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Prepared by D. Higinbotham (Jefferson Lab), E. Piasetzky (Tel Aviv Univ.) and M. Strikman (Penn. State Univ.) for the DNP webpage
The structure of nuclei is determined by the nature of the strong force: strong repulsion at short distances and strong attraction at moderate distances. This force makes the nucleus a fairly dilute system and allowed calculations that treated the nucleus as a collection of hard objects in a mean field to describe many of the properties of nuclear matter. Of course, this simple picture has limitations, as the nucleons should be thought of as waves that can strongly overlap for short periods of time. These states of strongly overlapping wave functions are commonly referred to as nucleon-nucleon short-range correlations, and recent inclusive experiments have suggested that about 20% of all nucleons in carbon are in such a state at any given time [1,2].
New exclusive data [3-6] have sought to confirm that the inclusive scaling data are in fact due to short-range nucleon-nucleon correlations . In the exclusive experiments, removing one fast nucleon from the nucleus, using a high-momentum probe, effectively breaks a pair and releases the second nucleon of the correlation, as shown in Fig. 1. Such experiments have been performed on the carbon nucleus at Brookhaven [3,4] and Jefferson Lab [5,6]. These experiments showed that recoiling nucleons with a momentum above the Fermi sea level in the nucleus are part of a correlated pair, and the experiments observed the same strength of proton-neutron correlations. Jefferson Lab’s experiment was also able to observe the proton-proton pairs and, with matched acceptance detectors, determine the ratio of neutron-proton to proton-proton pairs to be nearly 20, as shown in Fig. 2. Calculations explain the magnitude of this neutron-proton to proton-proton ratio as being due to the short-range tensor part, or nucleon-nucleon spin-dependent part, of the nucleon-nucleon force [8-10].
Isolating the signatures of short-range correlations opens new avenues for the exploration of nucleon-nucleon interactions at short distances, addressing in particular the long-standing question of how close nucleons have to approach before the nucleons’ quarks reveal themselves and nucleonic degrees of freedom can no longer be used to describe the system. These studies can influence calculations of extremely dense systems, such as neutron stars, where short-range nucleon-nucleon correlations cause the protons and neutrons to interact strongly, thus enhancing the high-momentum component of the proton momentum distribution and leading to changes in the expected physical properties of the system [11,12]. The study of short-range correlations in nuclei also provides a new way to study the short-range repulsive part of the nucleon-nucleon interaction. Future experiments will attempt to do so via both inclusive and exclusive reactions [13,14].
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