Experimental study of the behavior of the Bjorken sum at very low Q2

We present new data on the Bjorken sum (Gamma) over bar (p-n)(1)(Q(2)) at 4-momentum transfer 0.021 <= Q(2) <= 0.496 GeV2. The data were obtained in two experiments performed at Jefferson Lab: EG4 on polarized protons and deuterons, and E97110 on polarized He-3 from which neutron data were extracted. The data cover the domain where chiral effective field theory (chi EFT), the leading effective theory of the Strong Force at large distances, is expected to be applicable. We find that our data and the predictions from chi EFT are only in marginal agreement. This is somewhat surprising as the contribution from the Delta(1232) resonance is suppressed in this observable, which should make it more reliably predicted by chi EFT than quantities in which the Delta contribution is important. The data are also compared to a number of phenomenological models with various degrees of agreement. (C) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

Automated summary

This study presents new data on the Bjorken sum. The data and predictions from chi EFT are marginally consistent, and the data are compared to various phenomenological models.