Gamma Factory: A New Experimental Paradigm for CERN's HL-LHC--FCC-ee Transition
The Gamma Factory (GF) proposal \cite{Krasny:2015ffb} is motivated by the recognition of a largely untapped potential of the CERN accelerator complex to enable a new research programme at the intersection of particle, nuclear, atomic, fundamental, and applied physics. The central concept is to produce, accelerate, cool, and store atomic beams of highly relativistic partially stripped ions in the LHC, which would serve as an effective atomic trap. The internal degrees of freedom of these ions are then resonantly excited using laser photons. In the GF scheme, laser-cooled atomic beams serve both as high-precision probes and as low-emittance beam sources for high-luminosity LHC operation in the ion-ion collision mode. Interactions between laser pulses stored in Fabry--Perot cavities and circulating ion beams give rise to high-energy, highly collimated, and polarised secondary photon beams. Their expected intensities exceed those of existing gamma-ray sources by several orders of magnitude. These photon beams can further be used to generate unprecedented-intensity, tertiary beams of polarised electrons, positrons, muons, neutrons, radioactive ions, and flavour- or CP-tagged neutrinos. Furthermore, under a specific configuration, the same photon-driven processes may be exploited in an energy-production scheme generating the requisite plug-power for LHC operation. Together, cold relativistic atomic beams, high-intensity photon beams, and tertiary beams constitute a versatile experimental platform capable of opening a wide range of new scientific opportunities at CERN. By exploiting existing accelerator infrastructure and available state-of-the-art laser technologies, the GF offers a path to a cost-effective and timely programme capable of sustaining experimental innovation and bridging the gap between the HL-LHC era and the future FCC era.