/ Phenomenology-HEP arXiv:0808.0066

Bjorken Sum Rule and pQCD frontier on the move

Pasechnik, Roman S. (Dubna, JINR) ; Shirkov, Dmitry V. (Dubna, JINR) ; Teryaev, Oleg V. (Dubna, JINR)

Published in: Phys.Rev.
Year: 2008
Vol.: D78
Page No: 071902
Pages: 9

Abstract: We study the interplay between higher orders of perturbative QCD (pQCD) expansion and higher twist contributions by analyzing recent $0.1 <Q^2 < 3 {\rm GeV}^2$ Jefferson Lab data on the Generalized Bjorken Sum Rule function $\Gamma_1^{p-n} (Q^2)$. The inclusion of the higher-order pQCD corrections is absorbed, with good numerical accuracy, by change of the normalization of the higher-twist terms. {\it Item}, we found that the 'denominator' form of the two-loop effective (running) coupling constant is more suitable at low $Q^2$ than the standard 'PDG' one. At the same time, the inclusion of the third and fourth loops reduces the quality of the fit, which could be a signal of the asymptotic character of the pQCD series at $\alpha_s \sim 0.3-0.4$. Its convergence may be improved by making use of the ghost-free Analytic Perturbation Theory (APT). The values of the higher-twist coefficients extracted using APT approach provide better convergence of the higher-twist series than the standard pQCD. As a main result, we achieved a good quantitative description of the data down to $Q\simeq$ 350 MeV.

Web-Page: http://inspirebeta.net/record/792073/files/Graph-Q.png; http://inspirebeta.net/record/792073/files/Graph-Q_dual.png; http://inspirebeta.net/record/792073/files/Graph-Q_loops.png

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