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

TL;DR

This study investigates QCD coherence effects in hadronic Z decays by analyzing particle correlations with restricted momenta, testing theoretical predictions, and comparing experimental data with Monte Carlo models and deep-inelastic scattering results.

Contribution

It provides experimental evidence on particle correlations with restricted momenta, challenging the LPHD hypothesis and testing QCD predictions in a high-statistics dataset.

Findings

Correlations follow QCD predictions down to pTapprox 1 GeV

Discrepancies observed at lower transverse momenta and for pcut to 0 GeV

Monte Carlo models underestimate correlations at low momenta

Abstract

QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, pT<pTcut, where pT is defined with respect to the thrust axis, or restricted absolute momenta, p equiv |p| < pcut, 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 pTcut decreases. The expected correlation pattern is indeed observed down to pTcut approx 1GeV but not at lower transverse momenta. Furthermore, for pcut to 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 pTcut and pcut but underestimate their magnitudes at the lowest momenta. The e+e- data are also compared to the measurements in deep-inelastic e+p collisions. Our study indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.