Hadron production and QCD coherence at LEP

Abstract

Two analyses are presented based on the high statistics sample of hadronic Z0 decays recorded at LEP with the OPAL detector. First, Bose-Einstein correlation functions are studied in intervals of the average pair transverse momentum and of the pair rapidity, in order to study possible correlations between the pion production points and their momenta. The Yano-Koonin and the Bertsch-Pratt parameterizations are fitted to the measured correlation functions to estimate the space-time parameters of the source. The source rapidity is found to scale approximately with the pair rapidity, and both the longitudinal and transverse source dimensions decrease for increasing average pair transverse momenta, indicating that the source of pions expands during particle emission. In the second analysis, QCD coherence effects are studied measuring the correlations of particles with restricted transverse momenta. Due to colour coherence, the multiplicity distribution of particles with restricted transverse momenta is predicted to become Poissonian as the cut in transverse momentum decreases. The expected correlation pattern is observed for cuts down to ≈ 1 GeV but not at lower transvers momenta. This study indicates difficulties with the Local Parton-Hadron Duality (LPHD) hypothesis when applied to many-particle inclusive observables of soft hadrons.