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FCC-μe: High Energy, High Luminosity Muon-Electron Collider & Neutrino Factory at CERN

This paper proposes the FCC-μe concept, a high-energy, high-luminosity muon-electron collider and neutrino factory to be integrated with the FCC-ee facility at CERN, detailing its scientific motivations, technical layout from ultra-cold anti-muon sources to the main ring, and its potential for advancing Higgs physics and neutrino science.

Original authors: K. Piotrzkowski

Published 2026-10-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: K. Piotrzkowski

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the subatomic world, particles called muons are often described as heavy cousins of the electron. They are unstable, decaying in a fraction of a second, which makes them incredibly difficult to study or use as tools for discovery. For decades, physicists have dreamed of smashing muons together with electrons at energies high enough to recreate the conditions of the early universe, hoping to uncover the hidden rules that govern matter. The challenge has always been keeping these fleeting particles alive long enough to accelerate them and guide them into a collision. Now, a new proposal suggests a way to turn this dream into reality by adapting existing plans for a massive particle collider in Europe, offering a fresh path to answer some of the most profound questions in physics.

The paper outlines a concept called FCC-µe, a facility designed to collide beams of muons and electrons at the same time a standard electron-positron collider is running. Imagine a giant ring where two different types of particle beams race in opposite directions, meeting at specific points to create high-energy crashes. In this new design, a beam of anti-muons, accelerated to energies between one and two trillion electron volts, would crash head-on into an electron beam. This setup is not just a theoretical exercise; it builds directly on recent ideas for a muon collider and leverages the infrastructure of a planned facility known as the FCC-ee. The researchers propose that this could be done concurrently with the standard electron collisions, meaning the machine would be working double-duty, delivering a unique type of high-energy data without needing to shut down its primary operations. However, the paper also notes that a "stand-alone" running of FCC-µe, which might be of great value, is also considered.

The primary motivation for building such a machine is to see things that other colliders cannot. While the standard electron collider is excellent for precision measurements, it lacks the raw energy needed to create pairs of Higgs bosons, the particles responsible for giving other particles their mass. By colliding muons and electrons at these higher energies, scientists could directly observe the Higgs boson interacting with itself. This is a crucial missing piece of the puzzle, as understanding how the Higgs talks to itself could reveal whether the universe is stable or if there are deeper, unknown forces at play. Furthermore, this specific collision type offers a cleaner environment than other methods. Because the initial particles are different, certain background noise that usually obscures rare events is naturally absent, allowing for a sharper view of new physics, such as the potential existence of supersymmetric particles or violations of fundamental symmetry rules.

To make this happen, the paper details a complex but feasible engineering plan. It starts with a source of ultra-cold anti-muons, which are then accelerated through a booster system before being injected into a main storage ring. The layout involves a large accelerator ring, roughly six kilometers in circumference for the main stage, where the muons are stored and circulated. A key feature of this design is the interaction point where the beams meet. Unlike standard colliders where the beams split apart immediately after a collision, the muon and electron beams here collide head-on and stay together for a short distance before being separated. This requires a larger central tunnel and special shielding to protect the detectors from the intense spray of particles created when muons decay. The design also accounts for the fact that muons are unstable; the system must be fast enough to accelerate and collide them before they vanish, a challenge that recent advances in magnet technology and beam handling have begun to solve.

The performance calculations suggest that this machine would be a powerhouse. With a beam of anti-muons carrying two trillion electron volts of energy, the facility could achieve collision rates that produce billions of events per second. The researchers estimate that over a few years of operation, they could gather enough data to measure the Higgs boson's self-interaction with a precision close to that of the HL-LHC, which would significantly improve if the anti-muon beam is polarized. If the machine were upgraded to use even more advanced magnets, it could reach energies of five trillion electron volts, opening the door to even more exotic discoveries. The paper also notes that this setup would naturally produce intense beams of neutrinos, a ghostly particle that rarely interacts with matter, providing a unique and powerful source for neutrino science without any extra cost.

Beyond the immediate physics goals, the proposal serves as a vital testbed for future technologies. The magnets and superconducting materials required to guide the five-trillion-electron-volt beam are the same ones needed for a future 100-kilometer proton collider, meaning this project would help prove the technology for the next generation of machines. The author concludes that the ability to store and collide these anti-muon beams is not just a step toward a single experiment, but a fundamental capability that could define the future of high-energy physics. By turning a theoretical concept into a concrete engineering plan, the paper suggests that the era of muon-electron collisions is closer than ever, promising a new window into the deepest secrets of the universe.

Technical Summary: FCC-µe Concept

Problem and Motivation
The paper addresses the limitations of the proposed Future Circular Collider in electron-positron mode (FCC-ee), specifically its inability to reach the center-of-mass energies required for direct studies of Higgs boson self-interactions. While the FCC-ee is limited to lower energies, the FCC-µe concept proposes a concurrent facility capable of colliding ultra-cold anti-muons ( $\mu^+$ ) with the existing FCC-ee electron beams ( $e^-$ ). This configuration aims to achieve center-of-mass energies ( $\sqrt{s}_{\mu e}$ ) between 1 and 2 TeV (and potentially up to 1.9 TeV with a 5 TeV muon beam) with average luminosities exceeding $10^{34} \text{ cm}^{-2}\text{s}^{-1}$ .

The primary scientific motivations identified are:

Higgs Self-Interaction: Direct access to Higgs boson pair production ( $\mu^+ e^- \to \mu^+ e^- HH$ ), which is inaccessible at FCC-ee energies.
Higgs Couplings: Competitive measurements of Higgs couplings via single Higgs production through gauge boson fusion.
New Physics: Unique sensitivities to charged lepton flavour violation (CLFV) and supersymmetric particle production mechanisms.
Background Reduction: Unlike $e^+e^-$ or $\mu^+\mu^-$ colliders, the $\mu^+e^-$ initial state lacks $s$ -channel annihilation processes, significantly reducing background levels (specifically the $s$ -channel $ZH$ background).
Neutrino Science: The facility would simultaneously function as a powerful neutrino factory, producing intense, collimated $\nu_e$ and $\bar{\nu}_\mu$ beams.

Methodology and Concept
The FCC-µe concept is built upon the $\mu$ TRISTAN proposal and recent developments in the LH $\mu$ C (LHC Muon Collider) framework. The proposed layout involves:

Source: An ultra-cold $\mu^+$ source (referencing J-PARC capabilities) capable of achieving transverse normalized emittance ( $\epsilon_n$ ) well below 1 $\mu$ m.
Acceleration: A $\mu^+$ booster system containing two linacs (inherited from the LHeC concept) with a circumference of approximately 9 km.
Storage: A main muon storage ring (approx. 6 km circumference) capable of storing 2 TeV beams, with a future stage considering 5 TeV beams using High-Temperature Superconducting (HTS) magnets.
Collision: Head-on collisions between the stored $\mu^+$ beam and the FCC-ee electron beam at the PL interaction point. The facility is designed for concurrent operation with FCC-ee, though "stand-alone" running is also considered.

The interaction region (IR) design differs significantly from standard $e^+e^-$ IRs. It features a larger central beam pipe (approx. 2 cm radius) to accommodate the separation of the non-colliding positron beam and the $\mu^+$ beam, which separates at a larger distance from the IP. Special shielding ("nozzles") is required to protect the central detector from high-energy positrons resulting from upstream muon decays.

Key Contributions and Results
The paper presents a preliminary performance assessment and layout for the FCC-µe:

Luminosity Performance: Assuming a conservative $\mu^+$ yield (10% of the "ultimate" $\mu$ TRISTAN proposal) and a 4 MW proton driver, the facility is projected to achieve a peak luminosity of $3.2 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ for a 2 TeV $\mu^+$ beam colliding with 120 GeV electrons ( $\sqrt{s} \approx 0.98$ TeV). The average luminosity is estimated at roughly 38% of the peak.
Higgs Pair Sensitivity: Monte Carlo studies for $W$ $W$ -boson fusion ( $WW \to HH$ $W W \to H H$ ) suggest that the signal cross-sections and irreducible backgrounds are comparable to those of a $\mu^+\mu^-$ $μ^{+} μ^{-}$ collider at the same energy.
- At $\sqrt{s} = 1$ TeV with 1 ab $^{-1}$ integrated luminosity, the precision on the Higgs trilinear self-coupling ( $\lambda_3$ ) is expected to approach that of the High-Luminosity LHC (HL-LHC).
- At $\sqrt{s} = 2$ TeV with 3 ab $^{-1}$ , a precision of approximately 20% is expected for non-polarized anti-muons, with significant improvement anticipated if 80% beam polarization is achieved.
Beam Parameters: The design utilizes a $\mu^+$ train of 80 bunches separated by 250 ns with a repetition rate of 12.5 Hz. The beam sizes at the interaction point are matched to the electron beam, with $\sigma_x \approx 4 \mu$ m and $\sigma_y \approx 0.6 \mu$ m.
Neutrino Output: The facility will generate intense neutrino beams in three directions (along two booster linacs and the main ring straight section) with energies ranging from 1 GeV to the top beam energy.

Significance and Outlook
The paper concludes that the FCC-µe offers a unique, high-energy, high-luminosity platform that complements the FCC-ee program. Its significance lies in:

Direct Access to High-Energy Physics: Enabling the first direct measurements of Higgs self-interactions and probing CLFV processes at the TeV scale.
Technological Demonstration: The 5 TeV stage would serve as a demonstration of HTS magnet technology for the future FCC-hh project.
Neutrino Science: Providing a "free" but potent neutrino factory for Europe, offering unique research opportunities in neutrino physics.
Future Storage Rings: The paper posits that the development of ultra-cold anti-muon beams is relevant for any future high-energy storage ring, not just the FCC.

The author emphasizes that while the concept relies on existing proposals ( $\mu$ TRISTAN, LH $\mu$ C), the concurrent operation with FCC-ee minimizes additional beam-beam effects, making the integration feasible. The facility is presented as a viable extension of the CERN physics program that maximizes the scientific return of the FCC infrastructure.

Technical Summary: FCC-µe Concept

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