The proposed Future Circular electron-positron Collider (FCC-ee), a massive new particle accelerator that could help scientists to uncover new information about the fundamental makeup of the universe, can also help spur innovation in other scientific areas, according to experts participating in a workshop at CERN this November.
The workshop gathered around 100 experts from diverse scientific fields, with the focus on the FCC-ee's vast potential beyond particle physics, exploring applications that could revolutionise photon science, material studies, biological research, positronium physics, nuclear science, and more.
The proposed FCC-ee would be based at the current CERN site, with the new near-100 km tunnel around three times the size of the current Large Hadron Collider (LHC). The proposal includes the first stage construction of the FCC-ee, which would begin operations in the 2040s, and the secondary phase FCC hadron-hadron (FCC-hh) beginning operations from the 2070s.
The ongoing FCC feasibility study, which was launched by the CERN Council to investigate the technical and financial viability of the project, is expected to conclude by early 2025.
Unveiling the FCC-ee's capabilities
The workshop, which took place on November 28 – 29 and was titled Other Science Opportunities at the FCC-ee Circular Collider, began with presentations on the FCC-ee's accelerator complex and beam parameters. Experts such as Hannes Bartosik from CERN, along with colleagues from CEA and DESY, discussed the collider's components. These include damping rings, injector linacs, boosters, and collider rings. They highlighted the achievable beam parameters in various operation modes and addressed limitations such as space charge effects, which are mitigated by the collider's high beam energy.
Revolutionising photon science
One of the most exciting prospects discussed was the FCC-ee's potential in photon science. Sara Casalbuoni from the European XFEL proposed profiting from the FCC-ee booster operation time at its 20 GeV injection energy to produce photon beams in the 50–200 keV range, with brilliances vastly surpassing existing sources. Using undulators at higher energies could extend photon energies into the tens or hundreds of MeV. Casalbuoni also suggested using the high-energy injector linac to drive a Free Electron Laser (FEL), generating intense photon pulses ideal for advanced imaging techniques.
Marco Stamponi from the Paul Scherrer Institute (PSI) emphasised that the FCC-ee's photon beams could push imaging technologies to new heights. Techniques such as high-energy time-resolved ptychographic imaging and scanning Compton X-ray microscopy would benefit, enabling studies of larger, heavier, or sensitive materials with unprecedented resolution.
High-energy photons through laser Compton scattering
Illya Drebot from Italy’s National Institute for Nuclear Physics (INFN) Milano discussed laser Compton backscattering applications at the FCC-ee, including polarimetry, bunch intensity control, and generating high-energy photons.
Laser Compton scattering off FCC-ee beams could produce photons at energies up to 100 GeV, vastly exceeding current facilities. This capability could open new research avenues in quantum chromodynamics (QCD) and nuclear physics.
Riccardo Negrello from the University of Ferrara explored using the FCC-ee's unique 20 GeV positron beam with a crystalline undulator to produce photons in the 5–50 MeV range. Armen Apyan from the A. Alikhanyan National Laboratory highlighted coherent bremsstrahlung in crystals as a method to generate high-energy, linearly polarized photons and polarised positrons, intensifying at higher energies.
Paolo Crivelli from ETH Zürich evaluated scenarios for dark matter searches using FCC-ee injectors, suggesting that slow extraction from the damping ring could explore new parameter spaces. Another option involves using the booster as a stretcher ring. Ivo Schulthess from DESY advocated probing strong-field quantum electrodynamics (QED) through electron-laser interactions, using the FCC-ee to benchmark theoretical models and explore new physics with intense beamstrahlung photons.
Laura Bandiera from INFN Ferrara proposed utilising intense electromagnetic fields in crystals to explore strong-field QED. Directing high-quality 20 GeV beams through crystals could open new research areas, including studying electron and positron anomalous magnetic moments. Her team is also investigating crystal-based positron sources for the FCC-ee.
Luca Serafini from INFN Milano discussed full inverse Compton scattering, where 255 keV photons collide with high-energy electrons to yield photons at the particle's full energy. This process enables precise photon energy boosts, with potential applications in accelerators and astrophysics. The extreme acceleration involved could lead to the emission of Unruh radiation at temperatures around 100 million Kelvin—a phenomenon yet to be experimentally confirmed.
Advancing positron applications
The FCC-ee's intense positron source opens doors for positron applications. Benjamin Rienäcker from the University of Liverpool discussed efforts to create a positronium Bose-Einstein condensate (BEC), requiring positronium densities currently unattainable. The FCC-ee could provide the necessary positron flux, potentially leading to a gamma-ray laser emitting coherent 511 keV photons.
Marcel Dickmann emphasised positrons as ideal probes for studying material defects, with applications across various industries. Antoine Camper from the University of Oslo highlighted experiments requiring dense positron clouds, suggesting methods to maximize positron rates using complete deceleration techniques.
Multipurpose applications: Radionuclide production and neutron sources
The workshop also covered the FCC-ee's potential for producing medically relevant radionuclides. Charlotte Duchemin from CERN assessed the production of isotopes like Ra-225/Ac-225 and Mo-99/Tc-99m, with yields surpassing current facilities. This could provide alternatives as nuclear reactors phase out, impacting medical diagnostics and treatments.
Frank Gunsing from the University of Paris-Saclay discussed using the FCC-ee's electron beam to drive a neutron source, potentially succeeding CERN's n_TOF facility. This would ensure the continuity of crucial neutron research, contributing significantly to experimental nuclear data.
The workshop concluded with an animated discussion. A short pulse neutron source for n_TOF-type nuclear physics could be based on the FCC-ee electron linac. Both this linac-driven neutron source and also beamstrahlung photonuclear production of neutrons could be used for activation measurements (Maxwellian average stellar spectra).
For radionuclides the emphasis can be on R&D, on production, and on comparison with the production at reactors. The focus shall be on where the production at FCC-ee would be unique, e.g. the desired cross-sections for photonuclear reactions at high energies could be obtained at the FCC-ee. Using beamstrahlung for photon science and for new physics searches both require further follow up.
Conclusions and future directions
The workshop highlighted the FCC-ee's vast potential beyond its primary mission. Operating the booster as a light source at various energies could enable groundbreaking studies like exploring pygmy resonances. The collider could test theoretical models in strong-field QED and facilitate studies in photon-photon scattering. The intense positron source opens up the possibility of forming a positronium Bose-Einstein condensate, potentially leading to a gamma-ray laser. Utilising FCC-ee injectors and beam dumps could explore new dark matter parameter spaces, complementing existing experiments. Moreover, the FCC-ee offers unique capabilities for producing critical radionuclides, impacting medicine and research as nuclear reactors phase out.
The collaborative efforts showcased the importance of interdisciplinary research in unlocking the FCC-ee's full potential. All presentations and discussions are available on the workshop's website: Other Science Opportunities at FCC-ee. Four topical expert groups will continue working over the next months along the directions identified during the meeting.
An upcoming event, "Storage Rings and Gravitational Waves" (SRGWmb2025), taking place on February 10 and 11 2025 will explore using the FCC-ee for gravitational wave detection. Details are at SRGWmb2025.