Space-time geometry of small and large collision systems at LHC energies

Abstract

Identified-hadron (PID) 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 Pb-Pb TCM is adopted with minor changes from a recent analysis of PID hadron spectra from 5 TeV $p$-Pb collisions. The object of study is evidence for jet suppression in small and large collision systems as indicating quark-gluon plasma (QGP) formation there. Conventional methods have included Pb-Pb centrality determination via classical Glauber model and evidence for high-$p_t$ suppression sought via spectrum ratio $R_\text{AA}$. In the present study alternative geometry determination via ensemble-mean $\bar p_t$ data reveals that the number of participant nucleons in central Pb-Pb collisions is about 1/3 of the Glauber estimate. 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. The new results indicate that exclusivity 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. The effect on parton fragment (jet) distributions on $p_t$ is similar to projectile-proton fragment distributions on pseudorapidity from fixed-target $p$-A experiments where low-$\eta$ densities scale with A while high-$\eta$ densities are consistent with $p$-$p$ collisions. $p$-Pb and Pb-Pb spectra similarly analyzed reflect the same physics given different geometries. Jet suppression related to QGP formation is not evident.