2026/06/03 by A. C. Canbay, S. Sultansoy, F. Zimmermann
Physics and Astronomy · #hep-ex #hep-ph #physics.acc-ph #physics.ins-det
We present the design of a high-energy polarized gamma-ray facility based on Compton backscattering (CBS) of laser pulses off the FCC-ee full-energy booster beams in the Z, WW, ZH and tt modes. Saturating the safe value of the kinematic parameter κ=4.35 fixes the laser wavelength in each mode and yields backscattered photons up to ωmax=148 GeV. The conversion point operates on the booster cycle structure: fully parasitically on the 0.1 s top-up flat-tops (f\rm CBS=10-8 per bunch crossing, cumulative electron loss 3×10-6 per cycle) or, as the baseline, in dedicated extended-flat-top fills inside the idle windows between top-up cycles (f\rm CBS=10-6, beam loss below 1% per cycle). The collider luminosity is unaffected in both scenarios, and the operational laser pulse energies lie in the sub-millijoule to few-millijoule range. Photon selection is performed event-by-event with a pair spectrometer on the high-energy Compton edge; for the unpolarized booster beam the Compton polarization transfer limits the achievable band-averaged circular polarization, and the selection is set to |⟨ S2⟩|=0.90. We project the sensitivity to the polarized gluon distribution Δg(x) via open-charm photoproduction γp→ ccX on an NH3 dynamically polarized target, including NLO QCD corrections via K-factors and propagating polarized-PDF uncertainties through the 100 Monte Carlo replicas of NNPDFpol2.0. The projected total precision is δ(Δg/g)\rm tot≃ 1.8-3.0×10-2, a factor of 4-7 below the total uncertainty of the most precise existing direct measurement (HERMES), at four values of ⟨ x⟩ in the medium-x region 0.07≤ x≤ 0.19. The facility would set the dominant constraint on Δg in this region, complementary to the low-x reach of the Electron-Ion Collider.