Exploring New Frontiers: Unlocking the FCC-ee’s Potential Beyond Particle Physics

Experts convene at CERN to unveil groundbreaking applications for the Future Circular electron-positron Collider.

On November 28–29, 2024, CERN hosted a pivotal workshop that brought together around 100 experts from diverse scientific fields, both in person and online. The focus was the Future Circular electron-positron Collider (FCC-ee), a proposed accelerator designed to be a powerhouse for Higgs, electroweak, and top quark studies. However, this workshop delved into the FCC-ee’s vast potential beyond particle physics, exploring applications that could revolutionize photon science, material studies, biological research, positronium physics, nuclear science, and more. The workshop reviewed the activities of four working groups, namely: (1) Photon and photon science applications, coordinated by John Byrd (formerly ANL), Sara Casalbuoni, and Frank Zimmermann; (2) HEP applications, convened by Gianluigi Arduini, Jörg Jäckel (U. Heidelberg), and Gunar Schnell (Basque U., Bilbao); (3) positron applications, coordinated by Benjamin Rienäcker with Michael Doser (CERN); and (4) multipurpose applications of the electron/positron beams and beamstrahlung photons, guided by Marco Calviani.

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 PSI emphasized that the FCC-ee’s photon beams could push imaging technologies to new heights. Techniques like 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.

This figure presents the peak photon brilliance in the 10-200 keV energy range at the FCC-booster, compared with the EuXFEL, proposed EuXFEL upgrade, and planned PETRA IV, as shown by Sara Casalbuoni (EuXFEL).

High-Energy Photons through Laser Compton Scattering

Illya Drebot from INFN Milano discussed laser Compton backscattering applications at the FCC-ee, including polarimetry, bunch intensity control, and generating high-energy photons. Frank Zimmermann predicted that 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 polarized 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 utilizing 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 emphasized 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. Utilizing 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.The four established groups will continue working over the next months along the directions identified during the meeting.
An upcoming event, “Storage Rings and Gravitational Waves” (SRGWmb2025), will explore using the FCC-ee for gravitational wave detection. Details are at SRGWmb2025.