Interplay of soft and hard processes and hadron pT spectra in pA and AA collisions -: art. no. 034901

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 p(T) (hard) by multiple scattering while suppressed at low p(T) (soft) by absorptive correction with the transition occurring at around a scale p(0) similar to 1 - 2 GeV/c that separates soft and hard processes. Around the same scale, the p(T) spectra in pp collisions also change from an exponential form at low p(T) to a power-law behavior at high p(T). At very large p(T)>>p(0), the nuclear enhancement is shown to decrease as 1/p(T)(2). Implications of these nuclear effects on the study of jet quenching, parton thermalization, and collective radial flow in high-energy AA collisions are discussed.