Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei

TL;DR

This study provides the first direct measurements of proton-proton and neutron-proton short-range correlations in heavy nuclei, revealing a dominance of np pairs and supporting theoretical models with implications for nuclear structure understanding.

Contribution

It presents the first direct measurement of SRC pairs in heavy nuclei and compares results with theoretical models, highlighting a lower pp to np pair ratio than previously thought.

Findings

np pairs dominate SRC in heavy nuclei

Measured pp to np ratio is about 3% after corrections

Theoretical models agree with experimental data

Abstract

We measured the triple coincidence A(e,e'np) and A(e,e'pp) reactions on carbon, aluminum, iron, and lead targets at Q2 > 1.5 (GeV/c)2, xB > 1.1 and missing momentum > 400 MeV/c. This was the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for Single-Charge Exchange effects decreased the SRC pairs ratio to ~ 3%, which is lower than previous results. Comparisons to theoretical Generalized Contact Formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale- and scheme-dependence in cross section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.