APACIC++, A PArton Cascade In C++, version 1.0

TL;DR

APACIC++ presents a novel C++ Monte Carlo framework for e+e- jet production that merges arbitrary matrix elements with parton showers, enabling consistent multijet simulations over a wide energy range. It implements two shower ordering schemes (virtuality and angle) and Lund-string hadronization via Pythia, with initial-state radiation modeled by structure functions and interfaces to multiple external ME generators. The paper details a rich program structure (apacic, cascade, tree, n_sudakov, dsec, hadron) and a flexible ME integration approach (xs_sum, xsee, xsee_tools) to achieve a coherent jet evolution and fragmentation pipeline, including vectorized data management and HEPEVT compatibility. It also provides installation and run guidelines, test-run outputs, and notes future work toward ISR refinement, gamma-related processes, cluster fragmentation, and hadron-hadron collisions.

Abstract

APACIC++ is a Monte-Carlo event-generator dedicated for the simulation of electron-positron annihilations into jets. Within the framework of APACIC++, the emergence of jets is identified with the perturbative production of partons as governed by corresponding matrix elements. In addition to the build-in matrix elements describing the production of two and three jets, further programs can be linked allowing for the simultaneous treatment of higher numbers of jets. APACIC++ hosts a new approach for the combination of arbitrary matrix elements for the production of jets with the parton shower, which in turn models the evolution of these jets. For the evolution, different ordering schemes are available, namely ordering by virtualities or by angles. At the present state, the subsequent hadronization of the partons is accomplished by means of the Lund-string model as provided within Pythia. An appropriate interface is provieded. The program takes full advantage of the object-oriented features provided by C++ allowing for an equally abstract and transparent programming style.

Figures (9)

• Figure 1: Scetch of an $e^+e^-$--annihilation into jets. The wiggly lines represent the photons of the initial state radiation, the thick shaded blob stands for the hard subprocess, here the production of three jets. The secondary parton radiation accounts for the inner--jet evolution, whereas the fragmentation is indicated by the ellipses with further hadronic decays indicated.
• Figure 4: Scetch 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 Markhov chain, which translates into the class tree. The basic building blocks, the binary decays, in turn are realized with knots. Thus a tree contains a list of linked knots.
• Figure 5: Scetch of the mapping between the hard cross sections as provided within xsec and the further treatment of the final state via tree. The translating class xsee includes their derivatives, too. In fact these derivatives contain the interfaces to the matrix element generators yielding the cross sections. The interfaces are organized as a list within xsec.
• Figure 6: Scetch of the interplay of the fundamental classes when running APACIC++. Depicted are the two central classes of APACIC++ , apacic and cascade, together with their most important methods and how they cooperate. The communication and relationships of the two steering classes with the other main parts of APACIC++ are indicated, and the methods of the other classes responsible for the contact with apacic and cascade are shown.
• Figure 7: Scetch of the interplay of classes governing the initialization, evaluation and running of cross sections for the hard subprocess.