Program Files

• A header file, .h in the include/Pythia8 subdirectory, where the public interface of the class is declared, and inline methods are defined.
• A source code file, .cc in the src subdirectory, where most of the methods are implemented.

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  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.