Multiplicity Difference between Heavy and Light Quark Jets Revisited

A peculiar prediction of perturbative QCD, obtained within the Local Parton Hadron Duality (LPHD) framework, is that the multiplicity difference δQl between heavy and light quark jets produced in e+e- annihilation is energy independent. In the Modified Leading Logarithmic Approximation (MLLA) the corresponding constant is derived in terms of a few experimentally measurable quantities. While the energy independence of δQl has been successfully verified experimentally for b-quarks up to the highest LEP2 energy, its numerical prediction (δblMLLA = 5.5 ± 0.8) overestimates the experimental results. The work presented in this talk, done in collaboration with Yuri L. Dokshitzer, Valery A. Khoze and Wolfgang Ochs, shows that in the light of new experimental results and the improvement in the understanding of the experimental data, this prediction needs indeed a revision. We now find δbl = 4.4 ± 0.4, in better agreement with experiment, and we shaw that the remaining difference can be attributed largely to next-to-MLLA contributions, an important subset of which are identified and evaluated. The situation with charmed quarks is also reviewed