PYTHIA and the preoccupied proton

TL;DR

This paper challenges the assumptions of the PYTHIA Monte Carlo model for high-energy proton collisions, proposing that proton interactions are exclusive and that the two-component model better explains observed data.

Contribution

It introduces the concept of a 'preoccupied proton' and argues against the multiple interactions assumption in PYTHIA, favoring a $p$-N exclusivity model supported by data analysis.

Findings

Glauber estimates of binary collisions are overestimated compared to TCM.

Data conflicts with the assumption that a proton can interact with multiple nucleons simultaneously.

Evidence supports the 'all or nothing' $p$-N collision model.

Abstract

The PYTHIA Monte Carlo (PMC) has been applied broadly to simulations of high-energy $p$-$p$ and $p$-$\bar p$ collisions. The PMC is based on several assumptions, such as that most hadrons result from jet production (multiple parton interactions or MPIs), that $p$-$p$ centrality is relevant and that color reconnection (CR) strongly influences fragmentation to jets. An alternative description is provided by the two-component (soft + hard) model (TCM) of hadron production. TCM analysis of $p$-Pb ensemble-mean-$p_t$ data reveals centrality trends quite different from those estimated via a geometric Glauber model based on the eikonal approximation. Glauber estimates of binary-collision number are three times TCM estimates. Detailed study of $p$-Pb data conflicts with a basic Glauber assumption -- that a projectile proton may interact simultaneously with multiple target nucleons. Instead, in both $p$-$p$ and $p$-A collisions, a $p$-N collision once initiated is exclusive of other possible interactions (during that collision), and within the collision any pair of participant partons may interact -- a $p$-N collision is thus "all or nothing." In this presentation the PMC is challenged by an assortment of contradictory data, and evidence for $p$-N exclusivity is reviewed to make a case for the "preoccupied proton" of the title.