order in a renormalization-group invariant description of the strong nucleon-nucleon interaction.

• Low-energy constants are calibrated to phase shifts, sub-leading corrections are computed in distorted-wave perturbation theory, and we employ momentum-cutoff values 500 and 2500 MeV.
• We find a steady order-by-order convergence and realistic descriptions of scattering observables up to a laboratory scattering energy of approximately 100 MeV.
• We also compare perturbative and non-perturbative calculations for phase shifts and cross sections and quantify how unitarity is gradually restored at higher orders.