Space-time Geometry of Small and Large Collision Systems

Abstract

Identified-hadron spectra from 2.76 TeV Pb-Pb and $p$-$p$ collisions are analyzed via a two-component (soft + hard) model (TCM) of hadron production in high-energy nuclear collisions. The object of study is evidence for jet suppression in small and large collision systems. Conventional methods include Pb-Pb centrality determination via classical Glauber model and evidence for high-$ p_t$ suppression sought via spectrum ratio $R_\text{AA}$. Previous $p$-Pb studies questioned the validity of the classical Glauber model. In the present study A-A geometry is determined instead via ensemble-mean $\bar p_t$ data. Based on certain features of Pb-Pb spectra the validity of the factorization assumption is also questioned. The entire jet contribution is therefore treated without factorization in ratio to a $p$-$p$ spectrum model as reference. These new results indicate that exclusivity (a nucleon may only interact with one nucleon ``at a time'') and time dilation (experienced by participant partons) play an essential role in jet production not incorporated in Glauber model or hard-component factorization. The combination determines an effective number of N-N collisions per participant nucleon given specific Pb-Pb centrality: multiple collisions if associated with low-$x$ (slow) partons, a single collision if associated with high-$x$ (fast) partons experiencing strong time dilation. The effect on parton fragment (jet) distributions on $p_t$ may be misinterpreted as jet suppression, but is similar to projectile-proton fragment distributions on pseudorapidity from fixed-target $p$-A experiments where low-$η$ densities scale with A while high-$η$ densities are consistent with $p$-$p$ collisions. $p$-Pb and Pb-Pb spectra similarly analyzed reflect the same physics given different geometries. Actual jet suppression related to QGP formation is not evident.