Extracting the spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable Σ 2 z has been measured for the first time in the Δ ( 1232 ) resonance region using a circularly polarized photon beam incident on a longitudinally polarized proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γ E 1 E 1 = − 2.87 ± 0.52 , γ M 1 M 1 = 2.70 ± 0.43 , γ E 1 M 2 = − 0.85 ± 0.72 , and γ M 1 E 2 = 2.04 ± 0.43 , in units of 10 − 4 fm 4 .