Higher order QCD effects and particle density in full phase space from highp T interactions at the ISR

Abstract A comparison of QCD parton models with events including a high transverse momentum trigger particle is performed. The data were obtained with the Split Field Magnet (SFM) detector at the CERN ISR. The effective intrinsic transverse momentum, 〈k T 〉, of a parton is found to be about 1 GeV/c from an analysis of inclusive cross sections based upon lowest order parton-parton scattering. Large values of 〈k T 〉 are also needed to obtain a good description of the particle density in the azimuthal hemisphere opposite to the trigger. The measured recoil of the spectator jets tends to require much smaller values of 〈k T 〉. The model is improved by including higher-order QCD effects. Thus, starting from 〈k T 〉=0.45 GeV/c, an effective 〈k T 〉 of about 1 GeV/c at hard scattering is generated through perturbative parton showers, which provides a consistent description of the measurements in full phase space. This analysis shows therefore that well-known problems of lowest-order QCD calculations can be resolved by inclusion of higher-order effects.