Electron-Ion Collider in China

Abstract: Lepton scattering is an established ideal tool for studying inner structure of small particles such as nucleons as well as nuclei. As a future high energy nuclear physics project, an Electron-ion collider in China (EicC) has been proposed. It will be constructed based on an upgraded heavy-ion accelerator, High Intensity heavy-ion Accelerator Facility (HIAF) which is currently under construction, together with a new electron ring. The proposed collider will provide highly polarized electrons (with a polarization of $\sim$80%) and protons (with a polarization of $\sim$70%) with variable center of mass energies from 15 to 20 GeV and the luminosity of (2-3) $\times$ 10$^{33}$ cm$^{-2}$ s$^{-1}$. Polarized deuterons and Helium-3, as well as unpolarized ion beams from Carbon to Uranium, will be also available at the EicC. The main foci of the EicC will be precision measurements of the structure of the nucleon in the sea quark region, including 3D tomography of nucleon; the partonic structure of nuclei and the parton interaction with the nuclear environment; the exotic states, especially those with heavy flavor quark contents. In addition, issues fundamental to understanding the origin of mass could be addressed by measurements of heavy quarkonia near-threshold production at the EicC. In order to achieve the above-mentioned physics goals, a hermetical detector system will be constructed with cutting-edge technologies. This document is the result of collective contributions and valuable inputs from experts across the globe. The EicC physics program complements the ongoing scientific programs at the Jefferson Laboratory and the future EIC project in the United States. The success of this project will also advance both nuclear and particle physics as well as accelerator and detector technology in China.

• The paper presents the EicC proposal focused on precision 3D tomography of nucleon and nuclear structure using TMDs and GPDs.
• It details an innovative accelerator design featuring a polarized electron and ion ring operating at 15–20 GeV with high luminosity and polarization.
• The study underlines potential breakthroughs in understanding partonic distributions, nuclear effects, and exotic hadrons critical to advancing QCD research.

Overview of the Electron-Ion Collider in China (EicC) Proposal

The paper outlines the proposal for the Electron-Ion Collider in China (EicC), emphasizing its potential impact on nuclear and particle physics through the study of electron-nucleon and electron-nucleus interactions. EicC is positioned to complement existing and future facilities like Jefferson Laboratory and the Electron-Ion Collider (EIC) in the USA, with a focus on specific energy regimes and polarization capabilities. The study of partonic structures, exotic hadrons, and the fundamental origins of mass and spin constitute the primary scientific objectives proposed for EicC.

Key Scientific Goals

• Nucleon Structure: EicC aims to perform precision measurements of the 3D tomography of nucleons, both in momentum space through Transverse Momentum Distributions (TMDs) and in spatial configurations through Generalized Parton Distributions (GPDs). This includes flavor decomposition of sea quarks and understanding gluon contributions across different kinematic regions.
• Intrinsic Properties of Nuclei: Discrepancies between free and bound parton distributions (nPDFs) will be addressed, which can help elucidate nuclear effects like the EMC effect, shadowing, and anti-shadowing, especially in the sea quark and gluon sectors.
• Exotic Hadrons: EicC will focus on photoproduction studies, free from triangle singularities that complicate resonance interpretations, to investigate states such as the $P_c$, $Z_c$, and potential hidden-bottom hadrons.

Accelerator Design and Parameters

The proposed collider features a polarized electron ring and ion ring, aiming for high luminosity at center-of-mass energies between 15 and 20 GeV. The innovative figure-8 design of the ion ring mitigates polarization loss via depolarization resonances, thus maintaining high polarization for protons and light ions. Key parameters include:

• Center-of-Mass Energy Range: 15-20 GeV
• Luminosity: Targeting ≥2.0 × 10$^{33}$ cm$^{-2}$ s$^{-1}$
• Polarization: ~80% for electrons and ~70% for protons

Technological Innovations

To achieve its technical goals, particularly high luminosity, EicC will employ advanced electron cooling using an energy recovery linac (ERL) to counteract beam emittance growth. This is pivotal for reaching the beam quality required for experimental precision. Additionally, integration of a full-acceptance detector supports comprehensive data acquisition across angular ranges and assists in extrapolating detailed reaction dynamics.

Potential Impact and Future Directions

The EicC is poised to make significant contributions to the understanding of nucleon and nuclear structure, providing crucial insights into the nonperturbative regime of QCD. It is also positioned uniquely to explore the dynamics of exotic hadrons and the manifestation of QCD phenomena in bound and free nuclear matter.

As an infrastructural companion, not a competitor, to the proposed EIC in the USA, EicC will help fill scientific gaps, particularly in moderate-x regions pertinent to the sea quark distributions and gluonic content. The facility aims to serve as a leading-edge platform for interdisciplinary research, potentially advancing fundamental theoretical frameworks as well as applications in accelerator technology and detector design.

Conclusion

The Electron-Ion Collider in China is strategically designed to address long-standing questions in high-energy nuclear physics and QCD, contributing to both experimental advances and theoretical physics discourse. The facility's potential to unlock new discoveries in nucleon structure, nuclear dynamics, and exotic hadron physics is substantial, augmented by its innovative technical approach and alignment with global research pipelines.