Onset of the repulsive core in short-range correlated pairs
In plain words
Close nucleon pairs are mostly proton-neutron pairs because of the tensor force (a force depending on spin orientation). At still shorter distance all pairs should feel a hard repulsion that treats them alike, and locating this switch tests the nuclear force at short range.
Precise statement
In hard two-nucleon knockout $\left(e, e' p N\right)$ on nuclei from 4He to 208Pb, measure the $\mathrm{pp}/\mathrm{np}$ pair ratio as a function of relative momentum $k$ from $0.3\text{ to }1.0\,\mathrm{GeV}/c$. Answer: the momentum at which the ratio rises from about $1/20$ toward the spin-isospin-independent value, and whether NN interactions (chiral EFT and phenomenological) reproduce it within the generalized contact formalism.
What would settle it
High-statistics $(e, e' p N)$ data above $k = 0.6\,\mathrm{GeV}/c$ at JLab $12\,\mathrm{GeV}$ with final-state-interaction corrections.
Status in the literature
CLAS data indicated a rise of the pp/np ratio above about 600 MeV/c (Schmidt et al., Nature 2020), with limited statistics at the highest momenta.