Nuclear modification factors for identified hadrons from 5 TeV $p$-Pb collisions and their relation to the Cronin effect

Abstract

Nuclear modification factors (NMFs) are spectrum ratios rescaled by an estimate of the number of binary N-N collisions $N_{bin}$ within an A-B collision. NMFs from more-central A-A collisions have been interpreted to indicate formation of a quark-gluon plasma (QGP) when compared with results from control $p$-A or $d$-A collisions. However, subsequent analyses of such control systems are now also interpreted to indicate QGP formation, calling into question proper interpretation of NMFs. An additional complication is the nature of the so-called ``Cronin effect'' contribution to NMF structure that is not well understood. In the present study a two-component model of hadron production (TCM) is applied to identified-hadron (PID) $p_t$ spectra from 5 GeV $p$-Pb collisions extending up to 20 GeV/c. Hard components (jet fragment distributions) are accurately isolated and their evolution with collision centrality parametrized. The TCM is then applied to NMFs without rescaling by $N_{bin}$, allowing direct comparisons between NMF evolution and hard-component evolution with centrality. To address the Cronin effect the TCM is applied to fixed-target $p$-A spectra from the Chicago-Princeton (C-P) collaboration, the origin of the Cronin effect. Inferred C-P spectrum hard components are quantitatively consistent with extrapolation of jet-related structure from higher energies. As a general conclusion spectrum ratios such as NMFs are difficult to interpret, whereas direct differential analysis of isolated spectra may be interpreted simply and accurately.