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, ${\delta }_{\pi }=[27.1\pm 2.2(\mathrm{stat}+\mathrm{syst}{)}_{-2.4\mathrm{}}^{+2.8\mathrm{}}\left(\mathrm{model}\right)]\times {10}^{-4\mathrm{}}{\mathrm{fm}}^4$, 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, $\overline{\alpha }-\overline{\beta }=[10.11\pm 1.74(\mathrm{stat}+\mathrm{syst}{)}_{-0.86\mathrm{}}^{+1.22\mathrm{}}\left(\mathrm{model}\right)]\times {10}^{-4\mathrm{}}{\mathrm{fm}}^3$, agrees with the previous world average of data below 155 MeV. Our value for $\overline{\alpha }+\overline{\beta }=[13.23\pm 0.86(\mathrm{stat}+\mathrm{syst}{)}_{-0.49\mathrm{}}^{+0.20\mathrm{}}\left(\mathrm{model}\right)]\times {10}^{-4\mathrm{}}{\mathrm{fm}}^3$ is consistent with a recent reevaluation of the Baldin sum rule.