Some physics of small collision systems

TL;DR

Recent experimental results from small collision systems suggest the possible formation of a dense, flowing medium similar to quark-gluon plasma, challenging traditional interpretations and emphasizing the need for refined analysis methods.

Contribution

This paper critically examines conventional interpretations of small collision system data within a two-component model, highlighting that p-Pb collisions are simple superpositions of p-N interactions.

Findings

p-Pb collisions are linear superpositions of p-N collisions

N-N collisions in small systems follow simple, consistent rules

More insights into basic QCD in small systems are needed with improved analysis

Abstract

In recent years certain experimental results from small collision systems (e.g. p-p, d-Au, p-Pb) at the RHIC and LHC have been reinterpreted as evidence for formation therein of a dense flowing medium (QGP) despite small collision volumes. Systems that had been assigned as simple references (e.g. cold nuclear matter) for larger A-A collisions would then no longer play that role. This presentation examines conventional interpretations of certain data features in the context of a two-component (soft+hard) collision model. Specific topics include centrality determination for p-Pb collisions, interpretation (or not) of nuclear modification factors, significance of claims for strangeness enhancement, and interpretation of the "ridge" in p-p collisions. For p-p and p-Pb data analysis results indicate that p-Pb collisions are simple linear superpositions of p-N collisions, and N-N collisions within small systems generally follow simple and consistent rules. However, there is more to be learned about "basic" QCD in small systems with improved analysis methods.