Reconciling the Contour-Improved and Fixed-Order Approaches for $\tau$ Hadronic Spectral Moments I: Renormalon-Free Gluon Condensate Scheme

TL;DR

This paper introduces a renormalon-free gluon condensate scheme that improves the convergence of perturbative expansions in $ au$ hadronic spectral moments, resolving longstanding discrepancies between fixed-order and contour-improved approaches.

Contribution

The authors develop a simple, perturbative subtraction-based scheme for a renormalon-free gluon condensate, applicable to various observables, and demonstrate its effectiveness in resolving FOPT-CIPT discrepancies.

Findings

The scheme resolves the FOPT-CIPT discrepancy in $ au$ spectral moments.

It improves the convergence of perturbative expansions with large non-perturbative corrections.

The scheme enhances the theoretical reliability of high-precision strong coupling determinations.

Abstract

We propose a simple and easy-to-implement scheme for a renormalon-free gluon condensate (GC) matrix element, which is analogous to implementations of short-distance heavy-quark mass renormalization schemes existing in the literature already for a long time. Because the scheme is based on a perturbative subtraction at the level of the matrix element, with a freely adaptable infrared factorization scale, it can be implemented with little effort for any observable where the GC is relevant. The scheme depends on the renormalon norm of the GC which has to be supplemented independently. We apply the scheme to the fixed-order (FOPT) and contour-improved (CIPT) perturbative expansions of $\tau$ hadronic spectral function moments. These expansions exhibit a long-standing discrepancy for moments used in high-precision determinations of the strong coupling in the commonly used GC scheme that is not renormalon-free. We show that the scheme is capable of resolving the FOPT-CIPT discrepancy problem. At the same time, the perturbative behaviour of the moments that previously showed bad convergence properties and for which the non-perturbative corrections from the GC are sizeable, is substantially improved. The new GC scheme may provide a powerful theoretical tool for future phenomenological applications.