Program Files
 
 
The code is subdivided into a set of files, mainly by physics 
task. Each file typically contains one main class, but often 
with a few related helper classes that are not used elsewhere in 
the program. Normally the files come in pairs. 
 
- A header file, .hin theinclude/Pythia8subdirectory, where the public interface of the class is declared, 
and inline methods are defined.
- A source code file, .ccin thesrcsubdirectory, where most of the methods are implemented.
During compilation, related dependency files,.d, and 
compiled code, .o are created in the tmp 
subdirectory. 
 
 
In part the .xml documentation files in the 
xmldoc subdirectory have matching names, but the match 
is broken by the desire to group topics more by user interaction than 
internal operation. On these pages the function of the different code 
files is summarized. Currently, each .xml file is also 
translated into an .html one in the 
htmldoc subdirectory, to allow easy viewing of the 
contents in a web browser, and an .php one in 
phpdoc, for more sophisticated interactivity 
if that subdirectory is installed on a web server. 
 
 
Here is the current list of files, ordered alphabetically, with a brief 
description of contents. 
 
file   Analysis   
contains routines to analyze events. Currently it can do sphericity, 
thrust, Lund/Jade/Durham jet clustering, cone-jet finding, and the 
kT, anti-kT and Cambridge/Aachen algorithms for 
hadron colliders. 
   
 
file   Basics   
contains some basic facilities of general use: a random number 
generator Rndm, a four-vector class Vec4, and a 
histogram class Hist. 
   
 
file   BeamParticle   
contains information on all partons extracted from one of the two 
beams. Defines modified parton distributions accordingly during the 
showering and multiparton interactions processing, thereby extending on 
the one-particle-inclusive distributions defined by the previous class. 
Finds the internal structure for a beam remnant. 
   
 
file   BeamRemnants   
adds primordial kT to the set of hard subsystems, 
and combines these subsystems with the two beam remnants to provide 
the overall energy-momentum picture. Also ties together all the 
colour lines into consistent singlets. 
   
 
file   BeamShape   
generates momentum spread of beams, and spread of collision vertex 
position. 
   
 
file   BoseEinstein   
provides a simple method to impose Bose-Einstein correlations on 
pairs of identical mesons. 
   
 
file   ColourReconnection   
implements several models that describe how the colour flow between 
outgoing partons can be changed by nonperturbative effects. 
   
 
file   ColourTracing   
finds out how partons are colour-connected in (more or less long) chains. 
   
 
file   DeuteronProduction   
provides a simple method to produce deuterons and anti-deuterons from 
events containing protons, neutrons, anti-protons, and anti-neutrons. 
   
 
file   Event   
contains the event record, which basically is a vector of particles. 
This file also contains the Particle class, used by 
Event. Pythia uses two Event 
objects, one for the process-level record (process) and 
one for the complete event (event). 
   
 
file   FJcore   
contains the external FJcore package, providing the coure functionality 
of the FastJet jet fiding package. 
   
 
file   FragmentationFlavZpT   
contains the classes for describing the fragmentation steps in 
flavour and in longitudinal and transverse momentum. 
   
 
file   FragmentationSystems   
defines some containers of parton systems, for use in 
the fragmentation routines. 
   
 
file   HadronLevel   
turns the parton-level event above into a set of outgoing particles, 
by applying string fragmentation (with special treatment for low-mass 
systems) and secondary decays, and optionally Bose-Einstein corrections. 
   
 
file   HadronScatter   
contains the beginning of a machinery for final state hadronic 
rescattering. 
   
 
file   HelicityBasics   
basic classes for the handling of helicities in tau lepton decays. 
   
 
file   HelicityMatrixElements   
helicity-dependent decay matrix elements for the tau lepton. 
   
 
file   HiddenValleyFragmentation   
hadronization in models with a hidden sector that contains an unbroken 
SU(N), which gives confinement. The model and code is largely 
derived from the normal fragmentation classes. 
   
 
file   History   
methods to reconstruct the imagined shower history of a 
matrix-element-generated multiparton configuration, as part of 
the CKKW-L matrix element merging. 
   
 
file   Info   
is a simple container that gives access to some information on the 
nature of the current process, such as Mandelstam variables. 
Also contains a small database for errors and warnings encountered 
during program execution. 
   
 
file   JunctionSplitting   
processes a colour singlet system containing several (anti)junctions 
such that it can be split into several systems each containing 
(at most) one junction. 
   
 
file   LesHouches   
gives the possibility to feed in parton configurations for the 
subsequent event generation. One base class is defined, with containers 
for initialization and event information, that can be read from 
Pythia. Derived classes allow for a few different cases. 
   
 
file   LHAFortran   
is a header file only, for a class derived from the above LesHouches 
one, to be used for runtime interfacing to Fortran programs, such as 
PYTHIA 6. 
   
 
file   Merging   
Wpapper class to interface matrix element merging schemes with Pythia. 
   
 
file   MergingHooks   
intercede in the normal shower evolution to construct the relevant 
Sudakov form factor suppressions as part of the CKKW-L matrix element 
merging. 
   
 
file   MiniStringFragmentation   
performs string fragmentation in cases where the colour singlet 
subsystem mass is so small that one or at most two primary hadrons 
should be produced from it. 
   
 
file   MultipartonInteractions   
performs multiparton interactions. 
   
 
file   ParticleData   
contains a database of all necessary particle data (masses, names, ..) 
and decay channels. 
   
 
file   ParticleDecays   
performs the decays of all normal unstable hadrons and leptons, i.e. 
in mass up to and including b bbar systems. It is not 
intended for decays of electroweak resonances, like Z^0. 
   
 
file   PartonDistributions   
contains parton distribution functions for the proton and electron. 
Currently very simple, with only two p parametrizations 
and one e ditto available, but it is possible to link in 
external sets. 
   
 
file   PartonLevel   
turns the (hard) process above into a complete set of partons, by 
adding initial- and final-state radiation, multiparton 
interactions, and beam remnants. 
   
 
file   PartonSystems   
keeps track of which partons belong to which partonic subsystem, 
i.e. one of the multiparton (semi)hard interactions with associated 
showers. 
   
 
file   PhaseSpace   
selects a point in phase space for the hard-process generation, 
optimized separately for each process to give improved Monte Carlo 
efficiency. 
   
 
file   ProcessContainer   
packages the information on a given subprocess, combining the 
phase-space selection and cross-section evaluation machineries 
with some statistics information. Also sets up the list of processes 
to be studied in a run. 
   
 
file   ProcessLevel   
handles the generation of the (hard) process that sets the character 
of the event. This involves either using internally implemented 
processes or linking to Les Houches information. The latter can 
be by runtime interfaces or by reading in a file. This step also 
includes resonance decays. 
   
 
file   Pythia   
is the main class, that administrates the whole event generation 
process by making use of all the others classes. Objects of most 
other classes reside (directly or indirectly) inside Pythia, 
so only a Pythia object needs to be explicitly instantiated 
and addressed   by the user. 
   
 
file   PythiaComplex   
is only a .h file, containing a typedef for 
double precision complex numbers. 
   
 
file   PythiaStdlib   
contains most of the Stdlib headers used in PYTHIA 8, 
with using directives. It defines M_PI if 
this is not already done. Also a few simple inline methods: 
pow2(x), pow3(x), pow4(x), 
pow5(x) and pow6(x) for small integer 
powers, and sqrtpos(x) where a max(0., x) 
ensures that one does not take the square root of a negative number. 
The non-inlined string toLower(const string, bool trim = true) 
converts a strimg to lowercase, and by default also removes initial and 
trailing blanks or escape characters. It returns a new string, while the 
inlined extension void toLowerRep(string, bool trim = true) 
replaces the string in place. Finally the non-inlined 
GammaReal(x) gives the Gamma function value of 
a real argument. 
   
 
file   ResonanceDecays   
decays resonances as part of the hard-process stage, in many cases 
(but not all) including angular correlations between the decay products. 
   
 
file   ResonanceWidths   
encodes some properties of resonances, in particular the dynamic 
calculation of widths. 
   
 
file   RHadrons   
handles the production and decay of hadrons formed by long-lived 
gluinos, stops or sbottoms. 
   
 
file   Settings   
contains a database of all flags, modes, parameters and words that 
determine the performance of the generator. Initial values are obtained 
from the contents of the .xml files, but these values can 
then be changed by the user. 
   
 
file   SigmaCompositeness   
contains the cross sections and matrix elements for production of 
some particles in compositeness scenarios, specifically excited 
fermions. 
   
 
file   SigmaEW   
contains the cross sections and matrix elements for electroweak 
processes involving photons, Z^0's and W^+-'s. 
   
 
file   SigmaExtraDim   
contains the cross sections and matrix elements for processes in 
scenarios involving extra dimensions. 
   
 
file   SigmaGeneric   
contains the cross sections and matrix elements for some generic 
processes, to be used as building blocks for a few BSM scenarios. 
   
 
file   SigmaHiggs   
contains the cross sections and matrix elements for Higgs production. 
   
 
file   SigmaLeftRightSym   
contains the cross sections and matrix elements for particle production 
in left-right-symmetry scenarios, specifically righthanded Z 
and W bosons and doubly-charged Higgs bosons. 
   
 
file   SigmaLeptoquark   
contains the cross sections and matrix elements for leptoquark production. 
   
 
file   SigmaNewGaugeBosons   
contains the cross sections and matrix elements for a Z'^0, 
a W^+- and a horizontal gauge boson R^0. 
   
 
file   SigmaOnia   
contains the cross sections and matrix elements for charmonium and 
bottomonium production. 
   
 
file   SigmaProcess   
contains the base class and derived classes for the evaluation of 
different matrix elements. Also keeps track of allowed incoming 
parton configurations and their cross sections, including parton 
densities. In order to keep this file from becoming too big, actual 
cross sections are found in several separate files of derived classes: 
SigmaQCD, SigmaEW, SigmaOnia, 
SigmaHiggs, SigmaSUSY, 
SigmaNewGaugeBosons, SigmaLeftRightSym, 
SigmaLeptoquark, SigmaCompositeness, 
SigmaExtraDim and SigmaGeneric. 
   
 
file   SigmaQCD   
contains the cross sections and matrix elements for soft and hard 
QCD processes. 
   
 
file   SigmaSUSY   
contains the cross sections and matrix elements for Supersymmetric 
processes. 
   
 
file   SigmaTotal   
contains parametrizations of total, elastic and diffractive hadronic 
cross sections. 
   
 
file   SLHAinterface   
handles the communication between the SusyLesHouches 
classes and Pythia. 
   
 
file   SpaceShower   
performs spacelike initial-state transverse-momentum-ordered 
shower evolution. 
   
 
file   StandardModel   
contains the running alpha_strong, with Lambda 
matching at flavour thresholds, the running alpha_em, 
CKM mixing matrices, and a few other parameters such as 
sin^2(theta_W). 
   
 
file   StringFragmentation   
performs string fragmentation of a given set of partons. 
   
 
file   StringLength   
calculates the string-length lambda measure for various 
colour topologies, including junction ones. 
   
 
file   SusyCouplings   
stores the various couplings used for SUSY cross sections and 
decays, as calculated from input e.g. based on the SUSY Les Houches 
Accord. 
   
 
file   SusyLesHouches   
contains information on SUSY parameters and particle data as specified 
by the SUSY Les Houches Accord. 
   
 
file   SusyResonanceWidths   
encodes some properties of supersymmetric resonances, similar to 
what ResonanceWidths does for other resonances. 
   
 
file   TauDecays   
the main routines for handling tau lepton decays with helicity 
information. 
   
 
file   TimeShower   
performs timelike final-state transverse-momentum-ordered 
shower evolution. 
   
 
file   UserHooks   
Provides a way for a user to study the event at a few intermediate 
stages of evolution, to reject the event as a whole or to modify 
its cross-section weight. 
   
 
file   WeakShowerMEs   
provides some matrix elements used for ME corrections of W 
and Z weak gauge boson emission in both initial- and final-state 
parton showers.