First Extraction of a Spin-Polarizability of the Proton

A proton spin-polarizability characterizing backward Compton scattering has been extracted from a dispersion analysis of data between 33 and 309 MeV. This backward spin-polarizability, "delta" = 27.1 +/-2.2(stat+sys) +2.8/-2.4(model) in units of 10^-4 fm^4, differs significantly from theoretical estimates which are dominated by pi0 t-channel exchange, and indicates a new contribution from the non-perturbative spin-structure of the proton. This new value for "delta" removes an apparent inconsistency in the difference of charge polarizabilities extracted from data above pi-threshold. Our global result, alpha-beta = 10.11 +/-1.74(stat+sys) +1.22/-0.86 (model) in units of 10^-4 fm^3, is consistent with the previous world average of data below 155 MeV. Our value for alpha+beta = 13.23 +/-0.86(stat+sys) +0.20/-0.49(model) is consistent with a recent re-evaluation of the Baldin sum rule.