Systematic study of highpThadron spectra inpp,pA,andAAcollisions at ultrarelativistic energies

Abstract

High-${p}_{T}$ particle spectra in $p+p(\overline{p}+p),$ $p+A,$ and $A+B$ collisions are calculated in a parton model with QCD-inspired hard scattering and evolution in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic ${k}_{T}$ and its broadening in $p+A$ and $A+B$ collisions due to initial multiple parton scattering are found to be very important at low energies $(\sqrt{s}<50$ GeV). Comparisons with S+S, S+Au, and Pb+Pb data with different centrality cuts show that the differential cross sections of large ${p}_{T}$ pion production ${(p}_{T}>1\mathrm{GeV}/c)$ in $A+B$ collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This suggests that hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at ${p}_{T}\ensuremath{\sim}2--10\mathrm{GeV}/c.$ However, there is no evidence of jet quenching or parton energy loss. Assuming this model of parton scattering, nuclear broadening and parton energy loss, one can exclude an effective parton energy loss ${\mathrm{dE}}_{q}/dx>0.01$ GeV/fm and a mean free path ${\ensuremath{\lambda}}_{q}<7$ fm from the experimental data of $A+B$ collisions at the SPS energies. Predictions for high ${p}_{T}$ particle spectra in $p+A$ and $A+A$ collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed.