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arxiv: 2503.20475 · v4 · submitted 2025-03-26 · ✦ hep-ex · physics.acc-ph

An electron-hadron collider at the high-luminosity LHC

Kevin David J Andr\'e , Laurent Forthomme , Bernhard Holzer , Krzysztof Piotrzkowski This is my paper

Pith reviewed 2026-05-22 22:44 UTC · model grok-4.3

classification ✦ hep-ex physics.acc-ph
keywords electron-hadron colliderLHeCHL-LHCenergy recovery linacRun5beam dynamicsconcurrent operationsdetector constraints
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0 comments X

The pith

A 20 GeV electron energy recovery linac can deliver electron-hadron collisions concurrently with the high-luminosity LHC during Run5.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents a concept for a lower-energy LHeC that uses a 20 GeV electron beam from an energy recovery linac to collide with the LHC proton beams. It reports optimizations of beam dynamics, accelerator technologies, and detector constraints needed to make this possible without stopping the main hadron-hadron program. The setup is framed as a phase-one facility that could operate during the planned Run5 period. A sympathetic reader would see this as a way to add electron-proton scattering capability to the existing LHC infrastructure with limited new construction. The work focuses on showing technical feasibility through concrete beam and detector studies.

Core claim

The paper claims that a 20 GeV ERL-based electron-hadron collider can be integrated with the HL-LHC for concurrent operations during Run5, with optimised beam dynamics, accelerator technologies, and detector constraints, thereby opening excellent research capabilities through the unique scientific potential of the proposed facility.

What carries the argument

The 20 GeV electron Energy Recovery Linac (ERL) and its beam optics integration with the HL-LHC ring for simultaneous electron-proton and proton-proton collisions.

If this is right

  • Electron-hadron collisions become available during the LHC Run5 period without halting the main hadron program.
  • Detector and beam constraints are shown to be manageable with current or near-term technologies.
  • The phase-one configuration provides a path to study electron-proton interactions at LHC energies.
  • ERL configurations are identified that fit within the planned LHC schedule and infrastructure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Such a facility could enable precision measurements of parton distributions that complement existing LHC data.
  • If the ERL integration succeeds, it would reduce the need for a fully separate electron ring in early stages.
  • The approach might inform similar concurrent lepton-hadron additions at future hadron colliders.

Load-bearing premise

Existing or near-term ERL technology and beam optics can reach the needed luminosity and stability without unacceptable interference or downtime when running at the same time as the HL-LHC.

What would settle it

A beam dynamics simulation or prototype test that shows the required luminosity cannot be maintained or that concurrent operation forces unacceptable downtime or instability in the HL-LHC proton beams.

Figures

Figures reproduced from arXiv: 2503.20475 by Bernhard Holzer, Kevin David J Andr\'e, Krzysztof Piotrzkowski, Laurent Forthomme.

Figure 1
Figure 1. Figure 1: Schematics of the single-pass LHeC layout at the Interaction Point 2 (IP2); with the optional return arcs 3 − 6 (dashed lines). 2 The LHeC IR The fundamental principle of an ERL provides an ideal condition for a staged particle accelerator. Still, a number of items have to be followed up, as beam parameters scale with energy, so while keeping the general layout of the ERL and the geometry of linacs and ret… view at source ↗
Figure 2
Figure 2. 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 [5]. scheme explained here, head-on collisions between the proton and electron beam are foreseen, to avoid luminosity reduction because of the geometric loss … view at source ↗
Figure 3
Figure 3. 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 [10]. 0 5000 10000 15000 20000 25000 Orbit S (m) 4 3 2 1 0 1 2 3 4 (%) ATLAS ALICE/LHeC C… view at source ↗
Figure 4
Figure 4. 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 [10]. the beam-beam interaction as well as the non-linear effect has been studied and long-range forces are not present: due to the fast beam separation scheme, the beam separation between elec… view at source ↗
Figure 5
Figure 5. 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, α ∗ = −0.2. this fact represents the pr… view at source ↗
Figure 6
Figure 6. Figure 6: Visualisation of the simulated beam-beam effect in phase space: in green the ellipse of the ideal configuration is shown. Due to the beam-beam effect, tails in the particle distribution are developing (blue spots) that have to be kept under control for full energy recovery performance. Left: 50 GeV case, right: 20 GeV. The lower energy of the 20 GeV electrons makes them more susceptible to the beam-beam fo… view at source ↗
Figure 7
Figure 7. Figure 7: Differential cross-sections in rapidity for the Higgs boson (left) and a single top quark (right), both produced through charged current, assuming the phase-one electron beam energy of 20 GeV, with and without electron polarisation, as well as for the final electron beam energy of 50 GeV. Higgs boson case remains compelling at 0.75 TeV (especially for the charged current events), as well as the single-top-… view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes a 'phase-one' LHeC concept using a 20 GeV ERL to deliver electron-hadron collisions concurrently with HL-LHC proton-proton operations during Run5. It reports on optimization studies for beam dynamics, accelerator technologies, and detector constraints, discusses ERL configurations, and outlines the scientific potential of such a facility.

Significance. If the integration and stability claims hold, the proposal would enable concurrent e-p physics at the HL-LHC without dedicated downtime, providing unique low-energy electron-hadron data alongside the pp program. The conceptual nature limits immediate impact, but successful validation could strengthen the case for extending LHC capabilities.

major comments (2)
  1. [Optimization studies and ERL configuration discussion] The central feasibility claim for concurrent Run5 operations rests on optimized beam dynamics and stability, yet the manuscript provides no quantitative luminosity values, beam-beam interaction margins, or full tracking simulation results (including RF compatibility and timing synchronization) to demonstrate that interference remains acceptable. This directly affects the load-bearing assumption that near-term ERL technology suffices.
  2. [Detector constraints section] Detector constraints and integration with HL-LHC infrastructure are discussed at a conceptual level, but no specific performance metrics (e.g., background rates, acceptance losses, or required modifications to existing detectors) are quantified to support the claim of minimal impact on concurrent pp running.
minor comments (2)
  1. [Abstract] The abstract refers to 'optimised' results without previewing any numerical outcomes or error estimates, which reduces clarity for readers expecting a feasibility assessment.
  2. [Accelerator technologies] Notation for beam parameters (e.g., energy, current, or emittance) should be defined consistently on first use to aid readability in the accelerator technology discussion.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback on our manuscript describing a phase-one LHeC concept with a 20 GeV ERL for concurrent operation during HL-LHC Run5. The comments correctly identify areas where the conceptual nature of the study limits quantitative support for the feasibility claims. We respond to each major comment below and indicate the revisions planned for the next version of the manuscript.

read point-by-point responses

  1. Referee: [Optimization studies and ERL configuration discussion] The central feasibility claim for concurrent Run5 operations rests on optimized beam dynamics and stability, yet the manuscript provides no quantitative luminosity values, beam-beam interaction margins, or full tracking simulation results (including RF compatibility and timing synchronization) to demonstrate that interference remains acceptable. This directly affects the load-bearing assumption that near-term ERL technology suffices.

    Authors: We agree that the manuscript is a conceptual study and does not contain the requested quantitative luminosity values, beam-beam margins, or full tracking simulations. The reported beam-dynamics optimizations were performed at a parameter-identification level rather than a complete performance evaluation. In the revised manuscript we will insert preliminary luminosity estimates derived from the optimized parameters, together with a qualitative discussion of beam-beam effects and RF/timing considerations, while explicitly noting that detailed tracking simulations lie beyond the present scope and are the subject of planned follow-up work. This revision will better substantiate the feasibility discussion without overstating the current results. revision: partial

  2. Referee: [Detector constraints section] Detector constraints and integration with HL-LHC infrastructure are discussed at a conceptual level, but no specific performance metrics (e.g., background rates, acceptance losses, or required modifications to existing detectors) are quantified to support the claim of minimal impact on concurrent pp running.

    Authors: The detector section is indeed presented at a conceptual level, as the primary emphasis of the paper is on accelerator aspects. Specific metrics such as background rates or acceptance losses would require dedicated detector simulations that were not carried out here. We will revise the manuscript to include order-of-magnitude estimates drawn from earlier LHeC detector studies, together with a statement that the design assumes only minimal modifications to the existing ATLAS or CMS detectors. This addition will provide clearer support for the claim of limited interference with pp running. revision: partial

Circularity Check

0 steps flagged

No circularity: conceptual feasibility study without derivations or fitted predictions

full rationale

The paper is a forward-looking feasibility discussion of an ERL-based electron-hadron collider concept. It reports on optimised beam dynamics, accelerator technologies, and detector constraints for concurrent HL-LHC operation but contains no mathematical derivations, parameter fits, predictions, or uniqueness theorems. No load-bearing steps reduce to self-citations, self-definitions, or inputs by construction. The analysis is self-contained as a conceptual proposal with no evident circular reasoning.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical derivations, fitted parameters, or new physical entities are described in the abstract; the work rests on standard accelerator-physics assumptions not enumerated here.

pith-pipeline@v0.9.0 · 5650 in / 994 out tokens · 33271 ms · 2026-05-22T22:44:17.564618+00:00 · methodology

discussion (0)

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Reference graph

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