CP Polarimetry with Linearly Polarized Photon Fusion and Double-Tagged Protons

Abstract

Double forward-proton tagging turns the forward detectors into event-by-event photon polarimeters because each measured proton recoil fixes the transverse momentum, and hence the linear-polarization axis, of the emitted photon. We show that this production-side polarimetry gives a decay-analyzer-independent measurement of the CP phase of a photon-coupled spin-zero resonance. We derive the leading-power photon-density contraction for a CP-mixed hard amplitude with scalar and pseudoscalar couplings. The CP phase appears as a translation of the second harmonic in the signed proton--proton azimuthal angle. This provides a compact production-side CP measurement for axionlike particles and more general spin-zero resonances.