Electron Ion Collider: The Next QCD Frontier - Understanding the glue that binds us all

Abstract: This White Paper presents the science case of an Electron-Ion Collider (EIC), focused on the structure and interactions of gluon-dominated matter, with the intent to articulate it to the broader nuclear science community. It was commissioned by the managements of Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility (JLab) with the objective of presenting a summary of scientific opportunities and goals of the EIC as a follow-up to the 2007 NSAC Long Range plan. This document is a culmination of a community-wide effort in nuclear science following a series of workshops on EIC physics and, in particular, the focused ten-week program on "Gluons and quark sea at high energies" at the Institute for Nuclear Theory in Fall 2010. It contains a brief description of a few golden physics measurements along with accelerator and detector concepts required to achieve them, and it benefited from inputs from the users' communities of BNL and JLab. This White Paper offers the promise to propel the QCD science program in the U.S., established with the CEBAF accelerator at JLab and the RHIC collider at BNL, to the next QCD frontier.

• The paper pioneers a detailed mapping of gluon distributions at low Bjorken x, revealing key insights into nucleon and nuclear structure.
• The paper employs advanced polarized beam techniques and cutting-edge detectors to probe spin-orbit correlations and hadronization processes in QCD.
• The paper outlines innovative experimental designs with high luminosity to study matter under extreme conditions, setting new benchmarks for QCD research.

Electron-Ion Collider: The Next QCD Frontier

The paper "Electron Ion Collider: The Next QCD Frontier" is an extensive white paper presented to articulate the scientific opportunities and objectives of an Electron-Ion Collider (EIC). This document is a culmination of collaborative work from numerous researchers and laboratories, designed to explore and expand the understanding of Quantum Chromodynamics (QCD), particularly in the context of gluons and their interactions within protons and nuclei.

Overview and Scientific Objectives

The EIC is envisaged as a next-generation facility that aims to significantly advance QCD studies by focusing on the gluon-dominated aspects of matter. Gluons, unlike photons in electromagnetism, carry color charge and interact with each other, introducing complexities that exist beyond quarks’ behavior. This paper argues that understanding gluonic interactions is not only crucial for a full comprehension of nucleon and nuclear structure but is also pivotal for elucidating phenomena such as the majority of visible mass in the universe and the dynamics of hadronic matter under extreme conditions.

Key Science Goals:

1. Mapping the Gluon Distribution: A primary goal is to chart how gluons are distributed inside nucleons and nuclei, both spatially and in momentum. This involves probing the gluon saturation phenomena expected at low momentum fractions (Bjorken $x$) and high densities.
2. Exploring Orbital Angular Momentum: The EIC will investigate the role of gluons in the nucleon's spin composition. Understanding spin-orbit correlations in nucleons will provide insights into how angular momentum is distributed and shared among quarks and gluons.
3. Investigating QCD Matter Under Extreme Conditions: By colliding electrons with various ions, the EIC aims to recreate conditions that test the behavior of gluons in high-density environments, thus improving our understanding of the early universe conditions just after the Big Bang.
4. Studying Nucleus and Collective Effects: The EIC will provide unprecedented insight into the complex interactions within nuclei. This includes studying phenomena such as nuclear shadowing and saturation effects, which have implications for the non-linear behavior of gluons.
5. Hadronization and Fragmentation Processes: Understanding how quarks and gluons transition into hadrons is another key goal, as this process is fundamental to the formation of recognizable matter from elementary particles.

Experimental Design and Technical Considerations

The EIC’s design capitalizes on the capabilities of existing accelerators, planning to add significant new features. Two primary designs have been proposed: eRHIC at Brookhaven National Laboratory and MEIC at Jefferson Laboratory, each utilizing current facilities but with major upgrades to accommodate the high energy and luminosity requirements.

Technical Highlights:

• Luminosity and Polarization: A critical feature of the EIC is its exceptional luminosity and the ability to use polarized beams. These capabilities are necessary to resolve the complex QCD phenomena and will distinguish the EIC from all existing and planned facilities globally.
• Versatile Experimental Setup: With a broad spectrum of ion species from protons to heavy nuclei and variable center-of-mass energies, the EIC is designed to be adaptable for a wide range of experimental conditions.
• Cutting-edge Detectors: Advanced detector technologies will be integrated to capture a comprehensive range of scattering events and ensure precision in measurements that were previously unattainable.

Implications and Future Outlook

The EIC promises substantial advances in QCD by providing a definitive probe into gluon dynamics and their contributions to the nucleonic and nuclear structures. Understanding gluon interactions at a fundamental level can potentially unravel key aspects of Standard Model physics and possibly reveal new physics beyond it.

Moreover, the success in studying dense gluon field environments may provide insights into nucleosynthesis processes and the conditions preceding the condensation of hadrons in the early universe. As the EIC pushes forward the frontier in QCD research, it is expected to establish the United States as a leader in nuclear physics, potentially influencing related fields such as astrophysics and cosmology.

In conclusion, the EIC represents a critical step in understanding the glue that binds us all—gluons and their profound impact on the structure and dynamics of matter at the most fundamental level. With its completion and operation, the scientific community anticipates unprecedented exploration into the strong force, complementing our knowledge of the fundamental forces shaping the universe.