## Sample Main Programs

Descriptions of available classes, methods and settings are all very good and useful. Ultimately they are necessary for you to be able to fine-tune your runs to the task at hand. To get going, however, nothing helps like having explicit examples to study. This is what is provided in the `examples` subdirectory, along with instructions how they should be run:

- `main00.cc` : does not exist, but it has been defined in the `Makefile`, so this name could be used for a simple first test run.
- `main01.cc` : a simple study of the charged multiplicity for jet events at the LHC (brief example fitting on one slide).
- `main01.py` : a Python interface equivalent of `main01.cc`.
- `main02.cc` : a simple study of the pT spectrum of Z bosons at the Tevatron (brief example fitting on one slide).
- `main03.cc` : a simple study of several different kinds of events, with the choice to be made in the `main03.cmnd` "cards file". Also shows how to plot histograms using the Python/Matplotlib/Pyplot solution.
- `main04.cc` : tests of cross sections, multiplicities and average transverse momenta for elastic, diffractive and nondiffractive topologies, using `main04.cmnd` to pick processes. For photoproduction one can use the alternative `main04_photons.cmnd` input.
- `main05.cc` : generation of QCD jet events at the LHC, with jet analysis using the `SlowJet` inclusive anti-kT sequential-recombination finder and the `CellJet` cone-jet finder.
- `main06.cc` : generation of LEP1 hadronic events, i.e. e^+e^- → gamma*/Z^0 → q qbar, with charged multiplicity, sphericity, thrust and jet analysis.
- `main07.cc` : set up a fictitious production process to a generic resonance, where you easily can compose your own list of (two-body) decay modes to a variety of final states. Also traces decay chains down to truly stable particles: gamma, e+-, p/pbar and neutrinos. Suitable for astroparticle applications, like neutralino pair annihilation, where cross sections are calculated separately in another program. Also shows how to plot histograms using the Python/Matplotlib/Pyplot solution.
- `main08.cc` : generation of the QCD jet cross section biased towards higher pT values, by two different techniques. Firstly, by splitting the run into subruns, each in its own pT bin, and adding the results properly reweighted. Two suboptions, with limits set either in the main program or by subrun specification in the `main08.cmnd` file. Secondly, by a continuous reweighting with a pT^4 bias in the selection, compensated by a 1/pT^4 event weight. Also inclusion of soft processes is illustrated, with subruns and weighted events.
- `main09.cc` : generation of two predetermined hard interactions in each event.
- `main10.cc` : illustration how userHooks can be used interact directly with the event-generation process.
- `main10.py` : a Python interface equivalent of `main10.cc`. Provides an example of how to derive PYTHIA classes in Python.
- `main11.cc` : a study of top events, fed in from the Les Houches Event File `ttbar.lhe`, here generated by PYTHIA 6.4. This file currently only contains 100 events so as not to make the distributed PYTHIA package too big, and so serves mainly as a demonstration of the principles involved.
- `main12.cc` : a more sophisticated variant of `main11.cc`, where two Les Houches Event Files (`ttbar.lhe` and `ttbar2.lhe`) successively are used as input. Also illustrating some other aspects, like the capability to mix in internally generated events.
- `main13.cc` : a streamlined version of `main12.cc`, where two Les Houches Event Files (`ttbar.lhe` and `ttbar2.lhe`) successively are used as input in `main13.cmnd` file.
- `main14.cc` : a systematic comparison of several cross section values with their corresponding values in PYTHIA 6.4, the latter available as a table in the code.
- `main15.cc` : loop over several tries, either to redo B decays only or to redo the complete hadronization chain of an event. Since much of the generation process is only made once this is a way to increase efficiency.
- `main16.cc` : put all user analysis code into a class of its own, separate from the main program; provide the "cards file" name as a command-line argument. Also exemplifies how Higgs mass, width and branching ratios can be set by hand.
- `main17.cc` : shows (a) how to use UserHooks to regularize onium cross section for pT → 0, and (b) how decays could be handled externally.
- `main18.cc` : shows how to write an event filter class, where you keep a vector of pointers to the subset of particles you want to study further. The event record itself remains unchanged.
- `main19.cc` : use several instances of Pythia, one for signal events and others for a variable number of pileup and "beam-gas" events, combined into one common event record. Illustrates how new Pythia instances can copy existing settings and particle data.
- `main20.cc` : shows how PYTHIA 8 can write a Les Houches Event File, using facilities potentially useful also for other programs to write an LHEF. See also `main64.cc`.
- `main21.cc` : an example how a single particle or various parton-level configurations can be input directly for hadronization, without being tied to the full process-generation machinery, e.g. to study the hadronization of junction topologies. Can also be used for single-resonance decays, with showers.
- `main22.cc` : shows how an external resonance can be implemented as a new class derived from a PYTHIA base class, and be used in an external process, both of them handed in for generation as with normal internal classes.
- `main23.cc` : shows how to write external classes, derived from PYTHIA base classes, that can be handed to PYTHIA for internal generation. The MIXMAX random number generator is this way compared with the default PYTHIA one. Explicit implementations are included for the generation of external beam momentum spread and vertex location, and for a simple scaling external parton distribution set.
- `main24.cc` : tests of internally implemented cross sections for Supersymmetric particle production, with SUSY spectrum defined in `slha2-example.spc` and settings in `main24.cmnd`. For illustration, an alternative example spectrum is also available, `sps1aWithDecays.spc`, which contains a decay table in SLHA format.
- `main25.cc` : input RPV-SUSY events from an LHEF file that contains an SLHA spectrum inside its header. The event file, `main25.lhe`, contains a sample events that illustrate how to arrange color tags in the presence of the color-space epsilon tensors that accompany baryon number violating event topologies.
- `main26.cc` : test program for processes in scenarios with large extra dimensions or unparticles.
- `main27.cc` : production of Kaluza-Klein gamma/Z states in TeV-sized extra dimensions.
- `main28.cc` : production of long-lived R-hadrons, that are forced to decay at a separate vertices and possibly with changed momenta.
- `main29.cc` : colour reconnection models studied for top production. Illustrates how to set up the user hooks in `include/Pythia8Plugins/ColourReconnectionHooks.h`, with several models not found in the standard PYTHIA library.
- `main30.cc` : example how to create a tailor-made copy of the ordinary event record, here with hard-process history tracing closer to the PYTHIA 6 conventions.
- `main31.cc` : exemplifies an improved matching of parton showers to LHEF-style input based on the POWHEG approach. The `main31.cmnd` allows to switch between several different matching options. It also allows to select input process, in this case either for the POWHEG-hvq program applied to top pair production [Cor10] or for QCD 2+3-jet events. The small samples of input events are stored in the `powheg-hvq.lhe` and `powheg-dijets.lhe` files, respectively.
- `main32.cc` : exemplifies MLM merging, either in the ALPGEN variant or in the Madgraph one, and with input events either from ALPGEN or from Madgraph, with relevant control cards stored in `main32.cmnd`. See Jet Matching for further details. Traditionally the ALPGEN output is split into one file with events and another with parameters and cross sections (unlike in LHEF). Here a sample of W + 3 jets events is stored in `main32.unw` and the parameters to go with it in `main32_unw.par`. Madgraph events are taken from the `w+_production_lhc_2.lhe` file in this case.
- `main33.cc` : demonstrates how to link the POWHEGBOX matrix element programs dynamically, bypassing the need for intermediate LHE files. Two special files are used in this option: `include/Pythia8Plugins/LHAPowheg.h` contains the LHAup class wrapper used to build the POWHEG plugin libraries, and `include/Pythia8Plugins/PowhegProcs.h` the simple class that facilitates loading the POWHEG plugins. In addition `main33.cmnd` contains the commands needed for POWHEGBOX to run the example.
- `main34.cc` : demonstrates how Madgraph5_aMC@NLO can be run "from within" Pythia, making use of the `LHAupMadgraph` wrapper/interface of Madgraph5_aMC@NLO and the Pythia jet matching facilities.
- `main34.py` : a Python interface equivalent of `main34.cc`. Demonstrates usage of a PYTHIA plugin within the Python interface.
- `main35.cc` : demonstrates how to generate quarkonia events with the external HelacOnia package interfaced to Pythia, and compare results with the internal implementation.
- `main36.cc` : demonstrates how to generate Deeply Inelastic Scattering events, e.g. in a HERA configuration.
- `main37.cc` : shows how LHEF version 3.0 files can be read and used to fill several histograms of the same property, but with different event weights.
- `main38.cc` : an extended version of `main37.cc`, where additionally it is shown how to extract many different kinds of LHEF version 3.0 information.
- `main39.py` : standalone Python code that parses the XML particle database and displays data for a requested particle.
- `main40.cc` : calculates the inclusive branching fractions for the Standard Model Higgs into quarkonia using the LETO timelike parton shower.
- `main41.cc` : similar to `main01`, except that the event record is output in the HepMC event record format. Requires that HepMC3 is properly linked. Note that the `hepmcout41.dat` output file can become quite big; so no example is included in this distribution.
- `main42.cc` : a streamlined version for the generation of events that are then stored in HepMC format, without any event analysis. That is, all physics studies will have to be done afterwards. The name of the input "cards file" (e.g. `main42.cmnd`) and output HepMC event file (e.g. `hepmcout42.dat`) are to be provided as command-line arguments. Requires that HepMC3 is properly linked. Note that the HepMC output file can become quite big; so no example is included in this distribution.
- `main43.cc` : a further extension of `main42.cc`, where subruns are used to process several consecutive LHEF, as in `main13.cc`, with information stored e.g in `main43.cmnd`. Other comments as for `main42.cc`.
- `main44.cc` : a legacy HepMC2 example, where subruns are used to process several consecutive LHEF, with information stored e.g in `main44.cmnd`. This example mainly serves as comparison to `main45.cc` which uses HepMC3.
- `main45.cc` : an example illustrating the generation of HepMC events using HepMC3, where subruns are used to process several consecutive LHEF. Input settings can e.g be stored in `main44.cmnd`. For users unfamiliar with HepMC3, note that `main44.cc` is basically a carbon copy, but uses HepMC2 instead, for comparison.
- `main46.cc` : an example illustrating the generation of HepMC events using the HDF5 LHA format (LHAHDF5).
- `main48.cc` : demonstrates how to use the `EvtGenDecays` class provided by `include/Pythia8Plugins/EvtGen.h` to perform decays with the `EvtGen` package. The `main48.cc` header contains special instructions how to configure PYTHIA for use with `EvtGen`.
- `main51.cc` : a test of the shape of parton densities, as a check prior to using a given PDF set in a generator. Requires that LHAPDF is properly linked. Also shows how to plot histograms (with logarithmic x scale) using the Python/Matplotlib/Pyplot solution.
- `main52.cc` : compares the charged multiplicity distribution, and a few other minimum-bias physics aspects, between default PYTHIA PDF and another one. Requires that LHAPDF is properly linked.
- `main53.cc` : tests the possibility to do backwards evolution from an incoming photon at the hard interaction. Requires that you link to a LHAPDF set that includes the photon PDF.
- `main54.cc` : compares the internal and LHAPDF implementations of the NNPDF 2.3 QCD+QED sets, for results and for timing. Requires that LHAPDF is properly linked.
- `main55.cc` : exemplifies how you can use the internal implementation of interpolation in an `lhagrid1.dat file`, without linking LHAPDF6. Also illustrates the topical issue of associated event properties for an intermediate spinless resonance in γ + γ → γ + γ at 750 GeV.
- `main61.cc` : exemplifies the generation of hard diffractive processes.
- `main62.cc` : illustrates how a user hook can be made to steer the angular distribution selection in resonance decays. The prime example would be if LHEF input, e.g. from Madgraph, contains undecayed resonances with helicity information. These would then be decayed isotropically by PYTHIA, but this example shows how one could do better. Some input in `main62.cmnd`.
- `main63.cc` : exemplifies how rare emissions can be enhanced in the shower.
- `main64.cc` : exemplifies how LHEF version 3 events can be written on an external file.
- `main68.cc` : exemplifies hard diffraction in the context of a photon-inside-lepton beam, like a HERA.
- `main69.cc` : exemplifies how to generate all relevant contributions for charged particle spectra in photon-photon and photon-proton collisions.
- `main70.cc` : exemplifies how to provide an external photon flux for photo-production processes.
- `main71.cc` : an example how the FastJet jet finding package can be linked to allow an analysis of the final state, in this case for a study of W + jet production.
- `main72.cc` : a comparison of SlowJet and FastJet jet finding, showing that they find the same jets if run under identical conditions, in this case for QCD jets.
- `main73.cc` : a comparison of jet properties on the parton and the hadron level, illustrating possibilities for larger control of which particles are used in the jet analyses.
- `main74.cc` : exemplifies how to use one of the contributed add-ons to the FastJet package. In this case the modified Mass Drop Tagger is used to improve the mass reconstruction of a boosted hadronically decaying Z^0.
- `main75.cc` : setup (in `main75.cmnd`) for Dark Matter production via an s-channel mediator, where a mono-jet pT spectrum is found with the FastJet package.
- `main76.cc` : simple setup for Dark Matter production in several different scenarios, as specified in `main76.cmnd`, notably with long-lived particle signatures.
- `main77.cc` : example on how double parton scattering events can be reweighted according to a different model than default in Pythia. Contributed by Boris Blok and Paolo Gunnellini.
- `main78.cc` : demonstrates how to generate different types of photon-initiated dilepton events in proton-proton collisions.
- `main80.cc` : do CKKW-L merging with a merging scale defined in kT. Input is provided by the `main80.cmnd` file and input LHE files. Very basic and pedagogical setup, suitable for tutorials.
- `main81.cc` : do CKKW-L merging with a merging scale defined in kT. Input is provided by the `main81.cmnd` file and the three data files `w+_production_lhc_0.lhe`, `w+_production_lhc_1.lhe` and `w+_production_lhc_2.lhe`.
- `main82.cc` : do CKKW-L merging with a user-defined merging scale function. Input is provided by the `main82.cmnd` file and the three data files `w+_production_lhc_0.lhe`, `w+_production_lhc_1.lhe` and `w+_production_lhc_2.lhe`.
- `main83.cc` : as `main82.cc` but with an additional cut on the lowest multiplicity allowed for the reclustered state. The same input as for `main82.cc` can be used.
- `main84.cc` : do CKKW-L merging with output in such a way that it can be used in subsequent RIVET analyses. Input is provided by the `main84.cmnd` file and the three data files `w+_production_lhc_0.lhe`, `w+_production_lhc_1.lhe` and `w+_production_lhc_2.lhe`.
- `main85.cc` : do CKKW-L merging, with HepMC event output. Input settings are provided by the `main85.cmnd` file. This example program allows the use of input Les Houches events that are regularised with only very minimal cuts, and on which Pythia itself should enforce the more restrictive merging scale cut. The example program can be used with the input files `w_production_tree_0.lhe`, `w_production_tree_1.lhe` and `w_production_tree_2.lhe`.
- `main86.cc` : do unitarised ME+PS (UMEPS) merging, with HepMC event output. Input settings are provided by the `main86.cmnd` file. This example program allows the consistent use of input Les Houches events that are regularised with only very minimal cuts, similar to `main85.cc`. The example program can be used with the input files `w_production_tree_0.lhe`, `w_production_tree_1.lhe` and `w_production_tree_2.lhe`. The program will produce positively and negatively weighted events. See UMEPS Merging for further details.
- `main87.cc` : do NL3 NLO merging, with inclusive NLO input, and with HepMC event output. Input settings are provided by the `main87.cmnd` file. This example program allows the consistent use of input Les Houches events that are regularised with only very minimal cuts, similar to `main85.cc`. The example program can be used with the tree-level input files `w_production_tree_0.lhe`, `w_production_tree_1.lhe`, `w_production_tree_2.lhe` and the inclusive POWHEG input files `w_production_powheg_0.lhe`, `w_production_powheg_1.lhe`. The program will produce positively and negatively weighted events. See NLO Merging (NL3 section) for further details.
- `main88.cc` : do unitarised NLO+PS (UNLOPS) merging, with inclusive NLO input, and with HepMC event output. Input settings are provided by the `main88.cmnd` file. This example program allows the consistent use of input Les Houches events that are regularised with only very minimal cuts, similar to `main85.cc`. The example program can be used with the tree-level input files `w_production_tree_0.lhe`, `w_production_tree_1.lhe`, `w_production_tree_2.lhe` and the inclusive POWHEG input files `w_production_powheg_0.lhe`, `w_production_powheg_1.lhe`. The program will produce positively and negatively weighted events. See NLO Merging (UNLOPS section) for further details.
- `main89.cc` : do matching/merging according to five alternative methods, simply by choosing which `.cmnd` file to read: `main89ckkwl.cmnd` for CKKW-L, `main89fxfx.cmnd` for FxFx, `main89mlm.cmnd` for MLM, `main89umeps.cmnd` for UMEPS, and `main89unlops.cmnd` for UNLOPS.
- `main91.cc` : shows how ROOT can be used for histogramming in a program that for the rest is structured like a normal PYTHIA run.
- `main93.cc` : streamlined event generation with possibility to output ROOT files, output HepMC files and run RIVET analyses, all by specifying output modes in a cmnd file, where also the event generator settings are specified. The example is run with command line options, run `./main93 -h` to see a full list. See ROOT Usage for information about ROOT output, RIVET Usage for information about RIVET and HepMC Interface for information about HepMC.
- `main94.cc` : use ROOT to visualize the particles produced by Pythia in (y,phi)-space.
- `main95.cc` : use ROOT to visualize different jet algoritms in (y,phi)-space. The jet clustering is done with FastJet. The produced figure was used in the article "50 years of Quantum Chromodynamics" in celebration of the 50th anniversary of QCD (EPJC).
- `main101.cc` : shows how the string shoving mechanism, part of the rope hadronization framework, can be set up and used to generate ridge effects.
- `main102.cc` : shows how flavour production is changed in the rope hadronization framework.
- `main103.cc` : shows how to use a filter to select a specific final state from resonance decays.
- `main111.cc` : simple pp collisions as in `main01.cc`, but using the Angantyr model for Heavy Ion collisions. Also shows how Rivet analyses can be set up easily using a special interface.
- `main112.cc` : p-Pb collisions at LHC energies, using the Angantyr model for Heavy Ion collisions, and analyzing events by centrality bins.
- `main113.cc` : Pb-Pb collisions at LHC energies, using the Angantyr model for Heavy Ion collisions, and analyzing events by centrality bins.
- `main121.cc` : set up automatic uncertainty band variations to PDFs and factorization and renormalization scales.
- `main151.cc` : compare the energy dependence of the average charged multiplicity between the simple treatment in rescattering and the full framework.
- `main152.cc` : compare multiplicities and pT spectra with or without rescattering, the former with or without rescattering between nearest neighbours along the string.
- `main153.cc` : simple generation of low-energy events.
- `main154.cc` : plot the energy dependence of the low-energy cross sections used in the hadronic rescattering framework.
- `main155.cc` : plot the energy dependence of the low-energy cross sections, specifically the contribution from resonances.
- `main156.cc` : perform parameterization of hadron widths and output the resulting tables.
- `main157.cc` : generate tetraquarks from the rescattering of a D0 and Dbar*0 beam.
- `main158.cc` : generate tetraquarks from the rescattering of D0 and Dbar*0 mesons produced in LHC events.
- `main161.cc` : gives an example of `PythiaParallelism` usage. This program is equivalent to `main01.cc`, but does event generation in parallel.
- `main162.cc` : equivalent to `main161.cc`, but is much more heavily commented to give more in-depth explanations of how the code works.
- `main162.py` : a Python interface equivalent of `main162.cc`.
- `main163.cc` : perform analyses in parallel using the `Parallelism:processAsync` setting.
- `main171.cc` : three scenarios for Hidden Valley particle production, with a selection of further possible variations.
- `main181.cc` : plot PDFs for a large number of hadrons.
- `main182.cc` : test switching between hadron beams on an event-by-event basis. Compare running times for different scenarios.
- `main183.cc` : simple example of how Pythia can be used to simulate a basic hadronic cascade in the atmosphere.
- `main184.cc` : as `main183.cc`, but using the `PythiaCascade` class to perform the separate collisions or decays, while the bookkeeping of the cascade evolution remains in the main program.
- `main185.cc` : an even simpler example of one collision or decay at a time, as performed in `PythiaCascade`.
- `main200.cc` : simple example of the VINCIA (or DIRE) shower model(s), on Z decays at LEP I, with some basic event shapes, spectra, and multiplicity counts.
- `main201.cc` : comparison of VINCIA and Pythia on inclusive jets at LHC, with option to run the two generators in parallel using OpenMP. See `main204.cc` for how to do this with the Parallelism framework.
- `main202.cc` : VINCIA setup for ttbar production at LHC, with measurement of run time and options to switch various shower and MPI/hadronization components on/off via command file.
- `main204.cc` : demonstrates the example of `main201` using the Parallelism framework.
- `main205.cc` : VINCIA setup for electroweak shower off a fictitious process for Z decay to neutrinos at high mass. (In the absence of a weak shower, these would not shower at all.) The VINCIA EW shower requires hard-process partons with assigned helicities. This is done via Pythia's MG5 matrix-element interface, which must be compiled and linked (using `configure --with-mg5mes`).
- `main206.cc` : previously `main203.cc` VINCIA setup for electroweak shower off high-pT dijets at the LHC. The VINCIA EW shower requires hard-process partons with assigned helicities. This is done via Pythia's MG5 matrix-element interface, which must be compiled and linked (using `configure --with-mg5mes`).
- `main207.cc` : VINCIA electroweak showers off an LHEF file for dark-matter annihilation. The VINCIA EW shower requires hard-process partons with assigned helicities. In this example, these are read in from the LHEF file.
- `main208.cc` : VINCIA setup for double-dissociative photon-initiated gamma gamma → mu+ mu- at LHC.
- `main280.cc` : do CKKW-L merging with the VINCIA sector shower with HepMC2 output to use in RIVET analyses. Also redirects all Pythia output to a log file to reduce terminal output.
- `main300.cc` : allows to steer Pythia from the command line and can produce HepMC files and allows for OpenMP parallelization. More documentation can be obtained by executing `./main300 --help`. The input file main300.cmnd further illustrates the use of DIRE.
- `main301.cc` : demonstrates the use of in-situ hadronization reweighting for variations of both kinematic and flavor hadronization parameters. The output compares multiplicity distributions from the default parameters to the reweighted output with the varied parameters and the output using the varied parameters without reweighting.
- `main333.cc` : illustrates the use of UserHooks to veto events after hadronization, but before any subsequent processes such as rescattering or Bose-Einstein.