Credit: AI (Not my best AI work but just pretend this is FCC’s tunnel)
If you’ve followed my posts, you already know I spend a lot of time thinking about a machine that does not exist yet: the Future Circular Collider (FCC). So it is fair to ask: why do we need another collider when the LHC is still running and has already given us the Higgs?
The Large Hadron Collider at CERN is currently the world’s most powerful accelerator. It is a 27 km ring buried deep under the French–Swiss border, accelerating two proton beams to 6.8 TeV each and smashing them together at a centre-of-mass energy of 13.6 TeV. ATLAS and CMS are the general-purpose detectors; LHCb focuses on flavour physics and CP violation; ALICE specialises in heavy-ion collisions.
Since first data in 2010, the LHC has delivered a long list of results: the 2012 Higgs discovery, precision measurements of electroweak parameters, top physics, flavour anomalies (and non-anomalies), and much more. The current upgrade, known as the High-Luminosity LHC (HL-LHC), aims to increase the luminosity by an order of magnitude and push the energy slightly higher. That will let us study rare processes in more detail and explore higher momentum scales, but only up to a point.
The key limitation of the LHC is simple: energy. With 13–14 TeV in proton–proton collisions, we are already in an impressive regime, but many interesting possibilities (heavy new particles, strongly-coupled Higgs sectors, very rare processes) could sit far beyond that. At the same time, even within the Standard Model, some properties of the Higgs, like its self-coupling, are extremely difficult to measure at the LHC.
The FCC proposal tries to address both precision and discovery by splitting the programme into two main stages:
- FCC-ee: a high-luminosity electron–positron collider.
- It would run at several centre-of-mass energies: the Z pole, WW threshold, a “Higgs factory” around 240–250 GeV, and the top-pair threshold near 365 GeV.
- Because e+e− collisions have a very clean initial state, FCC-ee could measure Higgs couplings and widths with per-mille level precision and perform extremely sharp electroweak tests.
- FCC-hh: a 100 TeV proton–proton collider in the same 90 km tunnel.
- This is the machine I focus on.
- With around seven times the LHC energy, it would be a genuine discovery machine for new heavy states and give direct access to processes like di-Higgs production in large numbers.
- It is also a nightmare experimentally: enormous backgrounds and hundreds of overlapping “pile-up” collisions in each event.
For Higgs physics, the roles are complementary. FCC-ee gives precision at relatively low energies: clean measurements of absolute couplings and the total width. FCC-hh pushes into the high-energy regime, where rare processes and high-pT tails become visible. Together, they can pin down whether the Higgs really behaves exactly like the Standard Model predicts.
My project sits within that story: I take one specific channel, Higgs produced in association with a Z boson (ZH), and ask whether, at FCC-hh, we can still do precision-style measurements in the middle of the chaos. If we can control boosted ZH at 100 TeV, it becomes a very sharp probe of the Higgs–gauge sector that ties nicely into the EFT picture from the first article.
So the short answer to “why another collider?” is:
- Precisely measuring the Higgs is not a solved problem; we have only scratched the surface.
- Some of the most interesting Higgs observables are either inaccessible or only weakly constrained at the LHC.
- A staged FCC programme would combine ultra-precise low-energy data with brute-force high-energy reach, which is exactly what you want if you suspect new physics might be hiding in subtle deformations of Higgs interactions rather than in obvious new resonances.
Noot in Circle 