Apacic++ 2.0, A Parton Cascade In C++

TL;DR

APACIC++ 2.0 enhances SHERPA's parton shower by enabling both initial- and final-state evolution and by providing improved merging with multi-leg tree-level matrix elements through a k_perp-based history reconstruction. The implementation relies on a virtuality-ordered shower with Sudakov form factors, backward evolution for incoming partons, and mass- and coherence-aware kinematics, all organized around a Tree/Knots data structure. The paper validates the approach against analytic Sudakov resummations and LEP1/Tevatron data, showing good agreement for jet rates, event shapes, and hadron-level observables after hadronisation. The work demonstrates robust ME-PS merging up to multiple jets and confirms APACIC++ 2.0 as a stable, integral shower module within SHERPA, suitable for realistic collider simulations.

Abstract

The new version of the parton shower module APACIC++ for the SHERPA event generator framework is presented. It incorporates some features, that are specific for the consistent merging with multi-particle matrix elements at tree-level. This publication also includes some exemplary results and a short description of the upgraded class structure of APACIC++, version 2.0.

Figures (20)

• Figure 1: The basic building blocks of shower emission are binary branchings, represented by Knots.
• Figure 2: Sketch of the mapping between radiation processes and the corresponding classes. The full radiation pattern is identified as a chain of $1\to 2$ processes, a Markov chain, which translates into the class Tree. A Tree in turn is realised as a list of linked Knots. The shower evolution of an event is represented by three Trees, one for the final state shower, and two for the initial state shower.
• Figure 3: Relation of the main classes of APACIC++.
• Figure 4: Cross section for the production of up to four jets (left) and the corresponding Sudakov weight that will be attached in the merging procedure (right) vs. the jet resolution $y$. Two/three/four jet topologies are depicted in red/green/blue, massless jet-configurations are shown with solid lines, jet configurations involving two massive $b$ quarks with $m_b=4.5$ GeV are shown with dashed lines.
• Figure 5: Cross section times Sudakov weight vs. the jet resolution $y$ for each jet configuration and the sum of them. The left plot shows massless jets, the right one displays jet configurations with two massive $b$ quarks.