The 10 TeV Horizon: A Perspective on the Future of High Energy Physics

The 10 TeV Horizon: A Perspective on the Future of High Energy Physics

As the High-Luminosity LHC era matures, the global high-energy physics (HEP) community is engaging in a deep strategic dialogue about the future. The recent US P5 Report helped crystalize a shared vision: the need to reach the 10 TeV parton center-of-mass energy scale.

This target represents the energy frontier required to explore physics beyond the Standard Model. However, reaching this scale requires navigating a complex landscape of technological readiness, economic constraints, and facility longevity. Coming from the Advanced Accelerator Concepts (AAC) community, I offer a perspective on how these different approaches stack up and where our technologies might best contribute.

1. The Physics Context: Defining the 10 TeV Goal

To evaluate future machines, it helps to look at the “effective” energy available for particle discovery.

• Proton Colliders (100 TeV): Because protons are composite particles, the collision energy is shared among constituent quarks and gluons. A 100 TeV proton collider (like the proposed FCC-hh or SPPC) effectively probes the 10-20 TeV energy scale at the constituent level.
• Lepton Colliders (10 TeV): Electrons and muons are fundamental particles, meaning the full beam energy is available for the interaction. Therefore, a 10 TeV lepton collider is generally considered the physics equivalent of a 100 TeV proton machine.

The Challenge: While 10 TeV is the clear target for the Energy Frontier, achieving this with leptons presents significantly different challenges than with protons.

2. Strategic Considerations for Linear Colliders

Linear Electron-Positron Colliders (like the ILC or CLIC) have long been studied as precision Higgs factories. However, when viewed through the lens of the 10 TeV long-term goal, they face distinct strategic hurdles compared to their circular counterparts.

Infrastructure and Upgradability A key advantage of circular collider proposals (like FCC-ee) is the “tunnel value.” Excavating a 90km tunnel is a massive investment, but it creates a permanent infrastructure asset. Once the electron machine completes its mission, the same tunnel can host a 100 TeV proton machine (FCC-hh), guaranteeing the facility’s relevance for decades. In contrast, a linear tunnel is optimized for a single pass. Upgrading a linear collider from a Higgs factory (~250 GeV) to the 10 TeV frontier is technically prohibitive due to the immense length required (potentially >100 km) and the physics of beamstrahlung (radiative energy loss), which becomes manageable only at lower energies or with novel particle species.

The “Middle Energy” Dilemma This leaves linear electron colliders in a difficult position: they are excellent precision machines at the 100s GeV scale, but extending them to the 10 TeV energy frontier—where the physics community aims to go next—requires a paradigm shift in technology rather than just scaling up.

The “Narrow Window” for Viability This leaves a very narrow path for linear colliders. To be justifiable, the footprint and cost of a linear Higgs factory must be radically reduced—perhaps to a small fraction (e.g., <20%) of a circular counterpart like FCC-ee or CEPC. Without such a disruptive cost advantage, a linear machine represents a significant strategic risk. As a single-purpose facility, it lacks the “infrastructure insurance” of a circular tunnel; if no new physics is found, the community is left with a dead-end facility rather than a stepping stone to the 10 TeV frontier.

3. The Global Landscape

Different regions are adopting different strategies to address these challenges.

Europe: The Infrastructure Approach (FCC)

CERN’s Future Circular Collider (FCC) proposal prioritizes infrastructure longevity. By starting with an e-e+ machine and planning for a proton machine later, it maximizes the return on the civil engineering investment. This is a robust, albeit expensive, path that relies on proven technology and CERN’s existing institutional strength.

China: The Window of Opportunity (CEPC)

The Circular Electron Positron Collider (CEPC) shares the same logic as the FCC—a circular Higgs factory followed by a super proton collider (SPPC). The primary challenge here is timing. For CEPC to secure its position as the world’s first Higgs factory, it needs to move to construction ahead of the FCC to maximize its strategic value to the global community.

USA: The Technology Leap (Muon Collider)

The US community is exploring a different path. Rather than committing to a massive new tunnel, interest is growing in the Muon Collider.

• The Logic: Muons, being heavy leptons, suppress synchrotron radiation. This allows for a circular 10 TeV machine that could essentially fit within the existing Fermilab site.
• The Status: This is an R&D-intensive “moonshot.” It requires mastering muon cooling (compressing the beam phase space). If successful, it offers a compact, cost-effective route to 10 TeV that bypasses the size constraints of proton rings and linear electron accelerators.

4. The Evolution of Advanced Accelerator Concepts (AAC)

For years, the AAC community (including Plasma Wakefield Acceleration) focused heavily on designing a compact linear collider. While we have achieved remarkable gradients (energy gain per meter), the stringent luminosity (collision rate) requirements of a particle physics collider remain a significant challenge.

A Pivot to Brightness However, this challenge has clarified where plasma acceleration truly shines. The unique strength of plasma-based schemes is not necessarily high average power (required for collider luminosity), but rather the ability to produce ultra-short beams with extreme peak brightness.

The Future: Photon Science This realization is driving a strategic evolution in our field. Rather than competing directly with conventional RF technology for collider luminosity, AAC is poised to revolutionize future light sources.

As we demonstrated in our recent paper, “Plasma wakefield accelerator simultaneously boosts electron beam energy and brightness,” plasma accelerators can act as an “Energy and Brightness Dual Transformer.” By simultaneously increasing energy and peak brightness in a single stage, this technology enables a new class of compact, next-generation Free Electron Lasers (FELs). This leverages plasma accelerators’ unique potential in chasing the Brightness Frontier and opens new doors for photon science and advanced materials research, playing to the inherent strengths of plasma technology while the high-energy physics community pursues the Muon and Proton paths for the Energy Frontier.