  The strong nuclear interaction between nucleons (protons and neutrons) is the
effective force that holds the atomic nucleus together. This force stems from
fundamental interactions between quarks and gluons (the constituents of
nucleons) that are described by the equations of Quantum Chromodynamics (QCD).
However, as these equations cannot be solved directly, physicists resort to
describing nuclear interactions using effective models that are well
constrained at typical inter-nucleon distances in nuclei but not at shorter
distances. This limits our ability to describe high-density nuclear matter such
as in the cores of neutron stars. Here we use high-energy electron scattering
measurements that isolate nucleon pairs in short-distance, high-momentum
configurations thereby accessing a kinematical regime that has not been
previously explored by experiments, corresponding to relative momenta above 400
MeV/c. As the relative momentum between two nucleons increases and their
separation thereby decreases, we observe a transition from a spin-dependent
tensor-force to a predominantly spin-independent scalar-force. These results
demonstrate the power of using such measurements to study the nuclear
interaction at short-distances and also support the use of point-like nucleons
with two- and three-body effective interactions to describe nuclear systems up
to densities several times higher than the central density of atomic nuclei.
