One of the big themes of my project is the idea that we can learn about new physics without ever producing the new particles directly. That is exactly what Effective Field Theory (EFT) is for.
In the Standard Model, we write down all the renormalisable interactions of the known particles. EFT takes a different view: instead of guessing the full high-energy theory, we say, “what if there are heavier particles we cannot see yet?” Their effects at lower energies can be captured by adding extra interaction terms to the Standard Model, built from the same fields but suppressed by some high energy scale. The heavier the new physics is, the more suppressed these terms are.
This is powerful because it lets us stay agnostic about the details. We do not need to commit to a particular model like supersymmetry or some specific dark sector. We just ask: how could the Higgs couplings be deformed in a way that is still consistent with symmetries, and how large are those deformations allowed to be?
For my project, I work with a specific EFT setup called the Higgs Effective Lagrangian (HEL). It is tailored to the Higgs and electroweak sector and is widely used in phenomenology. In practice, you start with the Standard Model Lagrangian and add a set of higher-dimensional operators. Each operator comes with a “Wilson coefficient” that tells you how strong that particular deviation from the Standard Model is.
The HEL splits these extra terms into several groups that affect different types of interactions:
- CP-even bosonic terms, which modify the Higgs kinetic term, its potential, and its couplings to W, Z and photons. This is the part I care about most.
- CP-odd terms, which flip the CP properties and can introduce new sources of CP violation.
- Fermionic terms, which change how the Higgs couples to quarks and leptons, or introduce dipole-like interactions.
- Pure gauge terms, which alter self-interactions between the gauge bosons.
In my analysis I focus mainly on a small set of Wilson coefficients that control Higgs–gauge interactions: usually written as cH, cHW, cHB and friends. Changing these numbers reshapes observables like the production rate of Higgs plus Z (ZH), and in particular the high transverse-momentum tails of those distributions.
One key point is that these operators do more than simply rescale a coupling. They can introduce momentum-dependent structures in the Higgs–Z–Z vertex, which means the effect grows with energy. At low energies, the Standard Model still dominates; but in boosted regimes – like very energetic Higgs plus Z events at a future collider – the EFT terms begin to show their hand.
That is exactly where the Future Circular Collider (FCC-hh) comes in. At around 100 TeV, Higgs and Z bosons can be produced with very high transverse momentum. My job is to see whether a realistic analysis could actually pick up those subtle shape changes and turn them into meaningful constraints on the Wilson coefficients.
In short:
- EFT and HEL let me parameterise “anything that could be going on” in the Higgs sector without choosing a specific model.
- The Wilson coefficients act as dials on different types of Higgs interactions.
- High-energy, boosted Higgs production is where those dials have the biggest visible effect.
The rest of my project is essentially about turning this clean theoretical idea into a messy, realistic collider analysis and asking: if nature has turned one of those dials, would we notice?
Noot in Circle 