The strong interaction is not well understood at low energies or for interactions with low momentum transfer. Chiral perturbation theory gives testable predictions for the nucleonic generalized polarizabilities, which are fundamental quantities describing the nucleon’s response to an external field. We report a measurement of the proton’s generalized spin polarizabilities extracted with a polarized electron beam and a polarized solid ammonia target in the region where chiral perturbation theory is expected to be valid. The investigated structure function g2 characterizes the internal spin structure of the proton. From its moments, we extract the longitudinal–transverse spin polarizability δLT and twist-3 matrix element and polarizability d2¯. Our results provide discriminating power between existing chiral perturbation theory calculations and will help provide a better understanding of this strong quantum chromodynamics regime.
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We would like to thank the Hall A technical staff, polarized target group, and the accelerator operators for their efforts and dedication. We would also like to thank J. M. Alarcón, V. Bernard, P. Bosted, E. Christy, A. Deur, E. Epelbaum, F. Hagelstein, H. Krebs, S. Kuhn, V. Lensky, U.-G. Meißner and V. Pascalutsa for their very helpful discussion and suggestions on this publication. This work was supported by the Department of Energy (DOE) under grants DE-FG02-88ER40410 (funding the University of New Hampshire Nuclear Physics group, received by K.S.), DE-FG02-96ER40950 (funding the University of Virginia Polarized Target Group, received by D.D.) and DE-AC02-06CH11357 (funding the Argonne National Lab group, received by P.R.). The Southeastern Universities Research Association operates the Thomas Jefferson National Accelerator Facility for the DOE under contract DE-AC05-06OR23177.
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