Speaker
Jean-Loic Kneur
(U)
Description
A variant of variationally optimized perturbation, incorporating renormalization group properties in a straightforward way, is used at three successive orders to calculate the nonperturbative ratio
$F_\pi/\Lambda$ of the pion decay constant and the basic QCD scale in the MSbar scheme. We demonstrate the good stability and (empirical) convergence properties of this modified perturbative series for this quantity. Using the experimental $F_\pi$ input value we determine
$\Lambda$ for $n_f=2$ and $n_f=3$ and estimate the theoretical uncertainties of the method.
Combining the $\Lambda(n_f=3)$ results with a standard perturbative evolution provides a new independent
determination of the strong coupling constant at various relevant scales, in particular $\alpha_S (m_Z)$, with an accuracy well comparable to recent single determinations of $\alpha_S$.
Primary author
Jean-Loic Kneur
(U)
