⚛️ phenomenology

Descoped and Upscoped FCC-ee Running Scenarios in the SMEFT

This paper evaluates how descoped (reduced beam power, fewer interaction points, or delayed top-quark runs) and upscoped (increased luminosity) FCC-ee running scenarios impact new physics sensitivity within the SMEFT framework, demonstrating that the top-quark run is critical to the collider's overall physics potential and its complementarity with HL-LHC measurements.

Original authors: Eugenia Celada, Elie Hammou, Jaco ter Hoeve, Marion O. A. Thomas

Published 2026-08-07

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Eugenia Celada, Elie Hammou, Jaco ter Hoeve, Marion O. A. Thomas

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

Imagine the universe as a giant, cosmic puzzle. For decades, scientists have been fitting together pieces called "particles"—tiny building blocks like electrons and quarks—to form the Standard Model, our best-ever picture of how everything works. But we know the picture isn't complete; there are gaps where gravity doesn't fit, and mysteries about why the universe has mass. To find the missing pieces, physicists build massive machines called particle colliders. These are like giant, high-speed racetracks where they smash particles together at incredible speeds to see what splinters fly out. The bigger the racetrack and the faster the crash, the more likely they are to find something new, something that hints at "New Physics" beyond our current understanding.

One such proposed racetrack is the Future Circular Collider (FCC-ee), a massive electron-positron machine planned for the future. The big question right now isn't just if we can build it, but how we should build it to get the most bang for our buck. Should we build a smaller, cheaper version that cuts corners? Or should we aim higher, spending more to get a bigger, faster machine? This paper acts like a cosmic accountant, crunching the numbers to see how different versions of this racetrack would perform in hunting for those missing puzzle pieces.

The Great Collider Budget Battle

Think of the FCC-ee as a massive, high-tech video game console that scientists want to build. The "Baseline" version is the dream machine: it has four powerful cameras (called Interaction Points) to record every crash, runs at maximum speed (50 MW of power), and includes a special, expensive level where they smash top-quarks together. This top-quark level is crucial because it helps them measure the "weight" of the top-quark with extreme precision, which is like calibrating the entire game's physics engine.

However, building dream machines is expensive. The European Strategy for Particle Physics suggested a "Descoped" version—a budget-friendly alternative. This version cuts the cameras down to two, lowers the power to 30 MW, and, most controversially, skips the top-quark level entirely to save money. The idea was: "Let's build the cheaper version first, and if we have extra cash later, maybe we can add the top-quark level."

The authors of this paper asked a simple but vital question: Does the budget-friendly version actually work as well as the dream machine? They used a mathematical toolkit called SMEFT (Standard Model Effective Field Theory), which is like a universal translator that converts tiny measurement errors into clues about new, heavy particles we haven't seen yet. They simulated how well different versions of the collider would perform in finding these clues.

The Top-Quark: The Missing Key

The most surprising finding is that the "budget" version fails spectacularly if it skips the top-quark run. Imagine trying to solve a jigsaw puzzle but refusing to look at the corner pieces. Without the top-quark run, the FCC-ee—even with all its other fancy features—ends up performing almost no better than a much older, smaller machine called LEP3.

The paper shows that the top-quark run is the "master key." It does two things:

It calibrates the machine: By measuring the top-quark's mass precisely, it removes a huge source of "fuzziness" (parametric uncertainty) in the other measurements.
It breaks the lock: In the complex math of the global analysis, many different theories look identical (these are called "flat directions"). The top-quark run provides unique data that shatters these illusions, allowing scientists to tell one theory from another.

The authors found that if you remove the top-quark run, the difference between a "2-camera" budget machine and a "4-camera" dream machine becomes almost irrelevant. Both are stuck in the same low-performance zone. However, if you add even a "staged" top-quark run (a smaller version added later), the performance jumps up significantly, allowing the machine to outperform the older LEP3 by a wide margin.

Bigger isn't Always Better (When it comes to Luminosity)

The paper also looked at "Upscoped" scenarios—what if we spent more money to make the machine even faster? They tested two ideas:

Uniform Boost: Increase the data collection (luminosity) by 20% across all energy levels.
Targeted Boost: Keep the lower levels the same but boost the top-quark run by 50%.

The result? Diminishing returns. The authors found that simply adding more data (more collisions) didn't improve the search for new physics as much as everyone hoped. Why? Because the bottleneck wasn't the number of collisions; it was the "noise" in the data.

In the global analysis, the biggest limit isn't how many times the particles collide, but the "systematic uncertainties" from the Large Hadron Collider (HL-LHC). Think of it like this: The FCC-ee is a super-precise microscope, but it's trying to look at a picture that was drawn by a shaky hand (the LHC data). No matter how much you zoom in or how many times you take a picture with the microscope, if the original drawing is shaky, you can't get a perfect result. The paper identifies the HL-LHC's systematic errors as the current "traffic jam" preventing the FCC-ee from reaching its full potential, noting that upscoping the FCC-ee yields only marginal gains because of this external limitation.

The Verdict: Don't Cut the Top-Quark

So, what's the takeaway for the future of particle physics?

The Top-Quark Run is Non-Negotiable: The paper explicitly argues that a "descoped" FCC-ee without the top-quark run is a bad deal. It wastes the machine's potential. Even a smaller, staged top-quark run is essential to unlock the physics potential.
More Data Isn't the Magic Bullet: Simply making the machine run faster or longer (increasing luminosity) won't fix the problem if the theoretical and systematic uncertainties remain high. The authors highlight that the HL-LHC's systematic errors are the current bottleneck, meaning that even a significantly more powerful FCC-ee would see limited improvements until these external uncertainties are addressed.
Complementarity is Key: The FCC-ee and the HL-LHC are not rivals; they are partners. The FCC-ee needs the HL-LHC to break the mathematical "flat directions" in the global fit. You can't just replace one with the other; they need to work together.

In the end, the authors suggest that if we have to choose between a cheaper machine with fewer cameras or a machine that skips the top-quark level, we should definitely keep the top-quark level. It's the difference between a machine that can barely see the edge of the puzzle and one that can actually solve it. The "budget" cuts might save money upfront, but they risk leaving the most important piece of the cosmic puzzle forever out of reach.

Technical Summary: Descoped and Upscoped FCC-ee Running Scenarios in the SMEFT

Problem Statement
The Future Circular Collider with electron-positron collisions (FCC-ee) is proposed as the next flagship facility for European particle physics. However, financial constraints have led to the consideration of a "descoped" version of the project, characterized by reduced beam power (30 MW vs. 50 MW), fewer interaction points (2 vs. 4), and the potential removal of the top-quark threshold run ( $\sqrt{s} \approx 365$ GeV). Conversely, an "upscoped" scenario is considered where increased beam power could allow for higher integrated luminosity. The central problem addressed is quantifying the physics impact of these alternative running scenarios on the sensitivity to New Physics, specifically within the framework of the Standard Model Effective Field Theory (SMEFT). The study aims to determine whether a reduced or enhanced FCC-ee programme can still outperform previous and concurrent experiments (LEP3 and the High-Luminosity LHC) and to identify the dominant sources of uncertainty limiting the physics reach.

Methodology
The authors perform a global SMEFT analysis using the open-source SMEFiT framework. The analysis covers:

Theoretical Framework: CP-even dimension-six operators in the Warsaw basis with $U(2)^5$ flavour symmetry, resulting in 61 independent Wilson coefficients. The analysis includes one-loop Renormalisation Group Equation (RGE) effects and considers both linear ( $O(\Lambda^{-2})$ ) and quadratic ( $O(\Lambda^{-4})$ ) EFT expansions.
Scenarios:
- Descoped: Two interaction points (2-IP) and four interaction points (4-IP) with reduced luminosity (36.5% and 60% of baseline, respectively) and no top-quark run. "Staged" scenarios are also modeled where a top-quark run is added later with partial luminosity.
- Upscoped: A uniform 20% luminosity increase across all energy runs, and a targeted 50% increase exclusively for the top-quark run.
Uncertainty Treatment: The study compares an "aggressive" theory uncertainty scenario (substantial future improvements to N4LO) and an "ideal" scenario (negligible theory uncertainties). Parametric uncertainties, particularly those dependent on the top-quark mass ( $m_t$ ), are propagated based on the presence or absence of the top-quark run. Systematic uncertainties for LHC/HL-LHC are treated as the limiting factor for global fits.
Analysis Type: Both individual fits (one Wilson coefficient at a time) and global marginalised fits (all 61 coefficients fitted simultaneously) are performed. The sensitivity is quantified via bounds on the new physics scale $\Lambda/\sqrt{c_i}$ .

Key Contributions and Results

Critical Importance of the Top-Quark Run:
The most significant finding is that the presence of a top-quark run is essential for the FCC-ee to outperform LEP3.
- Without the top run: In global marginalised fits, no variant of the descoped FCC-ee (with reduced luminosity but no top run) truly outperforms LEP3. The reduction in luminosity in descoped scenarios is less detrimental than the absence of the top run.
- With the top run: Including a staged top-quark run (even with reduced luminosity) significantly improves constraints, particularly in the marginalised fit. This is due to two factors: (1) the direct measurement of top electroweak couplings and (2) the reduction of parametric uncertainties on $m_t$ , which breaks correlations in the electroweak precision observables (EWPOs) at the Z-pole.
- Conclusion: A staged top-quark run should be considered a minimum requirement for any descoped FCC-ee programme to maximize physics potential.
Limited Impact of Luminosity Variations:
Increasing the integrated luminosity yields only marginal improvements in the global SMEFT fit sensitivity compared to the baseline, though the pattern of improvement depends on the scenario.
- While the uniform 120% luminosity increase generally provides better bounds for most coefficients (due to the enhanced Z-pole statistics), the targeted 50% increase for the top-quark run does not consistently outperform the uniform increase across all operators.
- This indicates that statistical uncertainties are not the dominant bottleneck. The "upscoped" scenarios result in bounds that change by only a few percent or modestly, suggesting that simply collecting more data at FCC-ee cannot overcome the current limitations in the global fit.
The HL-LHC Bottleneck and Complementarity:
The study identifies that the primary limitation for global SMEFT fits is not the FCC-ee statistics or theory uncertainties, but the systematic uncertainties of the HL-LHC.
- Flat Directions: The FCC-ee dataset alone suffers from degeneracies (flat directions) in the 61-dimensional parameter space. The HL-LHC is required to break these directions.
- Systematics Dominance: Removing HL-LHC systematic uncertainties drastically tightens the SMEFT bounds, whereas varying FCC-ee luminosity has a negligible effect.
- Complementarity: The FCC-ee and HL-LHC are fundamentally complementary. The FCC-ee cannot render the LHC obsolete; rather, the LHC provides the necessary inputs to disentangle the global SMEFT fit.
Individual vs. Global Fits:
- In individual fits, the FCC-ee Z-pole precision dominates, and theory uncertainties (missing higher-order corrections) are the primary limitation for many operators.
- In global marginalised fits, correlations between operators dilute the constraints, making the HL-LHC systematics the critical bottleneck.
Higgs Self-Coupling:
The sensitivity to the Higgs trilinear coupling ( $\delta\kappa_3$ ) is heavily dependent on the top-quark run. Without it, FCC-ee constraints on $\delta\kappa_3$ are comparable to LEP3. With the top run, the precision improves significantly (e.g., from $\sim 0.28$ to $\sim 0.16$ in the baseline scenario).

Significance and Claims
The paper claims to provide the first systematic assessment of the physics impact of descoped and upscoped FCC-ee scenarios within a consistent SMEFT framework. Its primary significance lies in:

Policy Guidance: Demonstrating that a "descoped" FCC-ee can retain substantial physics potential if a staged top-quark run is included, whereas removing the top run severely degrades the programme's value relative to LEP3.
Strategic Insight: Highlighting that increasing luminosity (upscoping) offers diminishing returns compared to securing the top-quark run, and that the physics reach is currently limited by HL-LHC systematics rather than FCC-ee statistics.
Complementarity: Establishing that the FCC-ee and HL-LHC are mutually dependent for a complete SMEFT analysis, with the LHC being essential to break flat directions in the global fit.

The authors conclude that while updated projections may refine these numbers, the qualitative hierarchy of importance—Top Run > Luminosity > HL-LHC Systematics—remains robust. They suggest that a physically meaningful set of operators for future fits may lie between the individual and full global fits explored here.

The Great Collider Budget Battle

The Top-Quark: The Missing Key

Bigger isn't Always Better (When it comes to Luminosity)

The Verdict: Don't Cut the Top-Quark

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