Strong interaction physics at the luminosity frontier with 22 GeV electrons at Jefferson Lab

Abstract

The purpose of this document is to outline the developing scientific case for pursuing an energy upgrade to 22 GeV of the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (TJNAF, or JLab). This document was developed with input from a series of workshops held in the period between March 2022 and April 2023 that were organized by the JLab user community and staff with guidance from JLab management (see Sect. 10). The scientific case for the 22 GeV energy upgrade leverages existing or already planned Hall equipment and world-wide uniqueness of CEBAF high-luminosity operations. CEBAF delivers the world’s highest intensity and highest precision multi-GeV electron beams and has been do so for more than 25 years. In Fall 2017, with the completion of the 12 GeV upgrade and the start of the 12 GeV science program, a new era at the Laboratory began. The 12 GeV era is now well underway, with many important experimental results already published, and an exciting portfolio Program Advisory Committee approved experiments planned for at least the next 8–10 years [1]. At the same time, the CEBAF community is looking toward its future and the science that could be obtained through a future cost-effective upgrade to 22 GeV. The great potential to upgrade CEBAF to higher energies opens a rich and unique experimental nuclear physics program that combines illustrious history with an exciting future, extending the life of the facility well into the 2030s and beyond. JLab at 22 GeV will provide unique, world-leading science with high-precision, high-luminosity experiments elucidating the properties of quantum chromodynamics (QCD) in the valence regime (x ≥ 0.1). JLab at 22 GeV also enables researchers to probe the transition to a region of sea dominance, with access to hadrons of larger mass and different structures. With a fixed-target program at the “luminosity frontier”, large acceptance detection systems, as well as high-precision spectrometers, CEBAF will continue to offer unique opportunities to shed light on the nature of QCD and the emergence of hadron structure for decades to come. In fact, CEBAF today, and with an energy upgrade, will continue to operate with several orders of magnitude higher luminosity than what is planned at the Electron-Ion Collider (EIC). CEBAF’s current and envisioned capabilities enable exciting scientific opportunities that complement the EIC operational reach, thus giving scientists the full suite of tools necessary to comprehensively understand how QCD builds hadronic matter. The physics program laid out in this document spans a broad range of exciting initiatives that focus on a common theme, namely, investigations that explore different facets of the nonperturbative dynamics that manifest in hadron structure and probe the richness of these strongly interacting systems. The central themes of this program are reviewed in Sect. 2 - Introduction. The main components of the research program are highlighted in Sects. 3 through 8, followed by Sect. 9, which provides a brief overview of the 22 GeV CEBAF energy-doubling concept. These sections outline the key measurements in different areas of experimental studies possible at a 22 GeV CEBAF accelerator in the existing JLab experimental end stations. They provide details on the key physics outcomes and unique aspects of the programs not possible at other existing or planned facilities. The 22 GeV physics program is being developed following three main principles: (a) identify the flagship measurements that can be done only with 22 GeV and their science impacts (Uniqueness); (b) identify the flagship measurements with 22 GeV that can extend and improve the 12 GeV measurements, helping the physics interpretation through multidimensional bins in extended kinematics (Enrichment); (c) identify the measurements with 22 GeV that can set the bridge between JLab12 and EIC (Complementarity). Even if a sharp separation among these three categories sometimes is difficult to maintain, we highlight the main points in the following.