22nd SPARC Topical Workshop

Resonant scattering of laser photons off relativistic ion beams can serve as an effective approach for the generation of high-energy photons, as the relativistic Doppler effect leads to a substantial frequency boost of the scattered photons. This idea is expected to be realized by the Gamma Factory project, supported by CERN’s Physics Beyond Colliders programme. The concept involves the interaction of laser photons with counter-propagating relativistic ion beams at the LHC, enabling the generation of high-intensity, narrow-band photons with energies of up to 400 MeV. Such a gamma-ray source would offer a broad range of applications in fundamental physics [1,2].
In this theoretical contribution, we investigate resonant photon scattering in the presence of external electromagnetic fields, which are strongly enhanced in a storage ring setup due to the high velocities of the ions. Using He-like calcium ions as a case study, our calculations show that realistic magnetic field strengths can substantially modify the total cross section, angular emission pattern, and polarization of the scattered photons. These findings point to potential applications within the Gamma Factory framework, such as the determination and calibration of the ion beam’s Lorentz factor [3].

[1] D. Budker, J. R. Crespo López-Urrutia, et al., Atomic physics studies at the Gamma Factory at Cern. Annalen der Physik, 532(8):2000204, 2020.
[2] D. Budker, J. C. Berengut, et al., Expanding Nuclear Physics Horizons with the Gamma Factory. Annalen der Physik, 534, 2100284, 2022.
[3] J. Richter, M. W. Krasny et al., Resonant photon scattering in the presence of external fields and its applications for the CERN Gamma Factory, Phys. Rev. A 111, 062820 (2025)