An electron-hadron collider at the high-luminosity LHC

This paper proposes a "phase-one" electron-hadron collider concept utilizing a 20 GeV Energy Recovery Linac to deliver concurrent collisions with the high-luminosity LHC during Run 5, detailing the necessary beam dynamics, accelerator technologies, and detector constraints to unlock unique scientific potential.

The Gist

Imagine the Large Hadron Collider (LHC) at CERN as a massive, 27-km circular racetrack where two streams of protons (heavy particles) zoom around in opposite directions, smashing into each other to reveal the secrets of the universe. This is the "High-Luminosity" version, meaning the crashes are incredibly frequent and intense.

This paper proposes adding a new, smaller racetrack right next to the main one, specifically for a different kind of experiment: crashing electrons (tiny, light particles) into the existing proton stream.

Here is the breakdown of their proposal, explained simply:

1. The "Phase-One" Shortcut

The original plan for this electron-proton collider (called LHeC) was to build a massive, high-tech machine that would take a long time and a lot of money to finish, likely waiting until after the current LHC program ends.

The authors propose a "Phase-One" shortcut. Instead of waiting, they suggest building a smaller, simpler version right now to run alongside the main LHC during its next major operating period (Run 5).

• The Analogy: Think of the main LHC as a Formula 1 race. The original plan was to build a brand-new, massive stadium for a different type of racing next door, but it would take a decade to build. This new proposal is like setting up a high-speed go-kart track right next to the F1 circuit. It's smaller, cheaper, and can start racing immediately while the F1 cars are still on the track.

2. How It Works: The "Energy Recovery" Elevator

The heart of this machine is a special accelerator called an Energy Recovery Linac (ERL).

• The Analogy: Imagine an elevator that carries a heavy box up to the top floor (accelerating the electron to 20 GeV). Instead of letting the elevator car drop down and waste energy, you use the weight of the car coming down to help power the elevator going up for the next passenger.
• In this machine, the electron beam is shot up to speed, smashes into the proton beam, and then is guided back through the same machine. As it goes back, it gives its remaining energy back to the machine (like the elevator dropping), which is then used to boost the next batch of electrons. This makes the process incredibly efficient and saves massive amounts of electricity.

3. Why 20 GeV? (The "Lite" Version)

The full version of this machine aims for 50 GeV (gigaelectronvolts) of energy. This proposal suggests starting with 20 GeV.

• Why? It's like choosing a "Lite" version of a video game. It's easier to build, costs much less (saving about 70 million Swiss Francs in materials alone), and can be ready much sooner.
• Even though it's "lower energy," it is still powerful enough to see things the current LHC cannot. It opens a window into a different part of the physics world that hasn't been explored since the HERA collider shut down years ago.

4. The "Traffic Control" Problem

One of the hardest parts of this project is keeping the electron beam and the proton beam from crashing into each other before they are supposed to. They need to travel side-by-side, then meet at one specific spot (the Interaction Point), and then separate again immediately.

• The Solution: The paper describes using a clever mix of magnets (like invisible hands) to gently push the electron beam away from the proton beam right after they collide. Because the electrons are so much lighter than protons, they bend easily. The design ensures they separate cleanly so they don't cause a traffic jam (which would ruin the experiment).

5. The Scientific "Treasure Hunt"

What will this machine actually find?

• The "X-Ray" of Matter: By smashing electrons into protons, scientists can take incredibly detailed "X-rays" of the inside of the proton. This helps them understand how the tiny building blocks (quarks and gluons) are arranged inside.
• The Higgs and Top Quark: Even at this lower energy, the machine is sensitive enough to study the Higgs boson and the top quark in a unique way that the main LHC cannot. It's like looking at a familiar object from a completely new angle.
• Nuclear Physics: It can also smash electrons into heavy atomic nuclei (like lead) to see how the rules of physics change when particles are crowded together inside a nucleus.

6. The Detector: A Shared Home

Usually, building a new collider means building a brand-new, massive detector (the "camera" that records the crashes).

• The Smart Move: The authors propose using the ALICE 3 detector, which is already being planned for the main LHC. They suggest adding a few extra parts (like a specific type of energy meter) to this existing design.
• The Benefit: This saves huge amounts of money and time. It's like buying a new camera lens for a camera you already own, rather than buying a whole new camera.

Summary

The paper argues that we don't need to wait for the "perfect" version of this electron-proton collider. By building a smaller, smarter, "Phase-One" version now, we can:

1. Save money and time.
2. Start doing unique science 10 years earlier than planned.
3. Use the experience gained to build the perfect, full-sized version later.

It's a "start small, learn fast, and get results now" strategy for exploring the deepest secrets of the universe.

Technical Summary

Technical Summary: An Electron-Hadron Collider at the High-Luminosity LHC

Problem and Motivation
The Large Hadron-electron Collider (LHeC) is proposed as a major upgrade to the High-Luminosity Large Hadron Collider (HL-LHC) at CERN, designed to collide an intense electron beam with the proton or ion beams of the HL-LHC. The original Conceptual Design Report (CDR) envisioned a 50 GeV electron beam delivered by a three-pass Energy Recovery Linac (ERL), with commissioning planned several years after the completion of the HL-LHC program. This paper addresses the need for an earlier, more cost-effective realization of electron-hadron physics. The authors propose a "phase-one" LHeC utilizing a lower-energy (20 GeV), single-pass ERL. This approach aims to deliver concurrent electron-hadron collisions during the planned Run 5 of the HL-LHC (starting ~2036), significantly reducing financial and personnel strain while enabling scientific output nearly a decade earlier than the final configuration.

Methodology
The study evaluates the feasibility of a 20 GeV single-pass ERL with a beam current of 60 mA, operating concurrently with the HL-LHC at Interaction Point 2 (IP2). The methodology involves:

• Accelerator Physics Optimization: Re-optimizing beam dynamics for the reduced energy, including the analysis of proton optics distortion, beam-beam effects (tune shifts and emittance growth), and transverse emittance matching between the electron and proton beams.
• Lattice and Interface Design: Adapting the existing LHeC layout by removing return arcs (3–6), beam spreaders, and re-combiners required for the 50 GeV multi-pass design. The study utilizes the existing HL-LHC proton lattice, modifying only the first proton quadrupole (Q1) to accommodate the electron beam separation.
• Separation Scheme Analysis: Modeling the separation of counter-rotating electron and proton beams using a combination of a weak detector dipole field and off-centered electron mini-beta quadrupoles to minimize synchrotron radiation and avoid parasitic collisions.
• Detector Integration: Assessing the compatibility of the proposed interaction region with the planned ALICE 3 detector, identifying necessary modifications (specifically the addition of a hadronic calorimeter) to support deep-inelastic scattering (DIS) and heavy particle reconstruction.
• Physics Reach Simulation: Calculating generator-level cross-sections for Higgs, top quark, and electroweak processes using MadGraph5 aMC@NLO, and estimating luminosity reach based on HL-LHC Run 5 parameters.

Key Contributions and Results

• Beam Dynamics and Stability: The study demonstrates that reducing the electron energy to 20 GeV significantly mitigates beam-beam effects on the proton beam. The resulting tune shift ($\Delta Q_{x,y} \approx 3.8 \times 10^{-4}$) is negligible compared to HL-LHC proton-proton interactions. While the lower energy makes electrons more susceptible to the proton's space-charge "pinch effect," the beam optics can be re-matched to maintain a high energy recovery efficiency of approximately 98%.
• Synchrotron Radiation and Cost: The 20 GeV design reduces the critical energy of synchrotron radiation to 15–20 keV and the total radiated power in the interaction region to well below 1 kW. This simplifies the interaction region design by eliminating the need for sophisticated protection masks. The removal of return arcs and associated hardware results in a material cost reduction of at least 70 MCHF and significantly lowers operational costs.
• Luminosity Performance: Assuming a 60 mA electron current and nominal HL-LHC proton parameters ($N_p = 2.2 \times 10^{11}$), the phase-one configuration achieves a luminosity of $6 \times 10^{33} \text{ cm}^{-2}\text{s}^{-1}$. This is sufficient to collect an integrated luminosity of 50 fb$^{-1}$ in a short timeframe, enabling precise parton density function (PDF) constraints.
• Physics Reach: At a center-of-mass energy of $\sqrt{s_{ep}} = 0.75$ TeV, the facility extends the kinematic reach beyond HERA. The paper reports significant cross-sections for Higgs boson production (via charged current) and single-top production, particularly with longitudinally polarized electron beams ($P_e = -0.8$). The facility is projected to measure Higgs couplings to W bosons ($\delta\kappa_W \approx 2.5\%$) and anomalous Wtb couplings with precision comparable to HL-LHC proton-proton measurements.
• Detector Synergy: The proposed configuration is highly compatible with the ALICE 3 detector design. The primary requirement is the addition of a hadronic calorimeter (HCAL) and extensions to the forward electromagnetic calorimeter. The "trigger-less" data streaming capability of ALICE 3 is identified as a critical asset for the LHeC.

Significance and Claims
The paper argues that a phased realization of the LHeC offers the optimal path forward. By deploying a 20 GeV single-pass ERL, the project can:

1. Accelerate Scientific Discovery: Provide unique electron-hadron collision data during Run 5, offering timely feedback to HL-LHC hadron-hadron experiments to improve precision measurements and enabling the study of new QCD phenomena at low $x$.
2. Optimize Future Design: The operational experience gained from the phase-one machine will inform the final design of the 50 GeV LHeC, particularly regarding RF-phase control and orbit stabilization systems.
3. Enable Unique Physics: The facility offers unique capabilities for studying electroweak physics, top quark properties, and Beyond the Standard Model (BSM) signatures (e.g., displaced vertices) with negligible pile-up. It also allows for comparative studies of electron-nucleus ($eA$) and nucleus-nucleus ($AA$) interactions using the same detector.
4. Complement Existing Projects: The paper notes that the LHeC's higher center-of-mass energy and wider $(x, Q^2)$ coverage complement the Electron-Ion Collider (EIC) at Brookhaven, which focuses on lower energies and spin physics.

The authors conclude that the technical feasibility of the single-pass design is supported by existing ERL test facilities and that the proposed "phase-one" LHeC represents a scientifically robust and financially prudent step toward the full realization of electron-hadron collider physics at CERN.