Analysis of identified-hadron spectra from fixed-target $\bf p$-A collisions and the nature of the Cronin effect

Abstract

In this study fixed-target spectra obtained by the Chicago-Princeton (C-P) collaboration at Fermilab in the mid seventies are analyzed with a two-component spectrum model (TCM) that has been applied successfully to a number of collision systems at the RHIC and LHC in the past. It is from C-P data that the Cronin effect was first inferred. TCM analysis leads to factorization of collision-energy and target A dependences. Over the energy range of C-P data energy dependence is restricted to model-function shapes on $p_t$ whereas A dependence is restricted to particle densities for three hadron species and their antiparticles. A dependence for soft and hard components varying separately as power laws $A^{\alpha_s}$ and $A^{\alpha_h}$ with fixed exponents is a central finding of this study. The trends $A^{\alpha(p_t)}$ inferred by the C-P collaboration resulted from treating spectra as monolithic which confuses the distinct $p_t$ (model functions) and A (particle densities) dependences. The Cronin effect resulting from that confusion is easily explained in a TCM context. Power-law trends for pions at 25 GeV are quantitatively compatible with trends at LHC energies. The relation of exponents $a_s$ and $a_h$ to $p$-A centrality is examined in detail. A Glauber model of $p$-A centrality seems invalid.