New Showers with transverse-momentum-ordering

TL;DR

Problem: improve coherence and merging of parton showers with fixed-order matrix elements in PYTHIA. Approach: two $p_T$-ordered routines for timelike and spacelike emissions, with explicit definitions of the evolution variable $p_T^2$ and mappings to $M^2$ and $Q^2$, plus dipole recoil and backwards evolution mechanics and matrix-element corrections. Contributions: detailed kinematic schemes for both sectors and notes on parameter retuning (e.g., Lambda$_ ext{QCD}$) to accommodate the new ordering. Significance: offers a path toward better phase-space coverage and more consistent predictions for LHC processes such as $W+n$ jets.

Abstract

Two new showering routines are introduced, one for timelike final-state showers and one for spacelike initial-state ones. They are both based on emissions ordered in approximate transverse momenta that can easily be translated to virtualities. Matrix-element corrections can then be imposed as in the existing Pythia routines.