The document of more than 70 pages finds that the Future Circular Collider (FCC-ee) – an electron–positron collider with a circumference of 90 kilometers and energies of about 100–400 GeV – is by far the best option for studying the world of particles. This aligns with years of discussions within the physics community on future accelerators, which will be formally concluded at the end of May.
"We just need to unlearn thinking in terms of bigger and higher energy for the discovery of new particles,” summarises Juan Rojo, Nikhef researcher and professor of theoretical physics at Vrije Universiteit Amsterdam. At first sight, the FCC-ee’s lower energy is a disappointment: it delivers about fifty times less energy than the current largest accelerator on Earth, the 27‑kilometer LHC in Geneva. But because electrons are point-like particles, their collisions are much cleaner than proton collisions at the LHC, making the conclusions more precise.
The study argues that this precision can be used in a systematic way. Rather than only searching directly for ever-heavier particles, the FCC-ee would be exceptionally good at collecting subtle, indirect clues for new physics. In the quantum world, particles and effects that are beyond direct reach can still influence measurable processes, and these contributions can be described using so-called effective field theories (EFTs).
Extensive studies of the feasible physics for the FCC-ee have been conducted in European strategy discussions. But these are often difficult to reproduce independently, says Rojo. The new study, on the other hand, is completely open source: anyone can verify the calculations and, if desired, also run simulations for other concepts.
On 22 May, the CERN Council will meet in Budapest to discuss the outcome of the European Strategy for Particle Physics (ESPP). If the Council endorses the preference for an FCC electron accelerator, a new phase will begin in which concrete funding and construction plans must be developed.