QCD coherence and correlations of particles with restricted momenta in hadronic Z decays

QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, p_T < p_T^cut, where p_T is defined with respect to the thrust axis, or restricted absolute momenta, p ≡ | p | < p^cut, using about four million hadronic Z decays recorded at LEP with the OPAL detector. The correlations are analyzed in terms of normalized factorial and cumulant moments. The analysis is inspired by analytical QCD calculations which, in conjunction with Local Parton-Hadron Duality (LPHD), predict that, due to colour coherence, the multiplicity distribution of particles with restricted transverse momenta should become Poissonian as p_T^cut decreases. The expected correlation pattern is indeed observed down to p_T^cut ≈ 1   GeV but not at lower transverse momenta. Furthermore, for p^cut → 0   GeV a strong rise is observed in the data, in disagreement with theoretical expectation. The Monte Carlo models reproduce well the measurements at large p_T^cut and p^cut but underestimate their magnitudes at the lowest momenta. The e⁺ e^- data are also compared to the measurements in deep-inelastic e⁺ p collisions at HERA. It is shown that for soft particles, the often assumed equivalence of a single hemisphere in e⁺ e^- annihilation with the current region in the Breit frame of a deep-inelastic collision may be misleading. Our study indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.