XCC: an X-ray FEL-based Γγ Compton Collider Higgs Factory

This report describes the conceptual design of a γγ 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 √(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 γγ collider designs, the sharply peaked γγ 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 γγ→ HH is twice that of e^+e^- → ZHH at √(s)=500 GeV. Design challenges are discussed, along with the R&D to address them, including demonstrators.