Higgs Physics with the X-Ray Free-Electron Laser Compton γγ Collider Concept

A dedicated γγ collider offers a physics program that both complements and extends a linear e⁺e⁻ machine. First, the Higgs boson is produced directly in the s-channel (unlike e⁺e⁻, which relies on the associated ZH process), sharpening sensitivity to its couplings. Second, photon beams allow precise control of initial-state polarization, enabling targeted CP studies. Third, di-Higgs production becomes accessible at a center-of-mass energy of ≈ 280 GeV (versus ≈ 550 GeV in e⁺e⁻), providing a direct probe of the Higgs self-interaction with sensitivity complementary to e⁺e⁻ running and to future hadron colliders with O(10) TeV partonic reach.

In this study, we investigate Higgs production in γγ collisions at a center-of-mass energy of 125 GeV using the X-ray Free-Electron Laser Compton Collider (XCC) concept, and present the first comprehensive single-Higgs at an XFEL-based e⁺e⁻ collider. Designed as a linear γγ Higgs factory, the XCC collides 62.6 GeV electron beams with 1 keV X-ray laser pulses to generate high-energy photons that produce the Higgs at threshold. In contrast to previous optical γγ concepts, recent advances in XFEL technology enable the use of X-ray photons, yielding a sharply peaked γγ center-of-mass spectrum and, in turn, significantly improved γγ → H yield. Our analysis methodology pairs a set transformer architecture operating on event-level particle point clouds with a genetic-algorithm optimizer for signal-background discrimination, achieving improved sensitivity over conventional approaches.

We report the projected sensitivities of σ(γγ → H) × Br(H → X) in all major hadronic, semi-leptonic, and leptonic final states, including H → ss̄, and compare their sensitivities with those of previously proposed optical γγ colliders as well as e⁺e⁻ → ZH. Our results demonstrate that an XFEL γγ collider can probe the Higgs sector with unprecedented high precision and enable new physics opportunities.