Conformal mapping of the Borel plane: going beyond perturbative QCD

TL;DR

This paper explores a conformal mapping approach to the Borel plane in QCD, proposing it as an alternative to the Operator Product Expansion for capturing nonperturbative effects, and demonstrates its potential for improved precision in strong coupling measurements.

Contribution

It introduces a conformal mapping method of the Borel plane in QCD, showing it can recover nonperturbative features and potentially replace the standard OPE.

Findings

Conformal mapping expansions contain an infinite number of terms when reexpanded in coupling.

These expansions exhibit nonperturbative features and are expected to be convergent.

Using this method, the strong coupling constant is estimated with improved accuracy.

Abstract

The power corrections in the Operator Product Expansion (OPE) of QCD correlators can be viewed mathematically as an illustration of the transseries concept, which allows to recover a function from its asymptotic divergent expansion. Alternatively, starting from the divergent behavior of the perturbative QCD encoded in the singularities in the Borel plane, a modified expansion can be defined by means of the conformal mapping of this plane. A comparison of the two approaches concerning their ability to recover nonperturbative properties of the true correlator was not explored up to now. In the present paper, we make a first attempt to investigate this problem. We use for illustration the Adler function and observables expressed as integrals of this function along contours in the complex energy plane. We show that the expansions based on the conformal mapping of the Borel plane go beyond finite-order perturbation theory, containing an infinite number of terms when reexpanded in powers of the coupling. Moreover, the expansion functions exhibit nonperturbative features of the true function, while the expansions have a tamed behavior at large orders and are expected even to be convergent. Using these properties, we argue that there are no mathematical reasons for supplementing the expansions based on the conformal mapping of the Borel plane by additional arbitrary power corrections. Therefore, we make the conjecture that they provide an alternative to the standard OPE in approximating the QCD correlator. This conjecture allows to slightly improve the accuracy of the strong coupling extracted from the hadronic $\tau$ decay width. Using the optimal expansions based on conformal mapping and the contour-improved prescription of renormalization-group resummation, we obtain $\alpha_s(m_\tau^2)=0.314 \pm 0.006$, which implies $\alpha_s(m_Z^2)=0.1179 \pm 0.0008$.