- JLab E94-010 collaboration
- Amarian et al., Phys. Rev. Lett. 89, 242301 (2002…
Since the generalized GDH sum is, in principle, calculable at any Q, it can be used to study the transition from the partonic to hadronic degrees of freedom of the strong force. However, the validity domains for chiral perturbation theory (χPT) at low Q and pQCD calculations at higher Q used to calculate the GDH sum do not overlap. The Bjorken sum is the flavor non-singlet part of the GDH sum. This leads to simplifications that may help in linking the (χPT) validity domain to the pQCD validity domain . Hence the Bjorken sum appears as a key quantity to study the hadron-parton transition. We used data from the Thomas Jefferson National Accelerator Facility (JLab) [5,6,7] to extract the Bjorken sum from Q = 0.16 to 1.1 GeV  (Fig. 1 panel A). At low Q, we can compare to χPT calculations [9,10]. The data agree well with models [11,12] and also with Eq. 1 calculated to third order in αs and to leading twist. The agreement between the data and the leading twist calculation down to quite low Q indicates that overall higher twist effects  are small.