XCC: An X-ray FEL-based $\gamma \gamma$ Compton Collider Higgs Factory

This report describes the conceptual design of a $\gamma \gamma$ Higgs factory in which 62.8 GeV electron beams collide with 1 keV X-ray free electron laser (XFEL) beams to produce colliding beams of 62.5 GeV photons. The Higgs boson production rate is 80,000 Higgs bosons per 10${}^7$ second year, roughly the same as the ILC Higgs rate at $\sqrt{s}$=250 GeV. The electron accelerator is based on cold copper distributed coupling (C${}^3$) accelerator technology. Unlike the center-of-mass energy spectra of previous optical wavelength $\gamma \gamma$ collider designs, the sharply peaked $\gamma \gamma$ center-of-mass energy spectrum of XCC produces model independent Higgs coupling measurements with precision on par with $e^{+}{e}^{-}$ colliders. For the triple Higgs coupling measurement, the XCC center-of-mass energy can be upgraded to 380 GeV, where the cross section for $\gamma \gamma \to HH$ is twice that of $e^{+}{e}^{-}\to ZHH$ at $\sqrt{s}$=500 GeV. Design challenges are discussed, along with the R&D to address them, including demonstrators.