<chapter name="Program Files"> 
 
<h2>Program Files</h2> 
 
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. 
<ul> 
<li>A header file, <code>.h</code> in the <code>include/Pythia8</code> 
subdirectory, where the public interface of the class is declared, 
and inline methods are defined.</li> 
<li>A source code file, <code>.cc</code> in the <code>src</code> 
subdirectory, where most of the methods are implemented.</li> 
</ul> 
During compilation, related dependency files, <code>.d</code>, and 
compiled code, <code>.o</code> are created in the <code>tmp</code> 
subdirectory. 
 
<p/> 
In part the <code>.xml</code> documentation files in the 
<code>xmldoc</code> 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 <code>.xml</code> file is also 
translated into an <code>.html</code> one in the 
<code>htmldoc</code> subdirectory, to allow easy viewing of the 
contents in a web browser, and an <code>.php</code> one in 
<code>phpdoc</code>, for more sophisticated interactivity 
if that subdirectory is installed on a web server. 
 
<p/> 
Here is the current list of files, ordered alphabetically, with a brief 
description of contents. 
 
<file name="Analysis"> 
contains routines to analyze events. Currently it can do sphericity, 
thrust, Lund/Jade/Durham jet clustering, cone-jet finding, and the 
<ei>kT</ei>, anti-<ei>kT</ei> and Cambridge/Aachen algorithms for 
hadron colliders. 
</file> 
 
<file name="Basics"> 
contains some basic facilities of general use: a random number 
generator <code>Rndm</code>, a four-vector class <code>Vec4</code>, and a 
histogram class <code>Hist</code>. 
</file> 
 
<file name="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> 
 
<file name="BeamRemnants"> 
adds primordial <ei>kT</ei> 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> 
 
<file name="BeamShape"> 
generates momentum spread of beams, and spread of collision vertex 
position. 
</file> 
 
<file name="BoseEinstein"> 
provides a simple method to impose Bose-Einstein correlations on 
pairs of identical mesons. 
</file> 
 
<file name="ColourReconnection"> 
implements several models that describe how the colour flow between 
outgoing partons can be changed by nonperturbative effects. 
</file> 
 
<file name="ColourTracing"> 
finds out how partons are colour-connected in (more or less long) chains. 
</file> 
 
<file name="DeuteronProduction"> 
provides a simple method to produce deuterons and anti-deuterons from 
events containing protons, neutrons, anti-protons, and anti-neutrons. 
</file> 
 
<file name="Event"> 
contains the event record, which basically is a vector of particles. 
This file also contains the <code>Particle</code> class, used by 
<code>Event</code>. <code>Pythia</code> uses two <code>Event</code> 
objects, one for the process-level record (<code>process</code>) and 
one for the complete event (<code>event</code>). 
</file> 
 
<file name="FJcore"> 
contains the external FJcore package, providing the coure functionality 
of the FastJet jet fiding package. 
</file> 
 
<file name="FragmentationFlavZpT"> 
contains the classes for describing the fragmentation steps in 
flavour and in longitudinal and transverse momentum. 
</file> 
 
<file name="FragmentationSystems"> 
defines some containers of parton systems, for use in 
the fragmentation routines. 
</file> 
 
<file name="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> 
 
<file name="HadronScatter"> 
contains the beginning of a machinery for final state hadronic 
rescattering. 
</file> 
 
<file name="HelicityBasics"> 
basic classes for the handling of helicities in tau lepton decays. 
</file> 
 
<file name="HelicityMatrixElements"> 
helicity-dependent decay matrix elements for the tau lepton. 
</file> 
 
<file name="HiddenValleyFragmentation"> 
hadronization in models with a hidden sector that contains an unbroken 
<ei>SU(N)</ei>, which gives confinement. The model and code is largely 
derived from the normal fragmentation classes. 
</file> 
 
<file name="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> 
 
<file name="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> 
 
<file name="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> 
 
<file name="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 
<code>Pythia</code>. Derived classes allow for a few different cases. 
</file> 
 
<file name="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> 
 
<file name="Merging"> 
Wpapper class to interface matrix element merging schemes with Pythia. 
</file> 
 
<file name="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> 
 
<file name="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> 
 
<file name="MultipartonInteractions"> 
performs multiparton interactions. 
</file> 
 
<file name="ParticleData"> 
contains a database of all necessary particle data (masses, names, ..) 
and decay channels. 
</file> 
 
<file name="ParticleDecays"> 
performs the decays of all normal unstable hadrons and leptons, i.e. 
in mass up to and including <ei>b bbar</ei> systems. It is not 
intended for decays of electroweak resonances, like <ei>Z^0</ei>. 
</file> 
 
<file name="PartonDistributions"> 
contains parton distribution functions for the proton and electron. 
Currently very simple, with only two <ei>p</ei> parametrizations 
and one <ei>e</ei> ditto available, but it is possible to link in 
external sets. 
</file> 
 
<file name="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> 
 
<file name="PartonSystems"> 
keeps track of which partons belong to which partonic subsystem, 
i.e. one of the multiparton (semi)hard interactions with associated 
showers. 
</file> 
 
<file name="PhaseSpace"> 
selects a point in phase space for the hard-process generation, 
optimized separately for each process to give improved Monte Carlo 
efficiency. 
</file> 
 
<file name="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> 
 
<file name="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> 
 
<file name="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 <code>Pythia</code>, 
so only a <code>Pythia</code> object needs to be explicitly instantiated 
and addressed   by the user. 
</file> 
 
<file name="PythiaComplex"> 
is only a <code>.h</code> file, containing a <code>typedef</code> for 
double precision complex numbers. 
</file> 
 
<file name="PythiaStdlib"> 
contains most of the <code>Stdlib</code> headers used in PYTHIA 8, 
with <code>using</code> directives. It defines <code>M_PI</code> if 
this is not already done. Also a few simple inline methods: 
<code>pow2(x)</code>, <code>pow3(x)</code>, <code>pow4(x)</code>, 
<code>pow5(x)</code> and <code>pow6(x)</code> for small integer 
powers, and <code>sqrtpos(x)</code> where a <code>max(0., x)</code> 
ensures that one does not take the square root of a negative number. 
The non-inlined <code>string toLower(const string, bool trim = true)</code> 
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 <code>void toLowerRep(string, bool trim = true)</code> 
replaces the string in place. Finally the non-inlined 
<code>GammaReal(x)</code> gives the <ei>Gamma</ei> function value of 
a real argument. 
</file> 
 
<file name="ResonanceDecays"> 
decays resonances as part of the hard-process stage, in many cases 
(but not all) including angular correlations between the decay products. 
</file> 
 
<file name="ResonanceWidths"> 
encodes some properties of resonances, in particular the dynamic 
calculation of widths. 
</file> 
 
<file name="RHadrons"> 
handles the production and decay of hadrons formed by long-lived 
gluinos, stops or sbottoms. 
</file> 
 
<file name="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 <code>.xml</code> files, but these values can 
then be changed by the user. 
</file> 
 
<file name="SigmaCompositeness"> 
contains the cross sections and matrix elements for production of 
some particles in compositeness scenarios, specifically excited 
fermions. 
</file> 
 
<file name="SigmaEW"> 
contains the cross sections and matrix elements for electroweak 
processes involving photons, <ei>Z^0</ei>'s and <ei>W^+-</ei>'s. 
</file> 
 
<file name="SigmaExtraDim"> 
contains the cross sections and matrix elements for processes in 
scenarios involving extra dimensions. 
</file> 
 
<file name="SigmaGeneric"> 
contains the cross sections and matrix elements for some generic 
processes, to be used as building blocks for a few BSM scenarios. 
</file> 
 
<file name="SigmaHiggs"> 
contains the cross sections and matrix elements for Higgs production. 
</file> 
 
<file name="SigmaLeftRightSym"> 
contains the cross sections and matrix elements for particle production 
in left-right-symmetry scenarios, specifically righthanded <ei>Z</ei> 
and <ei>W</ei> bosons and doubly-charged Higgs bosons. 
</file> 
 
<file name="SigmaLeptoquark"> 
contains the cross sections and matrix elements for leptoquark production. 
</file> 
 
<file name="SigmaNewGaugeBosons"> 
contains the cross sections and matrix elements for a <ei>Z'^0</ei>, 
a <ei>W^+-</ei> and a horizontal gauge boson <ei>R^0</ei>. 
</file> 
 
<file name="SigmaOnia"> 
contains the cross sections and matrix elements for charmonium and 
bottomonium production. 
</file> 
 
<file name="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: 
<code>SigmaQCD</code>, <code>SigmaEW</code>, <code>SigmaOnia</code>, 
<code>SigmaHiggs</code>, <code>SigmaSUSY</code>, 
<code>SigmaNewGaugeBosons</code>, <code>SigmaLeftRightSym</code>, 
<code>SigmaLeptoquark</code>, <code>SigmaCompositeness</code>, 
<code>SigmaExtraDim</code> and <code>SigmaGeneric</code>. 
</file> 
 
<file name="SigmaQCD"> 
contains the cross sections and matrix elements for soft and hard 
QCD processes. 
</file> 
 
<file name="SigmaSUSY"> 
contains the cross sections and matrix elements for Supersymmetric 
processes. 
</file> 
 
<file name="SigmaTotal"> 
contains parametrizations of total, elastic and diffractive hadronic 
cross sections. 
</file> 
 
<file name="SLHAinterface"> 
handles the communication between the <code>SusyLesHouches</code> 
classes and Pythia. 
</file> 
 
<file name="SpaceShower"> 
performs spacelike initial-state transverse-momentum-ordered 
shower evolution. 
</file> 
 
<file name="StandardModel"> 
contains the running <ei>alpha_strong</ei>, with <ei>Lambda</ei> 
matching at flavour thresholds, the running <ei>alpha_em</ei>, 
CKM mixing matrices, and a few other parameters such as 
<ei>sin^2(theta_W)</ei>. 
</file> 
 
<file name="StringFragmentation"> 
performs string fragmentation of a given set of partons. 
</file> 
 
<file name="StringLength"> 
calculates the string-length <ei>lambda</ei> measure for various 
colour topologies, including junction ones. 
</file> 
 
<file name="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> 
 
<file name="SusyLesHouches"> 
contains information on SUSY parameters and particle data as specified 
by the SUSY Les Houches Accord. 
</file> 
 
<file name="SusyResonanceWidths"> 
encodes some properties of supersymmetric resonances, similar to 
what <code>ResonanceWidths</code> does for other resonances. 
</file> 
 
<file name="TauDecays"> 
the main routines for handling tau lepton decays with helicity 
information. 
</file> 
 
<file name="TimeShower"> 
performs timelike final-state transverse-momentum-ordered 
shower evolution. 
</file> 
 
<file name="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> 
 
<file name="WeakShowerMEs"> 
provides some matrix elements used for ME corrections of <ei>W</ei> 
and <ei>Z</ei> weak gauge boson emission in both initial- and final-state 
parton showers. 
</file> 
 
</chapter> 
 
<!-- Copyright (C) 2020 Torbjorn Sjostrand --> 
