The Large Hadron-Electron Collider at the HL-LHC

Abstract: The Large Hadron electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High Luminosity--Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron-proton and proton-proton operation. This report represents an update of the Conceptual Design Report (CDR) of the LHeC, published in 2012. It comprises new results on parton structure of the proton and heavier nuclei, QCD dynamics, electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics in extending the accessible kinematic range in lepton-nucleus scattering by several orders of magnitude. Due to enhanced luminosity, large energy and the cleanliness of the hadronic final states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, the report represents a detailed updated design of the energy recovery electron linac (ERL) including new lattice, magnet, superconducting radio frequency technology and further components. Challenges of energy recovery are described and the lower energy, high current, 3-turn ERL facility, PERLE at Orsay, is presented which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution and calibration goals which arise from the Higgs and parton density function physics programmes. The paper also presents novel results on the Future Circular Collider in electron-hadron mode, FCC-eh, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies.

• The paper introduces an updated LHeC design employing a multi-turn energy recovery linac to achieve high-luminosity electron–proton collisions.
• It details technological advancements in superconducting radio frequency systems and energy recovery that enable concurrent electron-proton and proton-proton operations.
• The study emphasizes precision deep inelastic scattering to refine parton distribution measurements and explore beyond-standard-model physics.

Overview of The Large Hadron-Electron Collider at the HL-LHC

The paper "The Large Hadron-Electron Collider at the HL-LHC," sponsored by the LHeC and FCC-he Study Group, presents an updated conceptual design for the Large Hadron-Electron Collider (LHeC) positioned at the High Luminosity Large Hadron Collider (HL-LHC). This extensive documentation builds upon the initial design from 2012, incorporating advancements in accelerator technology and physics expectations that have evolved significantly over the last decade.

Technological Advancements and Design

The LHeC is conceptualized as a multi-turn energy recovery linac (ERL) capable of colliding an intense electron beam with proton or ion beams derived from the HL-LHC. The report outlines a detailed design incorporating modern technological advancements in superconducting radio frequency technology and explores the intricacies of energy recovery which is pivotal for sustainability in contemporary high-energy physics endeavors.

The report addresses the integration of an energy-efficient ERL configuration, allowing the LHeC to function with high instantaneous luminosity and minimal power consumption. Notably, the design accommodates concurrent electron-proton and proton-proton operations, posing no interference with ongoing LHC experiments.

Physics Potential and Experimental Goals

Primarily, the LHeC strives to extend the exploration of deep inelastic scattering (DIS) beyond previous limits, achieving an unprecedented precision in understanding the parton distribution functions (PDFs) of protons and nuclei. This effort is enhanced by the capability to analyze electroweak and QCD dynamics and investigate the strange quark density with unprecedented accuracy.

The LHeC's physics program ambitiously seeks to explore Higgs boson properties, test electroweak sector predictions, and open new doors to beyond-the-standard-model physics. With several chapters dedicated to the physics potential, the paper outlines experimental procedures and potential discoveries in regions of kinematic space previously inaccessible, particularly emphasizing the unique strength of a high-resolution probe into the internal structure and dynamics of nuclear matter.

Implications on Future Collider Physics

By significantly enhancing the kinematic reach and luminosity, the LHeC is positioned to revolutionize our understanding of nuclear particle physics. It provides a groundwork for future endeavors, such as the Future Circular Collider (FCC), promising a range of new opportunities by enabling unprecedented energy scales in lepton-nucleus collisions. This implies leveraging the advantages of high energy and luminosity to thoroughly investigate non-linear QCD dynamics and parton saturation phenomena at small x.

Longer-term and Strategic Implications

The implications of the LHeC extend beyond immediate physics outputs. The report underscores the strategic importance of this collider as a precursor to the envisioned FCC complex. Its development paves the way for high-energy collision studies necessary for plotting the future course of particle physics. With its energy-efficient operation model, the LHeC aligns with the emerging paradigm of ecological sustainability in scientific experimentation.

Conclusion

The LHeC project represents a prospective leap forward in our quest to understand fundamental constitutions of matter. It harnesses modern accelerator technologies to propel high-energy physics into new realms of precision and discovery. The meticulous discussion in this document outlines not only the technical feasibility of hosting the LHeC at CERN with current resources but also its strategic fit within the broader objectives of global particle physics research.