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The Higgs boson, discovered in 2012 at the Large Hadron Collider (LHC) at CERN in Switzerland, is a new and unique particle, unlike anything discovered before. The confirmation of its existence also confirmed our understanding of the universe consistent with the standard model of particle physics. But it also raised many questions about what other mysterious matter could and should be out there.
The HIGGSEFT project built on the Higgs discovery, deriving new information about the properties of the boson from LHC data, and laid the foundations to begin answering some of those fundamental questions. The project identified novel ways to maximise the chances of finding evidence of other as-yet undiscovered particles in future collider experiments via their interactions – or couplings – with other particles and forces.
‘To measure distances, one needs a ruler,’ says project coordinator Christophe Grojean of German particle accelerator centre DESY in Hamburg. ‘The ruler to measure the Higgs couplings had only two ticks: zero or one, meaning agreement or disagreement with the standard model. During the course of the project, we have defined more ticks on this ruler to continuously interpolate between 0 and 1.’
This ‘ruler’ – a formalism to describe the interactions more precisely – is fully compatible with the laws of quantum mechanics and special relativity. It is now being used by the ATLAS and CMS collaborations at CERN to analyse their measurements on the Higgs boson and gain new insights about this particle.
On target
‘So far, the LHC hasn’t revealed any sign of new physics beyond the standard model. That makes the study of the Higgs boson more and more important as a prime target for future projects,’ Grojean says.
High-energy physicists are currently brainstorming the next collider to build after the LHC, and the HIGGSEFT formalism has become a key instrument in helping the community gauge what approaches should produce the most promising results.
‘The approach advocated in the HIGGSEFT project offers the best way to parametrise our ignorance about physics at a higher scale, and thus draw some general and model-independent conclusions on the sensitivity of these planned machines,’ Grojean says.
The HIGGSEFT project was supported by the EU’s Marie Skłodowska-Curie Actions programme.