Gluons and the quark sea at high energies: distributions, polarization, tomography

Abstract: This report is based on a ten-week program on "Gluons and the quark sea at high-energies", which took place at the Institute for Nuclear Theory in Seattle in Fall 2010. The principal aim of the program was to develop and sharpen the science case for an Electron-Ion Collider (EIC), a facility that will be able to collide electrons and positrons with polarized protons and with light to heavy nuclei at high energies, offering unprecedented possibilities for in-depth studies of quantum chromodynamics. This report is organized around four major themes: i) the spin and flavor structure of the proton, ii) three-dimensional structure of nucleons and nuclei in momentum and configuration space, iii) QCD matter in nuclei, and iv) Electroweak physics and the search for physics beyond the Standard Model. Beginning with an executive summary, the report contains tables of key measurements, chapter overviews for each of the major scientific themes, and detailed individual contributions on various aspects of the scientific opportunities presented by an EIC.

• The paper demonstrates the interplay of gluon and sea quark distributions at high energies using advanced QCD approaches.
• It employs polarization measurements and SIDIS techniques to map the nucleon’s spatial structure and dynamics.
• The study provides a foundation for refining theoretical models and guiding future experiments at the Electron-Ion Collider.

The EIC Science Case: Overview and Implications

The Electron-Ion Collider (EIC) represents a significant step forward in exploring the fundamental structure of matter, providing unparalleled opportunities to study quantum chromodynamics (QCD) in detail. This report, stemming from a ten-week program at the Institute for Nuclear Theory, thoroughly articulates the scientific case for the EIC, focusing in particular on the dynamics of gluons and sea quarks at high energies.

Key Physics Goals and Experiments

1. Spin and Flavor Structure of the Proton: Understanding the spin contributions from gluons and quarks to the proton's total spin has been a focus for several decades. The EIC offers precise measurement capabilities, particularly for the polarized structure function $g_1^p(x,Q^2)$, to better determine the gluon helicity distribution $\Delta g$ down to small $x$ values. The program will significantly enhance our understanding of quark and anti-quark helicity distributions through semi-inclusive deep-inelastic scattering (SIDIS).
2. Three-Dimensional Structure of Nucleons and Nuclei: The EIC's capabilities in spatial imaging are instrumental for studying the transverse momentum and configuration space distributions of nucleons and nuclei. Advanced measurements in SIDIS will offer insights into the complex interplay between sea quarks, gluons, and their spatial distribution within nucleons.
3. QCD Matter in Nuclei: The planned electron-nucleus collisions at the EIC will extend parton saturation studies significantly beyond existing efforts, exploring high parton density regimes within nuclear environments. Such examinations will uncover details about gluon saturation thresholds and parton correlations within nuclei.
4. Electroweak Physics and Beyond the Standard Model: By comparing EIC results to known $Z$-pole measurements, studies at the EIC will potentially highlight discrepancies indicative of physics beyond the standard model. The exploration of weak mixing angles and flavor violation phenomena offers a unique extension to current electroweak analyses.

Theoretical Framework

The EIC research agenda leverages the fundamental principles of QCD, focusing on factorization theorems and perturbative calculations to bridge theoretical models with empirical data. With higher-order corrections available for many processes, the EIC's high precision demands equally sophisticated theoretical interpretations, particularly in the field of transverse momentum distributions (TMDs) and generalized parton distributions (GPDs).

Technological and Experimental Considerations

• Beam Design and Detector Layout: The EIC aims to utilize techniques such as crab crossing and ion beam cooling to achieve the requisite luminosities. Detector advancements are designed to support high-resolution measurements, particularly along the beam's transverse dimension, necessary for spatial imaging and jet detection in hadronic final states.
• Innovative Experimental Techniques: The capability to vary beam energies and include multiple species, including polarized beams, is crucial for comprehensive flavor separation studies and electroweak physics explorations.

Implications for Future AI and Research Directions

The EIC's experimental outputs will undoubtedly enhance our understanding of nuclear matter and fundamental forces, guiding the development of new computational models and refined AI techniques for data analysis in high energy physics. Furthermore, the EIC's results could potentiate significant progress in theoretical frameworks that interface traditional quantum field theory with emergent AI methodologies.

Anticipated improvements in theoretical accuracy and the ability to discern finer-scale details of partonic interactions at the EIC will likely inspire nuanced model validations and predictions, driving AI innovations in large-scale data handling and pattern recognition within complex datasets.

Conclusion

While it primarily focuses on advancing nucleon structure understanding through QCD, the EIC also presents opportunities to explore new physics realms. Hailed as an initiative of significant merit within nuclear physics, it promises to push current technological and experimental boundaries, providing groundbreaking insights into the multifaceted fabric of nucleonic matter.