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 over the past decades 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. It has been benefited profoundly from inputs by the users' communities of BNL and JLab. This White Paper offers the promise to propel the QCD science program in the US, established with the CEBAF accelerator at JLab and the RHIC collider at BNL, to the next QCD frontier. Preamble Editors' note for the second editionThe first edition of this White Paper was released in 2012. In the current (second) edition, the science case for the EIC is further sharpened in view of the recent data from BNL, CERN and JLab experiments and the lessons learnt from them. Additional improvements were made by taking into account suggestions from the larger nuclear physics community including those made at the EIC Users Group meeting at Stony Brook University in July 2014, and the QCD Town Meeting at Temple University in September 2014.Abhay Deshpande, Zein-Eddine Meziani and Jian-Wei Qiu November 2014 Editors' note for the third edition Since the 2nd release of this White Paper, the NSAC's Long Range Plan (2015) was successfully completed. The EIC is a major recommendation of the US nuclear science community. In the current release (version 3) we have fixed some minor remaining errors in the text, and have added a few new references. While the core science case for the EIC remains the same, the machine designs of both options, the eRHIC at BNL and the JLEIC at JLab keep evolving. In this 3rd release of the EIC White Paper instead of making substantial changes to the machine design sections (5.1 and 5.2), we give references to the most recent machine design documents.

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 over the past decades 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. It has been benefited profoundly from inputs by 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. Editors' Note for the Second EditionThe first edition of this White Paper was released in 2012. In the current (second) edition, the science case for the EIC is further sharpened in view of the recent data from BNL, CERN and JLab experiments and the lessons learnt from them. Additional improvements were made by taking into account suggestions from the larger nuclear physics community including those made at the EIC Users Group meeting at Stony Brook University in July 2014, and the QCD Town Meeting at Temple University in September 2014.

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 ∼80%) and protons (with a polarization of ∼70%) with variable center of mass energies from 15 to 20 GeV and the luminosity of (2–3) × 1033 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.

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