Coherent parton showers versus matrix elements - implications of PETRA/PEP data

Abstract

Traditionally, matrix elements have been used to provide the probability for multijet configurations in e+e− annihilation events. Some serious discrepancies between PETRA/PEP data and event simulation programs based on this approach have been discovered by the JADE and Mark II Collaborations. Instead we develop here a new coherent parton shower algorithm, well matched on to the three-jet matrix element, which provides a better description of existing data, in line with the agreement previously obtained between data and the Marchesini-Webber coherent parton shower program. A very low cutoff for the patron shower evolution, in the order of 1 GeV, seems indicated. The conclusion is that events with many partons, more than are accessible with present-day matrix elements, play an important role already at CM energies of 30 GeV. For extrapolations to higher energies, like TRISTAN/SLC/LEP, parton showers therefore offer more reliable predictions.