Determination of Lambda_QCD from the measured energy dependence of <1-Thrust>

Abstract

We directly fit the experimentally measured energy dependence of the average value of 1-Thrust, <1-T>, over the e^+e^- centre-of-mass energy range Q=14 - 172 GeV to the QCD expectation obtained by integrating up the evolution equation for d<1-T>/dlog Q in terms of <1-T>. We fit for Lambda_QCD, uncalculated O(alpha_S^3) and higher perturbative corrections parameterized by the scheme invariant rho_2, and the parameter lambda which characterizes the magnitude of the leading 1/Q power corrections anticipated for <1-T>. A 3-parameter fit yields Lambda_QCD=245^{+20}_{-17} MeV, rho_2=-16\pm 11 and lambda=0.27^{+0.12}_{-0.10} GeV, equivalent to alpha_S(M_Z)=0.1194 \pm 0.0014. In this approach, there is no error associated with the choice of the renormalization scale mu.

Figures (3)

• Figure 1: The average value of $1-T$ obtained experimentally datasiggi compared with the QCD expectation of eq. (\ref{['eq:intup']}). The solid line shows the fit to the data with $\rho_2 = \lambda = 0$ corresponding to $\Lambda_{\overline{MS}} = 266$ MeV. The long-dashed and short-dashed lines show the effect of altering $\Lambda_{\overline{MS}}$ by $\pm 30$ MeV.
• Figure 2: The average value of $1-T$ obtained experimentally datasiggi compared with the QCD expectation of eq. (\ref{['eq:intup']}). The dashed line shows the fit to the data with $\rho_2 = \lambda = 0$ while the result of the three parameter fit (to $\Lambda_{\overline{MS}}, ~\rho_2$ and $\lambda$) is shown as a solid line.
• Figure 3: The average value of the heavy jet mass obtained experimentally datasiggi compared with the QCD expectation of eq. (\ref{['eq:intup']}). The dashed line shows the fit to the data with $\rho_2 = \lambda = 0$ while the result of the two parameter fit (to $\rho_2$ and $\lambda$) using the value of $\Lambda_{\overline{MS}}$ obtained from the three parameter fit to $< \!\! 1-T \!\! >$ is shown as a solid line.