Evolution Effects in ... Fragmentation into Charmonium

Abstract

In Z 0 decay into prompt charmonium, i.e. charmonium not originating from B-meson decays, the most important contribution is expected to come from colour-octet mechanisms. However, previous fixed-order calculations of the colour-octet contribution contain large logarithms which, in a more complete treatment, should be resummed to all orders. We study this resummation by using a Monte Carlo QCD cascade model and find that the fixed-order colour-octet result is diminished by 15%. We compare the Monte Carlo calculations with results obtained by using analytical evolution equations. 1 Introduction The production of charmonium and bottomonium states in various processes, especially at high-energy colliders [1], has recently received considerable experimental and theoretical interest. New data have become available from p¯p [2], ep [3] and e + e \Gamma [4--6] colliders. Theoretically, it has been realized that quarkonium production at colliders is dominated by parton fragmentation ..