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(pi) = [27.1 +/- 2.2(stat + syst)(-2.4)(+2.8)(model)] x 10(-4) fm(4), differs significantly from theoretical estimates and suggests a new contribution from the nonperturbative spin structure of the proton. This delta(pi) value removes an apparent inconsistency in the difference of charge polarizabilities extracted from data above pi threshold. Our global result, <(alpha)over bar> - <(beta)over bar> = [10.11 +/- 1.74(stat + syst)(-0.86)(+1.22)(model)] x 10(-4) fm(3), agrees with the previous world average of data below 155 MeV. Our value for <(alpha)over bar> + <(beta)over bar> = [13.23 +/- 0.86(stat + syst)(-0.49)(+0.20)(model)] x 10(-4) fm(3) is consistent with a recent reevaluation of the Baldin sum rule.