The midpoint between dipole and parton showers

TL;DR

This paper introduces DIRE, a dipole-like parton-shower algorithm that orders emissions by a transverse-momentum variable in the soft limit, blending dipole coherence with DGLAP evolution and enabling a simple analytic structure. A Catani-Seymour-inspired modification of the splitting functions enables soft-anomalous-dimension consistency and allows a robust weight-based handling of negative splitting weights. The authors provide two independent public implementations (Pythia and Sherpa), validate the code with cross-checks, and demonstrate good agreement with LEP and LHC data, including merged calculations. The work offers a unifying framework that facilitates precise comparisons between showers and analytic resummations, and sets the stage for higher-order matching and improved qT resummation in event generation.

Abstract

We present a new parton-shower algorithm. Borrowing from the basic ideas of dipole cascades, the evolution variable is judiciously chosen as the transverse momentum in the soft limit. This leads to a very simple analytic structure of the evolution. A weighting algorithm is implemented, that allows to consistently treat potentially negative values of the splitting functions and the parton distributions. We provide two independent, publicly available implementations for the two event generators Pythia and Sherpa.

Figures (9)

• Figure 1: Kinematics in the initial-state parton splitting process $a\to \{aj\}j$.
• Figure 2: Validation in $e^+e^-\to\text{hadrons}$ and $\tau^+\tau^-\to[h\to\text{hadrons}]$
• Figure 3: Validation in $e^-q\to e^-q$ and $\tau^-g\to\tau^-g$
• Figure 4: Validation in $q\bar{q}\to e^+e^-$ and $gg\to\tau^+\tau^-$
• Figure 5: DIRE predictions in comparison to LEP data from Pfeifenschneider:1999rz.