🔬 physics

Report on the Advanced Linear Collider Study Group (ALEGRO) Workshop 2026

This report summarizes the 7th ALEGRO workshop held in March 2026, which brought together the international community to review progress in advanced wakefield accelerators, discuss the design of a future 10 TeV linear collider, and explore applications ranging from high-energy physics to free-electron lasers and AI-driven accelerator control.

Original authors: L. Verra, P. Muggli, B. Cross, E. Adli, R. Babjak, T. Barklow, F. Bencivenga, C. Benedetti, C. Benedetti, M. Buscher, S. S. Bulanov, A. Caldwell, G. Chen, O. Chubenko, R. D'Arcy, S. Diederichs, K. Dow

Published 2026-07-13

📖 8 min read🧠 Deep dive

CC BY 4.0

Original authors: L. Verra, P. Muggli, B. Cross, E. Adli, R. Babjak, T. Barklow, F. Bencivenga, C. Benedetti, C. Benedetti, M. Buscher, S. S. Bulanov, A. Caldwell, G. Chen, O. Chubenko, R. D'Arcy, S. Diederichs, K. Downham, J. Farmer, M. Ferrario, A. Ferran Pousa, A. Formenti, M. Fuchs, S. Gessner, L. Giannessi, Z. Gong, J. Grames, T. Grismayer, H. Jaworska, L. Ji, V. Kain, S. Karkare, A. Knetsch, C. Lindstrom, N. Lopes, N. Lopes, F. Massimo, C. Masciovecchio, P. Oliveira, T. Opferkuch, J. Osterhoff, J. Osterhoff, S. Pagan Griso, G. Penco, J. Power, A. Pukhov, T. P. Rakitzis, A. Rastogi, L. Reichwein, C. B. Schroeder, L. O. Silva, A. Sinn, D. Sofikitis, D. Storey, M. Thévenet, M. Trovò, M. Turner, M. Vranic, F. Willeke, T. Wilson, M. Wing, W. Zhang, Chuan Zheng

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

The Race to Build a 10-Teeter-Totter: A Report on the Future of Particle Smashing

Imagine you want to smash two tiny marbles together to see what's inside. To do this, you need a machine that can fling them at each other with enough speed to break them apart. Right now, our best machines are like giant, circular rollercoasters (the Large Hadron Collider) that take up a huge amount of space. But scientists are dreaming of a new kind of machine: a straight, super-fast track that could smash particles together with 10 TeV (ten trillion electron volts) of energy. That's like taking the energy of a speeding train and squeezing it into a single particle!

This report is a summary of a workshop called ALEGRO 2026, where scientists gathered to discuss how to build this "10 TeV Collider" using a wild new trick called Wakefield Acceleration. Instead of using giant radio waves to push particles (like a traditional train engine), they use "waves" created by lasers or other particle beams to surf the particles to incredible speeds.

Here is the story of where we stand, what we've tried, and what we still need to figure out.

The Big Dream: A 10 TeV Wakefield Collider

The main goal is to build a linear collider that reaches 10 TeV. Why? Because smashing particles at this energy level might reveal new secrets of the universe that our current machines can't see.

The report suggests that Wakefield Accelerators (AWA) are the best way to do this. Think of a traditional accelerator like a long, slow elevator. A Wakefield accelerator is like a surfer catching a massive wave. If you drop a heavy object (a "driver" beam or a laser pulse) into a pool of plasma (a hot soup of charged particles), it creates a wake. If you drop a second, lighter object (the "witness" beam) right behind it, it can surf that wake and gain massive speed in a very short distance.

The Catch: We are still in the "R&D" phase. We haven't built the 10 TeV machine yet. The report explicitly states that while we have made amazing progress, we are not there yet. We need to solve several "global questions" before we can even start building.

The Three Big Questions Scientists Are Still Pondering

The report highlights that the community is still debating the best way to build this machine. They haven't ruled anything out, but they are asking:

Can we accelerate positrons? (Positrons are the "anti-electrons"). We can accelerate electrons easily, but getting positrons to surf the wave without losing their shape is still a huge challenge.
Round vs. Flat? Should the particle beams be round like a coin or flat like a sheet of paper? The report suggests that while flat beams are great for traditional machines, plasma accelerators naturally make round beams. We need to figure out which shape works best for a 10 TeV smash.
How do we connect the stages? Since one "surf" isn't enough to reach 10 TeV, we need to chain many stages together. The report suggests that connecting these stages without losing the beam's quality is a major hurdle we are currently working on.

The "Middle Ground" Projects: Testing the Waters

While the 10 TeV machine is the "moonshot," the report highlights several "mid-term" projects that are closer to reality. These are like the training camps for the Olympic team.

HALHF (Hybrid Asymmetric Linear Higgs Factory): This is a clever idea to build a machine that makes Higgs bosons (a type of particle) now, without waiting for the 10 TeV machine. The idea is to use a traditional machine to make positrons and a plasma machine to make electrons. It's a "hybrid" approach. The report notes that this concept is being studied seriously, but it still needs to prove it can run at the high speeds (repetition rates) needed to be useful.
ALiVE and LEP3: Another idea is to use proton beams to drive plasma waves. This could boost the energy of an existing machine (LEP3) to reach higher energies. The report suggests this is an "exciting option" if the European plan for a circular machine (FCCee) doesn't happen.
AWAKE: This is a real experiment at CERN using protons to drive plasma waves. The report says they are working on demonstrating that this can produce high-quality electron beams for experiments, potentially by the mid-2030s.

The "Surfing" Challenges: What's Hard About It?

The report uses some vivid analogies to explain why this is so hard.

The "Flat Beam" Problem: Imagine trying to keep a flat sheet of paper flat while it's being blown by a chaotic wind. In plasma, the forces can twist the beam, mixing its shape. The report presents simulations showing that if the beam isn't perfectly controlled, it can lose its "flatness," which ruins the collision quality. They are working on ways to stop this mixing.
The "Staging" Problem: Imagine trying to pass a baton in a relay race, but the runners are moving at the speed of light and the track is made of hot plasma. You have to take the driver beam out and put a fresh one in, all while keeping the surfer beam perfectly aligned. The report suggests a new "lattice" (a set of magnetic lenses) that might solve this, but it needs to be tested.
The "Positron" Problem: Positrons are tricky. The report explicitly states that no suitable scheme has been found yet to accelerate positrons in a plasma way that is good enough for a collider. This is a major gap.

The "Real-World" Stuff: Lasers and Reliability

You can't just have a cool idea; you need a machine that works 24/7. The report discusses the reality of running these facilities.

Lasers: To drive the plasma waves, you need massive lasers. The report notes that current lasers are great for research but need to be much more reliable and powerful (running at high repetition rates) to be useful for a collider.
AI and Automation: The report suggests that Artificial Intelligence (AI) will be crucial. Just like weather forecasting has been revolutionized by AI, the control of these complex machines will likely need AI to keep everything running smoothly and to fix problems instantly.

The "What If" Scenarios

The report also looks at different types of collisions:

Electron-Electron ( $e^-e^-$ ): Since positrons are hard, maybe we just smash electrons into electrons? The report suggests this is a viable path, but we need to understand the physics better.
Gamma-Gamma ( $\gamma\gamma$ ): What if we turn the electrons into light (gamma rays) and smash the light? The report suggests this is a "compelling" path that avoids the positron problem entirely. They are simulating how this would work and finding it promising.

The Bottom Line: Are We There Yet?

No. The report is very clear: we are in the "design study" phase.

What is proven? We have demonstrated that plasma accelerators can create high-quality beams. We have shown that we can drive them with lasers and particle beams. We have simulated how to connect stages.
What is suggested? The report suggests that a 10 TeV collider is possible, but it requires solving the positron problem, perfecting the staging, and building machines that run 24/7.
What is ruled out? The report doesn't rule out any specific technology, but it does rule out the idea that we can just "copy-paste" current technology. We need new designs for drivers, staging, and beam shapes.

The Timeline

The report paints a picture of the future:

Near-term (Next few years): We will see more experiments with "mid-term" applications like injectors for other machines and free-electron lasers (like EuPRAXIA@SPARC_LAB).
Mid-term (2030s): We hope to have working demonstrators for things like the AWAKE experiment and the HALHF concept.
Long-term (2040s and beyond): If everything goes well, we might see a decision on a 10 TeV collider. The report suggests that if we start the design study now, we could have a machine ready to build in the 2040s.

In short, the ALEGRO 2026 workshop was a gathering of dreamers and doers. They are mapping out the path to a machine that could change our understanding of the universe. They have the map, but the road is still under construction. The journey from "cool idea" to "world-changing machine" is long, but the report suggests it's a journey worth taking.

Technical Summary: Report on the Advanced Linear Collider Study Group (ALEGRO) Workshop 2026

Problem Statement
The particle physics community faces the challenge of defining the next generation of high-energy colliders capable of reaching the energy frontier (10 TeV and beyond) while managing the prohibitive costs and physical footprints of conventional radio-frequency (RF) accelerators. While the European Strategy for Particle Physics Update (ESPPU) currently prioritizes the circular FCC-ee, there is a critical need to evaluate Advanced and Novel Accelerators (ANA), specifically Advanced Wakefield Accelerators (AWA), as viable long-term solutions for linear colliders. The primary technical hurdles preventing the immediate deployment of AWA for High-Energy Physics (HEP) include the preservation of beam quality (emittance, energy spread, and polarization) during acceleration, the acceleration of positrons, the staging of multiple acceleration modules, and the transition from research-grade facilities to reliable, 24/7 user facilities. Furthermore, the interaction of ultra-short, high-intensity beams at the interaction point (IP) introduces complex beam-beam dynamics and strong-field Quantum Electrodynamics (QED) effects that challenge existing simulation tools.

Methodology
The ALEGRO 2026 workshop, held in March 2026 at INFN-LNF, served as a coordination mechanism for the global AWA community. The methodology involved a comprehensive review of the current state of the art across four main pillars:

Design Studies: A dedicated day was devoted to the "10 TeV Wakefield Collider Design Study," initiated in 2025, which evaluates physics cases, beam delivery systems, and detector concepts for $e^+e^-$ , $e^-e^-$ , and $\gamma\gamma$ collision modes.
Technology Assessment: Detailed reviews of Laser Wakefield Acceleration (LWFA), Plasma Wakefield Acceleration (PWFA), and Structure Wakefield Acceleration (SWFA) were conducted, focusing on driver technologies (high-power lasers, electron/proton beams), accelerating structures, and staging concepts.
Simulation and Modeling: The workshop highlighted the development and validation of advanced simulation codes (e.g., WarpX, OSIRIS, VLPL) to model beam-beam interactions, including strong-field QED effects and beamstrahlung, which are critical for 10 TeV energies.
Operational Readiness: Lessons were drawn from existing user facilities (e.g., EuPRAXIA@SPARC_LAB, EPAC, Vulcan 20-20, FERMI) to define the requirements for reliability, reproducibility, and automation necessary for HEP applications.

Key Contributions and Results

10 TeV Collider Physics and Design:
- The design study confirmed that a 10 TeV collider is a natural evolution of a linear Higgs factory. The physics case for $e^-e^-$ and $\gamma\gamma$ collisions was strengthened, noting that $\gamma\gamma$ colliders can bypass the positron acceleration bottleneck and offer enhanced cross-sections for vector boson scattering.
- Beamstrahlung: Contrary to previous assumptions that beamstrahlung is purely detrimental, the study found that at 10 TeV, the broad luminosity spectrum induced by strong beamstrahlung acts as an asset, effectively scanning a wide range of sub-nominal energies and enhancing the discovery reach for narrow resonances (e.g., $Z'$ bosons).
- Beam Shape: While flat beams are traditionally preferred to minimize beamstrahlung, the study suggests that round beams may be more suitable for plasma-based accelerators due to resonant emittance mixing issues in flat beams. Round beams offer higher geometric luminosity despite increased radiation.
Acceleration Technologies and Staging:
- LWFA: Progress was reported on hydrodynamic, optically-field-ionized (HOFI) channels, which avoid material damage and offer a path to collider-scale acceleration. A specific design for a 6.2 GeV stage using a 10 J laser driver was presented, achieving a wall-to-beam efficiency of ~8.2%.
- PWFA: The HALHF (Hybrid, Asymmetric, Linear Higgs Factory) concept was updated, proposing the use of RF accelerators for positrons and PWFA for electrons to bypass positron acceleration challenges. Significant progress was made in "staging" concepts, specifically the development of an achromatic interstage lattice using nonlinear plasma lenses to preserve emittance between plasma modules.
- SWFA: Structure Wakefield Acceleration (SWFA) utilizing short-pulse (ns) RF regimes in Ka-band (35 GHz) structures was identified as a path to achieving real-estate gradients of 500–700 MeV/m, potentially enabling a 10 TeV collider within a 10–20 km tunnel.
Mid-Term Applications and Injectors:
- Higgs Factories: Concepts like HALHF and ALiVE (driven by proton bunches and Fixed Field Accelerators) were presented as viable paths to Higgs factories, potentially utilizing proton drivers to reach high energies without staging.
- Injectors: Proton-driven plasma wakefield acceleration (AWAKE scheme) is being developed as a compact injector for the Electron-Ion Collider (EIC) and as a booster for LEP3 to reach the $t\bar{t}$ threshold.
- Light Sources: The EuPRAXIA@SPARC_LAB project is transitioning to implementation, aiming to demonstrate a Free-Electron Laser (FEL) driven by a plasma-accelerated beam, serving as a demonstrator for user-oriented plasma facilities.
Simulation and Beam-Beam Dynamics:
- The legacy 2D codes (GUINEA-PIG, CAIN) were found insufficient for 10 TeV regimes due to memory limits and lack of strong-field QED physics. The community is transitioning to the 3D, massively parallel code WarpX, which has been validated for beam-beam interactions and includes models for pair production (Breit-Wheeler, Bethe-Heitler) and beamstrahlung.
- New analytic theories for beamstrahlung at high energies ( $\Upsilon \gg 1$ ) were derived, showing self-similar luminosity spectra.
Operational Challenges and AI:
- The transition to 24/7 operation requires addressing laser stability, plasma reproducibility, and component maintainability. Automated alignment, adaptive optics, and spare-parts management strategies were discussed based on experiences with EPAC and Vulcan 20-20.
- Artificial Intelligence (AI) and Machine Learning (ML) are identified as critical for optimization, control, and differentiable simulation. The workshop emphasized the need for AI-native design practices for future machines.

Significance
The paper claims that the AWA community has matured significantly, moving from exploratory research to the design of demonstrator facilities and the conceptualization of a 10 TeV collider. The significance of the report lies in its synthesis of the physics case, technical feasibility, and operational requirements for a future linear collider. It establishes that:

Feasibility: A 10 TeV collider is technically conceivable using wakefield acceleration, with specific designs for $e^+e^-$ , $e^-e^-$ , and $\gamma\gamma$ modes.
Strategic Path: The development of mid-term applications (Higgs factories, injectors for CEPC/EIC, and FELs) is essential to mature the technology for the ultimate 10 TeV goal.
Community Coordination: The ALEGRO workshop serves as a vital mechanism for international coordination, bridging the gap between accelerator physics, plasma physics, and particle physics communities.
Readiness: While significant R&D remains—particularly in positron acceleration, staging, and high-repetition-rate drivers—the path forward is defined, with the 10 TeV Wakefield Collider Design Study providing a unified framework for future progress.

The report concludes that while funding and political decisions (such as the outcome of the ESPPU) will dictate the timeline, the scientific and technical foundations for a 10 TeV wakefield collider are being actively built, with the ultimate goal of creating a sustainable, compact, and affordable energy frontier machine.