A Large Hadron Electron Collider at CERN: Report on the Physics and Design Concepts for Machine and Detector

Abstract: The physics programme and the design are described of a new collider for particle and nuclear physics, the Large Hadron Electron Collider (LHeC), in which a newly built electron beam of 60 GeV, up to possibly 140 GeV, energy collides with the intense hadron beams of the LHC. Compared to HERA, the kinematic range covered is extended by a factor of twenty in the negative four-momentum squared, $Q^2$, and in the inverse Bjorken $x$, while with the design luminosity of $10^{33}$ cm$^{-2}$s$^{-1}$ the LHeC is projected to exceed the integrated HERA luminosity by two orders of magnitude. The physics programme is devoted to an exploration of the energy frontier, complementing the LHC and its discovery potential for physics beyond the Standard Model with high precision deep inelastic scattering measurements. These are designed to investigate a variety of fundamental questions in strong and electroweak interactions. The physics programme also includes electron-deuteron and electron-ion scattering in a $(Q^2, 1/x)$ range extended by four orders of magnitude as compared to previous lepton-nucleus DIS experiments for novel investigations of neutron's and nuclear structure, the initial conditions of Quark-Gluon Plasma formation and further quantum chromodynamic phenomena. The LHeC may be realised either as a ring-ring or as a linac-ring collider. Optics and beam dynamics studies are presented for both versions, along with technical design considerations on the interaction region, magnets and further components, together with a design study for a high acceptance detector. Civil engineering and installation studies are presented for the accelerator and the detector. The LHeC can be built within a decade and thus be operated while the LHC runs in its high-luminosity phase. It thus represents a major opportunity for progress in particle physics exploiting the investment made in the LHC.

• The paper introduces LHeC's innovative design for electron-proton collisions that extends DIS kinematics by up to twentyfold compared to HERA.
• It details comprehensive evaluations of both ring-ring and linac-ring configurations along with key infrastructure adaptations for CERN integration.
• The study projects significant advances in QCD precision and parton distribution measurements, paving the way for discoveries beyond the Standard Model.

A Large Hadron Electron Collider at CERN: Physics and Design Perspectives

The concept of the Large Hadron Electron Collider (LHeC) at CERN offers a novel exploration frontier in particle physics, promising to generate significant insights beyond those of the Large Hadron Collider (LHC) alone. The LHeC would achieve this by colliding a freshly constructed electron beam at energies ranging from 60 GeV to potentially 140 GeV with the high-intensity hadron beams of the LHC. Compared to the initial electron-proton collider, HERA, the LHeC extends the kinematic coverage significantly, providing a twenty-fold increase in negative four-momentum squared, $Q^2$, and inverse Bjorken $x$. Moreover, with a design luminosity of $10^{33} \, \text{cm}^{-2}\text{s}^{-1}$, the LHeC is anticipated to surpass the integrated luminosity of HERA by two orders of magnitude, thus promising a profound impact on precision measurements and discovery potential in both particle and nuclear physics.

Theoretical and Practical Implications

The LHeC's physics program is positioned to explore the energy frontier, complementing the LHC in its potential to uncover new physics beyond the Standard Model through high precision deep inelastic scattering (DIS). This will enable investigation of fundamental questions in strong and electroweak interactions. A particular focus will involve probing electron-deuteron and electron-ion scattering across a $(Q^2, 1/x)$ range extended by four orders of magnitude beyond previous lepton-nucleus DIS experiments. This expansion promises to drive novel investigations into neutron and nuclear structure, quark-gluon plasma initial conditions, and other quantum chromodynamic phenomena.

On a practical level, the LHeC design considers both ring-ring and linac-ring configurations for the collider, each offering unique advantages in terms of operational feasibility and scientific output. The ring-ring configuration benefits from established technology, while the linac-ring option carves out significant potential via energy recovery and enhanced polarisation capabilities for the electron beams.

Highlights and Challenges

The paper delineates a series of detailed optics and beam dynamics studies for both configurations, alongside technical considerations for the interaction region and infrastructure components like magnets, cryogenics, and radio-frequency systems. The extensive engagement with civil engineering and detector installation highlights the feasibility of integrating the LHeC into the existing CERN infrastructure, enabling contemporaneous operation with the LHC during its high-luminosity phase.

The LHeC offers a fertile ground for advancing the understanding of Quantum Chromodynamics (QCD) as it extends DIS studies into uncharted kinematic regions. The collider aims to extend the determination of parton distribution functions with unprecedented precision, including the accurate measurement of gluon and heavy-flavour distribution across a wide kinematic range. Additionally, the potential to probe the strong coupling constant $\alpha_s$ with remarkable precision addresses longstanding theoretical uncertainties, cementing the LHeC's role as a pivotal project for next-generation particle physics research.

Future Prospects

The research outlined in the LHeC design study signals a transformative step forward in both theoretical and experimental particle physics. The collider’s capacity to integrate with future upgrades to the LHC, possibly extending proton beam energies beyond 7 TeV per beam, establishes a long-term vision facilitating cutting-edge research into the deepest questions of matter and forces.

In conclusion, while the vision of the LHeC remains complex and ambitious, it simultaneously embodies a significant opportunity to advance theoretical physics, guided by empirical data gathering that was previously unattainable. The project encapsulates the spirit of international collaboration and technological innovation, setting a precedent for the exploration of the next layers of the high-energy frontier.