Renormalon-based resummation of Bjorken polarised sum rule in holomorphic QCD

Abstract

Approximate knowledge of the renormalon structure of the Bjorken polarised sum rule (BSR) ${\overline \Gamma}_1^{{\rm p-n}}(Q^2)$ leads to the corresponding BSR characteristic function that allows us to evaluate the leading-twist part of BSR. In our previous work \cite{pPLB}, this evaluation (resummation) was performed using perturbative QCD (pQCD) coupling $a(Q^2) \equiv \alpha_s(Q^2)/\pi$ in specific renormalisation schemes. In the present paper, we continue this work, by using instead holomorphic couplings [$a(Q^2) \mapsto {\mathcal A}(Q^2)$] that have no Landau singularities and thus require, in contrast to the pQCD case, no regularisation of the resummation formula. The $D=2$ and $D=4$ terms are included in the Operator Product Expansion (OPE) of inelastic BSR, and fits are performed to the available experimental data in a specific interval $(Q^2_{\rm min},Q^2_{\rm max})$ where $ Q^2_{\rm max}=4.74 \ {\rm GeV}^2$. We needed relatively high $Q^2_{\rm min} \approx 1.7 \ {\rm GeV}^2$ in the pQCD case since the pQCD coupling $a(Q^2)$ has Landau singularities at $Q^2 \lesssim 1 \ {\rm GeV}^2$. Now, when holomorphic (AQCD) couplings ${\mathcal A}(Q^2)$ are used, no such problems occur: for the $3 \delta$AQCD and $2 \delta$AQCD variants the preferred values are $Q^2_{\rm min} \approx 0.6 \ {\rm GeV}^2$. The preferred values of $\alpha_s$ in general cannot be unambiguously extracted, due to large uncertainties of the experimental BSR data. At a fixed value of $\alpha_s^{{\overline {\rm MS}}}(M_Z^2)$, the values of the $D=2$ and $D=4$ residue parameters are determined in all cases, with the corresponding uncertainties.