# EPS HEP 2013 Stockholm

17-24 July 2013
KTH and Stockholm University Campus
Europe/Stockholm timezone

## $\alpha_S$ from $F_\pi$ and Renormalization Group Optimized Perturbation

18 Jul 2013, 10:00
15m
E1 (KTH Campus)

### E1

#### KTH Campus

Talk presentation QCD

### 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 $\alpha_S$.

 Slides