Determination of event shape distributions and α ^b _s from$Z^0 \to b\bar b$ events at LEP

Abstract

In hadronic Z⁰ decays collected by the OPAL experiment at LEP, event shape variables such as jet rates, jet masses, thrust and the energy-energy correlation for$Z^0 \to b\bar b$ events are compared to those for all flavours using secondary vertex information to tag the b quarks. The measured distributions are found to be well described by anO"(α _s ) calculation for heavy quarks as well as by parton shower simulations. We also determine the ratio of the strong coupling constant for b quarks and all quarks, α ^b _s /α ^incl _s , from these distributions. We find

$$\alpha _s^b /\alpha _s^{incl} = 0.994 \pm 0.005_{ - 0.012}^{ + 0.010} ,$$

where the errors are the statistical and systematic errors. The result can be converted into the ratio for b quarks relative to the complementary flavours udsc α ^b _s /α ^udsc _s =0.992±0.007 ^+0.013_-0.015 .