If Europe decides to build a larger successor to the LHC accelerator, particle physicists will have to thoroughly rethink their methods of discovery. That is the conclusion reached by a group of ten authors in a study that has just been published online.
The document, which spans more than 70 pages, concludes that the proposed FCC-ee, an electron accelerator with a circumference of 90 kilometers, is by far the best option for studying the world of particles. This aligns with the outcomes of years of discussions within the physics community regarding new accelerators, which will be formalized at the end of May.
But this study should not be seen as just another vote for the FCC, says Nikhef theorist and VU physicist Juan Rojo, one of the authors. “FCC-ee offers the greatest potential, but in ways different from what we are used to in particle physics.”
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.
The proposal for the Future Circular Collider (FCC) currently envisions a circular accelerator with a circumference of 90 kilometers, which would collide electrons and positrons at energies of approximately 100–400 GeV.
That is fifty times less energy than the current largest accelerator on Earth, the 27-kilometer-wide LHC in Geneva, can deliver. However, electrons are point particles, which means the collisions will be cleaner and the conclusions more accurate than for protons in the LHC.
At first glance, that lower energy in a larger accelerator is a disappointment, says Rojo. “A few years ago, I was still skeptical about the FCC, because what could you do with fifty times less energy? We already know that energy range and don’t see any new physics there.”
This stands in stark contrast to the most common method for advancing particle physics: smashing particles with higher energy into each other, thereby revealing new, heavier particles. With limited energy, this means in practice that the experiments are primarily good at ruling out hypothetical particles. Discovering something new is a rarity.
But his earlier skepticism has completely vanished with this new study, says Rojo on behalf of his colleagues. They developed a systematic method to determine, for proposed accelerators, what the collisions actually reveal. And for the FCC-ee, the potential is far richer than assumed, the study shows.
“We just need to unlearn thinking in terms of bigger and higher energy for the discovery of new particles,” Rojo summarizes. “What the FCC-ee is likely to do exceptionally well is gather subtle clues for new physics. On one condition: that the theorists truly commit to that task and that experimenters accept that greater role for theory.”
Crucial to this reasoning is that in the world of particles, quantum laws apply, where quantities are not sharply defined but are diffuse. As a result, particles and effects that are beyond direct reach can still influence measurable particle processes. This can be captured in so-called effective field theories (EFTs).
Rojo calls the new study a plea for greater efforts to develop such theoretical techniques as far as possible. This is also the case within Nikhef, he says, where a strategy group has been considering a role in the arrival of a new accelerator since the beginning of this year. “I think Nikhef can play a leading role here. This is a signal to both theorists and experimentalists, who must jointly explore the full potential of the FCC.”
On May 22, the CERN Council will meet in Budapest to discuss the outcome of the European strategy discussion ESPP on the future of particle physics. If the CERN Council endorses the preference for an FCC electron accelerator, a new phase will begin in which actual funding and construction plans must be realized.
