Parton Distributions and Event Generators

TL;DR

This work integrates the NNPDF2.3QED LO parton set into the Pythia8 event generator to enable LO QCD⊗QED evolution with a photon PDF. It constructs the LO PDF set by combining NNPDF2.1 LO with the NLO photon PDF at a low scale and evolves it with LO QCD⊗QED, enforcing positivity and testing two values of $\alpha_s(M_Z)$. Implemented as an internal PDF set in Pythia8, the study shows that photon-initiated contributions to dilepton production are tiny near the Z peak but become substantial at high dilepton masses, highlighting their importance as backgrounds in new physics searches and their potential use to constrain the photon PDF. The work also provides practical access to the set via NNPDF, LHAPDF interfaces, and internal Pythia8 distributions, aiding accurate modeling of hadron-collider processes and underlying-event tunes.

Abstract

We present the implementation within the Pythia8 event generator of a set of parton distributions based on NNPDF methodology. We construct a set of leading-order parton distributions with QED corrections, NNPDF2.3QED LO set, based on the same data as the previous NNPDF2.3 NLO and NNLO PDF sets. We compare this PDF set to its higher-order counterparts, we discuss its implementation as an internal set in Pythia8, and we use it to study some of the phenomenological implications of photon-initiated contributions for dilepton production at hadron colliders.

Figures (6)

• Figure 1: The gluon PDF at LO, NLO and NNLO in the NNPDF2.3QED sets.
• Figure 2: The photon PDF at LO, NLO and NNLO in the NNPDF2.3QED sets.
• Figure 3: The small-$x$ gluon PDFs in the NNPDF2.3QED LO set for $\alpha_{s}=0.119$ and 0.130.
• Figure 4: Invariant mass distributions of the dilepton pair at the LHC 14 TeV, computed with NNPDF2.3QED LO and Pythia8. The contribution from the $q\bar{q}$ and $\gamma\gamma$ initiated subprocess are separately shown. No kinematical cuts have been applied.
• Figure 5: Same as Fig. \ref{['figPythia8']} but now in the high dilepton mass region. Realistic kinematical cuts have been applied to the events, see text.