Comparing the PYTHIA Monte Carlo to a two-component (soft + hard) model of hadron production in high-energy p-p collisions

Abstract

The PYTHIA Monte Carlo (PMC), first introduced more than thirty years ago, remains a popular simulation tool both for analysis of $p$-$p$ collision dynamics and for detector design and calibration. The PMC assumes that almost all produced hadrons result from parton-parton scatterings (interactions) described by pQCD (a hard component), and that multiple parton interactions per collision event (MPIs) are a common occurrence. In contrast, a two-component (soft + hard) model (TCM) of high-energy collisions, inferred inductively from a variety of data formats, attributes a majority of final-state hadrons to a soft component (projectile-nucleon dissociation) and a minority to a hard component representing minimum-bias dijet production (corresponding to measured jet spectra and fragmentation functions). The hard-component hadron yield is precisely proportional to the square of the soft-component yield over an interval corresponding to 100-fold increase in dijet production. The two data descriptions appear to be in conflict. This study presents a detailed comparison of the two models and their relations to a broad array of collision data. The PMC appears to disagree with some data, whereas the TCM provides an accurate and comprehensive data description.