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 309MeV. This backward spin polarizability, {delta}{sub {pi}}=[27.1{plus_minus}2.2(stat +syst){sup +2.8}{sub {minus}2.4}(model )]{times}10{sup {minus}4} fm {sup 4} , differs significantly from theoretical estimates and suggests a new contribution from the nonperturbative spin structure of the proton. This {delta}{sub {pi}} value removes an apparent inconsistency in the difference of charge polarizabilities extracted from data above {pi} threshold. Our global result, {ovr {alpha}}{minus}{ovr {beta}}=[10.11{plus_minus}1 .74(stat+syst){sup +1.22}{sub {minus}0.86}(model)]{times}10{sup {minus}4} fm{sup 3} , agrees with the previous world average of data below 155MeV. Our value for {ovr {alpha}}+{ovr {beta}}=[13.23{plus_minus}0 .86(stat+syst){sup +0.20}{sub {minus}0.49}(model)]{times}10{sup {minus}4} fm{sup 3} is consistent with a recent reevaluation of the Baldin sum rule. {copyright} {ital 1998} {ital The American Physical Society}