Interplay of soft and hard processes and hadron pT spectra in pA and AA collisions

Motivated by a schematic model of multiple parton scattering within the Glauber formalism, the transverse momentum spectra in pA and AA collisions are analyzed in terms of a nuclear modification factor with respect to pp collisions. The existing data at the CERN Super Proton Synchrotron energies are shown to be consistent with the picture of Glauber multiple scattering in which the interplay between soft and hard processes and the effect of absorptive processes lead to nontrivial nuclear modification of the particle spectra. Relative to the additive model of incoherent hard scattering, the spectra are enhanced at large pT (hard) by multiple scattering while suppressed at low pT (soft) by absorptive correction with the transition occurring at around a scale p0∼1-2 GeV/c that separates soft and hard processes. Around the same scale, the pT spectra in pp collisions also change from an exponential form at low pT to a power-law behavior at high pT. At very large pT>>p0, the nuclear enhancement is shown to decrease as 1/pT2. Implications of these nuclear effects on the study of jet quenching, parton thermalization, and collective radial flow in high-energy AA collisions are discussed