An electron-hadron collider at the high-luminosity LHC

TL;DR

The paper outlines a phase-one LHeC concept that pairs a $20\ { m GeV}$ energy-recovery linac with the HL-LHC to enable concurrent $ep$ and $pp$ collisions, achieving $\sqrt{s_{ep}}\approx0.75$ TeV and a luminosity around $L\sim6\times10^{33}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$ while maintaining high energy-recovery efficiency. It analyzes the ERL-based interaction region, beam dynamics, and detector needs, arguing for a compact, cost-efficient design with minimal additional radiation and robust beam-beam performance. The authors present a physics case emphasizing improved low-$x$ parton densities, Higgs/top/EW measurements, and unique BSM searches, aided by electron polarisation and low pile-up, plus strong synergies with exclusive two-photon processes and eA/AA studies. They advocate a staged implementation to deliver valuable science during Run5, while establishing a path toward a full LHeC, supported by ALICE3 detector adaptations and a feasible seven-year construction plan. Overall, the phase-one approach offers timely, high-impact physics and a practical route to realize the broader LHeC program.

Abstract

We discuss a concept of a lower-energy version of the Large Hadron-electron Collider (LHeC), delivering electron-hadron collisions concurrently to the hadron-hadron collisions at the high-luminosity LHC at CERN. Assuming the use of a 20 GeV electron Energy Recovery Linac (ERL), we report the results on the optimised beam dynamics, accelerator technologies, and detector constraints required for such a "phase-one" LHeC. Finally, we also discuss the ERL configurations and the possibility of delivering electron-hadron collisions during the planned {Run5} of the LHC, which opens excellent research capabilities - the unique scientific potential of the proposed facility is outlined.

Figures (7)

• Figure 1: Schematics of the single-pass LHeC layout at the Interaction Point 2 (IP2); with the optional return arcs $3-6$ (dashed lines).
• Figure 2: Different ERL projects worldwide that are already operational or in their commissioning / planning phase. The red and green markers indicate the baseline LHeC design and the ERL configuration described in this paper, respectively. Figure adapted from Hutton:2022kac.
• Figure 3: The interaction region of electron and proton beams: in black the mini-beta focusing scheme of the electrons is shown, including the detector dipole field that is part of the beam separation. Both are embedded within the free space of the HL-LHC proton inner triplet lattice, marked in blue and red. Figure adapted from tizi.
• Figure 4: Local distortion of the proton optics due to the influence of the electron quadrupoles before (red) and after a local compensation of the effect (blue) for the case of a 20 GeV electron energy. Figure extracted from tizi.
• Figure 5: Effect of the space charge force on the 20 GeV electron beam, or "beam-beam force". The beam size is reduced (leading to an additional enhancement of the luminosity, pinch effect) and re-matched to the ideal conditions before entering the return arc for energy recovery. The best re-matching was obtained for non-zero value of the slope of amplitude function at the IP, $\alpha^*=-0.2$.