Nucleon-Nucleon Short-Range Correlations
The nucleus can often be approximated as an independent collection of protons and neutrons confined in a volume, but for short periods of time, the nucleons in the nucleus can strongly overlap. This quantum mechanical overlapping, known as a nucleon-nucleon short-range correlation, is a manifestation of the nuclear strong force, which produces not only the long-range attraction that holds matter together, but also the short-range repulsion that keeps it from collapsing.
Direct observation of short-range correlations has been a challenge for nuclear physics, as other phenomena often mask the signal. Cross section ratios of inclusive scattering of heavy nuclei to 3He at Q2>1.4 [GeV/c]2 and as function of Brokjen x have shown scaling regions that have been interpreted as corresponding to two- and three-nucleon correlations . To directly observe high momentum pairs emerging from the nucleus, a triple coincident 12C(e,e'pN) experiment was designed to probe high Q2>1.5 [GeV/c]2, Brokjen x>1 and missing momenta greater than 300 [MeV/c]2.
The experiment found, for these special conditions, that every single knocked-out proton had a correlated partner . As predicted by theorists , the correlated nucleon pairs were predominately proton-neutron pairs with only a small fraction of proton-proton pairs. Since the nuclear density of a correlated pair is approximately five times larger than average nuclear matter, these results may give scientists new insight into dense nuclear systems such as neutron stars.
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