Determination of the QCD $\hat{I}›$-parameter and the accuracy of perturbation theory at high energies

Abstract

We discuss the determination of the strong coupling $\hat{I}{\pm }_{\overline{\mathrm{MS}}}^{}(m_{\mathrm{Z}})$ or equivalently the QCD $\hat{I}›$-parameter. Its determination requires the use of perturbation theory in $\hat{I}{\pm }_s(\hat{I}¼)$ in some scheme, $s$, and at some energy scale $\hat{I}¼$. The higher the scale $\hat{I}¼$ the more accurate perturbation theory becomes, owing to asymptotic freedom. As one step in our computation of the $\hat{I}›$-parameter in three-flavor QCD, we perform lattice computations in a scheme which allows us to non-perturbatively reach very high energies, corresponding to $\hat{I}{\pm }_s=0.1$ and below. We find that (continuum) perturbation theory is very accurate there, yielding a three percent error in the $\hat{I}›$-parameter, while data around $\hat{I}{\pm }_s\approx 0.2$ is clearly insufficient to quote such a precision. It is important to realize that these findings are expected to be generic, as our scheme has advantageous properties regarding the applicability of perturbation theory.

6.5 pages including figures and references; version accepted for publication in PRL