A Comparison of new MC-adapted Parton Densities

TL;DR

This paper evaluates how including MC-adapted PDFs in Pythia8 affects hadron-collider simulations and their agreement with CDF Run 2 data. It compares ten PDFs across minbias and hard QCD observables to identify where MC-adapted sets improve or degrade performance, and discusses technical issues related to grid extrapolation and momentum-sum rules. The results show that MC-adapted PDFs often yield closer agreement to data than ordinary LO/NLO PDFs, particularly in the small-$x$ gluon region, but no single PDF universally wins and NLO PDFs can be problematic in LO MCs. The study highlights the importance of careful tuning (e.g., $p_{ perp 0}^{Ref}$) and motivates broader tuning and validation for LHC-era simulations.

Abstract

A selection of the latest and most frequently used parton distribution functions (PDFs) is incorporated in Pythia8, including the Monte Carlo-adapted PDFs from the MSTW and CTEQ collaborations. This article examines the differences in PDFs as well as the effect they have on results of simulations and compare with data collected by the CDF experiment. Monte Carlo-adapted PDFs do a better job than leading- and next-to-leading order PDFs for many observables, but there is room for further improvements.

Figures (11)

• Figure 1: Up quark (a, b) and gluon (c, d) distributions at $Q^2=4$ GeV$^2$. Note the difference in horizontal and vertical scales
• Figure 2: Up quark (a, b) and gluon (c, d) distributions at $Q^2=10^{3}$ GeV$^2$. Note the difference in horizontal and vertical scales
• Figure 3: Rapidity distributions of the partons created in the $2 \rightarrow 2$ sub-process
• Figure 4: Rapidity distributions of charged particles after hadronization at minbias simulations with $E_{CM}=1960$ GeV
• Figure 5: Charged particle multiplicity distributions