From a research paper: “Probing Cold Dense Nuclear Matter”:
The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, where a proton is knocked-out of the nucleus with high momentum transfer and high missing momentum, show that in 12C the neutron-proton pairs are nearly twenty times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.
Nuclei are composed of bound protons and neutrons, referred to collectively as nucleons (the standard notation is p, n, and N, respectively). A standard model of the nucleus since the 1950s has been the nuclear shell model, where neutrons and protons move independently in well-defined quantum orbits in the average nuclear field created by their mutual attractive interactions. In the 1980s and 1990s, proton removal experiments using electron beams with energies of several hundred MeV showed that only 60-70% of the protons participate in this type of independent-particle motion in nuclear valence states. At the time, it was assumed that this low occupancy was caused by correlated pairs of nucleons within the nucleus. Indeed, the existence of nucleon pairs that are correlated at distances of several femtometers, known as long-range correlations, has been established (3), but these accounted for less than half of the predicted correlated nucleon pairs. Recent high momentum transfer measurements have shown that nucleons in nuclear ground states can form pairs with large relative momentum and small center-of-mass (CM) momentum due to the shortrange, scalar and tensor, components of the nucleon-nucleon interaction. These pairs are referred to as short-range correlated (SRC) pairs. The study of these SRC pairs allows access to cold dense nuclear matter, such as that found in a neutron star.
Experimentally, a high-momentum probe can knock a proton out of a nucleus, leaving the rest of the system nearly unaffected. If, on the other hand, the proton being struck is part of a SRC pair, the high relative momentum in the pair would cause the correlated nucleon to recoil and be ejected as well
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