Nucleon-nucleon correlations, short-lived excitations, and the quarks within

Abstract

This article reviews our current understanding of how the internal quark structure of a nucleon bound in nuclei differs from that of a free nucleon. We focus on the interpretation of measurements of the EMC effect for valence quarks, a reduction in the Deep Inelastic Scattering (DIS) cross-section ratios for nuclei relative to deuterium, and its possible connection to nucleon-nucleon Short-Range Correlations (SRC) in nuclei. Our review and new analysis (involving the amplitudes of non-nucleonic configurations in the nucleus) of the available experimental and theoretical evidence shows that there is a phenomenological relation between the EMC effect and the effects of SRC that is not an accident. The influence of strongly correlated neutron-proton pairs involving highly virtual nucleons is responsible for both effects. These correlated pairs are temporary high-density fluctuations in the nucleus in which the internal structure of the nucleons is briefly modified. This conclusion needs to be solidified by the future experiments and improved theoretical analyses that are discussed herein.

Search for three-nucleon short-range correlations in light nuclei

“…The inclusive 4 He/ 3 He cross section ratio is observed to be both x and Q 2 independent for 1.5 < x < 2, confirming the dominance of two-nucleon short-range correlations. For x > 2, our data support the hypothesis that a previous claim of three-nucleon correlation dominance was an artifact caused by the limited resolution of the measurement.…”

“…We apply a ±7 cm cut around the center of the target, removing > 99.9% of the events from target endcap scattering, as determined from measurements on empty target cells. One of the largest contributions to the systematic uncertainty comes from target density reduction due to heating of the 2 H, 3 He, and 4 He targets by the high-current electron beam. We made dedicated measurements over a range of beam currents and used the variation of the yield to determine the current dependence of the target density.…”

“…A 3.356-GeV electron beam with currents ranging from 40 to 120 μA impinged on nuclear targets, and scattered electrons were detected in two nearly identical high-resolution spectrometers (HRSs) [28]. Data were taken on three 20-cm cryogenic targets (liquid 2 H and gaseous 3 He and 4 He) and on thin foils of 12 C and 40,48 Ca. Each HRS consists of a pair of vertical drift chambers (VDCs) for particle tracking, two scintillator planes for triggering and timing measurements, and a gas Čerenkov counter and two layers of lead-glass calorimeters for particle identification [28].…”

“…Within the simple SRC model [10], the cross section is composed of scattering from one-body, two-body, etc., configurations, with the one-body (shell-model) contributions dominating at x ≈ 1, while 2N (3N ) SRCs dominate as x → 2(3). Taking ratios of heavier nuclei to 3 He allows a similar examination of the target ratios for x > 2, where the simple SRC model predicts a universal behavior associated with 3N SRCs-configurations where three nucleons have large relative momenta but little total momentum. 3N SRCs could come from either three-nucleon forces or multiple hard two-nucleon interactions.…”

“…Understanding the complex structure of the nucleus remains one of the major uncompleted tasks in nuclear physics, and the high-momentum components of the nuclear wave function continue to attract attention [1][2][3]. Momenta above the Fermi momentum are strongly suppressed in shell-model and mean-field calculations [4].…”