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.

• The paper shows that short-range nucleon-nucleon correlations modify quark configurations, underpinning the EMC effect observed in electron-nucleus scattering.
• It employs empirical models, light-front dynamics, and effective field theories to analyze high-momentum transfer and nucleon virtuality.
• The findings link nucleon modifications in SRC pairs to nuclear high-density fluctuations, motivating advanced experimental investigations.

An Essay on "Nucleon-Nucleon Correlations, Short-lived Excitations, and the Quarks Within"

The paper "Nucleon-Nucleon Correlations, Short-lived Excitations, and the Quarks Within" provides an extensive review of the interrelated phenomena observed in lepton-nucleus deep inelastic scattering (DIS) and quasi-elastic (QE) scattering. It explores nucleon-nucleon (NN) correlations in the nucleus, explicating how these short-range correlations (SRC) impact nucleon structure and result in the empirically observed EMC effect.

Central to the paper is the analysis of the EMC effect, characterized by the suppression of the nuclear parton distribution functions compared to those of free nucleons. Empirical evidence indicates that this effect correlates with the probability of finding nucleons in SRC pairs. The paper postulates that the nucleons within these high-momentum, high-virtuality pairs exhibit modified quark configurations due to the dense local nuclear environment, thus accounting for the EMC effect.

Technical details underscore the importance of considering both the virtuality and the momentum transfer present during electron-nucleus interactions. The interactions involve short-ranged nucleon configurations which are further described using a combination of empirical models and theoretical frameworks, including light-front dynamics and e↯ective field theories (EFT). The paper suggests that complex inter-nucleon forces and the consequent short-range interactions are critical to understanding not only nuclear binding but also the modification of nucleon structure functions under high-momentum transfer conditions.

The research emphasizes that both the EMC effect and SRC effects are likely influenced by high-density fluctuations within nuclei, where proton-neutron pairs form the majority of correlated pairs. This conclusion is supported by the observation of plateaus in the cross section ratio of various nuclei to deuterium at large values of $x_B$, indicative of SRC presence. The correlation between the strength of the EMC effect and the SRC scaling factors is supported by several experimental datasets and incites significant interest in further theoretical analysis.

The paper concludes by highlighting the need for advanced experimental techniques, such as the measurement of tagged structure functions and the exploration of EMC effects in asymmetric nuclei to further verify and expand upon their theoretical conclusions. The implications of these findings are profound, potentially affecting our understanding of confinement mechanisms in QCD and even informing astrophysical phenomena such as neutron star structure.

In future directions, the authors suggest more robust theoretical models are necessary to bridge the gap between mean-field approaches and SRC, alongside more extensive experimental evidence across different nuclear systems. Experimentation at facilities equipped to handle high-energy processes, such as Jefferson Lab, is pivotal to advancing our comprehension of how nucleonic matter behaves under extreme conditions, thereby addressing the broader questions affecting both nuclear physics and particle physics paradigms.