QCD results from the study of hadronicZ-decays

Abstract

The high statistics data collected from hadronic Z-decays have lead to remarkable progress in the understanding of the dynamics of strong interactions. In addition to precision measurements of the strong coupling constant, the colour factors dening the gauge structure of QCD were determined with large accuracy. They are found to agree with the expectation for an unbroken SU(3) symmetry, t h us establishing unambigously the non-abelian nature of QCD. Higher order eects in many cases are known to next-to-leading logarithmic precision. Coherence eects as predicted in the framework of the modied leading-log approximation and local parton-hadron duality are experimentally well established. Studies of the hadronization process favour a chain-like production mechanism which bridges the gap between perturbative QCD as described by coherent parton shower models and the nal state hadrons. Submitted to Physica Scripta

Averaging correlated data

“…In order to calculate an error estimate for the average, the eective size of the correlations has to be known. Here it is inferred from the data, using the requirement that the (correlated) 2 for the average is equal to its expectation value. The robustness of the method has been demonstrated in averaging actual measurements of QCD parameters.…”

“…If all errors are truly gaussian and if also the correlations between the individual measurements are described correctly by a c o v ariance matrix C, the optimal procedure for nding the average a of a set of resultsx is obtained by minimizing the 2 …”

“…The results are based on an analysis of kinematical correlations in 4-jet events, and again the highly correlated theoretical errors are the dominant uncertainties. The data taken from [2] are listed in table 2 together with the uncorrelated and the correlated averages. Also here the presence of correlations between the results manifest itself through the small 2 of the uncorrelated average.…”

Jet-evolution in the quark–gluon plasma from RHIC to the LHC

“…(33). We fix the normalization C ≃ 0.024 of the multiplicity n(Q 2 ) = C exp(a 0 ) with LEP data from e + e − [27,28].…”

Jets in QCD

“…The issue of coherence has become paramount in describing e + e − data, and models that do not implement it, at least approximately, are completely ruled out (see for example (36) and references therein). They are also strongly disfavoured in DIS.…”