Active polarized target: measurements of spin polarizabilities of the proton

Static (electric and magnetic) polarizabilities of the proton are known from measurements of Compton scattering of real photons at low energies. However, no experimental data exist about spin polarizabilities which describe a response of the proton spin to a varying electromagnetic field. These fundamental structure constants could be obtained most model-independently from measurements of the double-polarized Compton scattering below the pion threshold using a polarized photon beam and a polarized proton target. Using an active polarized target (polystyrene with a scintillator dopant as a target material) allows one to detect recoil protons in the target material itself, thus excluding the background from coherent Compton effect on heavier nuclei (¹²C and others), and provides precision data on spin polarizabilities