Update History

1. 8.312: 23 May 2024
2. 8.311: 08 March 2024
3. 8.310: 25 July 2023
4. 8.309: 16 February 2023
5. 8.308: 16 November 2022
6. 8.307: 25 February 2022
7. 8.306: 28 June 2021
8. 8.305: 03 May 2021
9. 8.304: 09 April 2021
10. 8.303: 01 September 2020
11. 8.302: 24 April 2020
12. 8.301: 30 October 2019

8.312: 23 May 2024

• The Sample Main Programs in the examples subdirectory have been completely reorganized. Over the years, an expansion of examples has been driven by the addition of new capabilities as well as by a strong user demand. As the old two-digit mainNN.cc structure filled up, empty slots were used more-or-less at random, before eventually also three-digit codes were introduced. Now all examples have been regrouped by topic as three-digit codes mainNNN.cc, with empty space within each group for future expansion. No program keeps its old code. Some new examples have been introduced and some existing ones have been modified. The matching and merging examples have seen a significant restructuring, where main162.cc and main164.cc provide omnibus versions for several approaches.
• Some related updates and cleanup in examples/Makefile.
• The PYTHIA 8.3 Worksheet has been updated and expanded.
• The new more QCD based color reconnection model, ColourReconnection:mode = 1 has been sped up by over a factor of two, thanks to Michal Kreps.
• Variable energy beams have been added to Angantyr. See main424.cc for details.
• New parameters BeamRemnants:heavyQuarkEnhance allows heavier quarks (s, c, b) to take more beam-remnant momentum than the lighter u and d ones. The same parameters also makes it more likely for a heavier quark not to be kicked backwards in low-mass systems. Thanks to Manuel Masip and Ivan Rosario Bonastre for pointing out the issue.
• Example of using Pythia as a custom shared library through Python has been added in main296.cc and main296.py.
• The option --version is now available for pythia8-config which will return the Pythia version, thanks to Christopher Dilks for the suggestion.
• A bug which caused popcorn destructive interference mechanism to not work has now been fixed.
• Added a new flag for neutrino PDFs to have them as unresolved particles by default as no other options currently exists. Minor related documentation update.
• Some bug fixes in less commonly used methods of the Settings and ParticleData classes.
• Tiny correction in configure that fixes issue with pythia8-config on some platforms, thanks to Gabriele Inghirami for reporting.
• Protect against potential division by zero in the Thrust method.
• Fixed an issue where cross-sections could be reported incorrectly when running a second hard process when the first hard process comes from an LHE file.
• Protection was added to Vincia FSR for the case where the charm quark mass is set to zero.

8.311: 08 March 2024

• Javira Altmann joins the author team.
• Stefan Prestel leaves as an active author.
• Reweighting for a subset of hadronization parameters, both kinematic and flavor related, is now available. See main301 for an example and Hadronization Variations for details.
• A number of changes have been made to the junction fragmentation procedure:
• Updated iterative procedure to find average junction rest frame that is more stable as it does not rely on convergence.
• Include special treatment of soft-leg cases (i.e. when the 120 degree junction rest frame does not exist), called pearl-on-a-string.
• Allow for fragmentation of pearl-on-a-string cases using a gluon-approximation, controlled by StringFragmentation:pearlFragmentation and StringFragmentation:pearlProbFactor.
• Improvements in the hadronization of low-mass junction string systems, contributed by Harsh Shah. Low-mass junction systems now are handled by the ministring fragmentation code. This results in fewer discarded events when using the QCD-based colour reconnection model in scenarios where many junction systems are formed.
• A number of changes to the QCD-based colour reconnection model:
• Include a new string-length measure that better takes into consideration heavy quarks.
• Include additional seperate parameter ColourReconnection:mPseudo, which formally had a combined definition with parameter ColourReconnection:m0.
• New option to limit the impact-parameter distance between dipoles that are allowed to reconnect in the QCD-based colour reconnection model, contributed by Harsh Shah. See ColourReconnection:dipoleMaxDist in Colour Reconnection for details.
• The inverse formation times for a three-dipole junction system were not correctly being initialized, while their colours were. This bug has now been fixed, although its impact should be minimal. Thanks to Michal Kreps for the fix.
• A number of updates and extensions to the string "closepacking" model:
• Include close-packing mechanism for Gaussian string breaks controlled by ClosePacking:doClosePacking, ClosePacking:tension, ClosePacking:tensionRatio and other ClosePacking parameters. See further on Close-packing.
• Include strangeness enhancement for string breaks next to a junction, controlled by StringFragmentation:strangeJunctions and StringFragmentation:strangeJuncFactor.
• Include popcorn destructive interference via simple diquark suppression controlled by ClosePacking:qqFacQ and ClosePacking:qqFacP.
• A new sub collision model (Angantyr:CollisionModel=4,5) has been added, wherein the projectile and target can fluctuate with different parameters, as one would expect in generic hadron-ion collisions.
• Usage of HeavyIon:SigFitDefPar is changed. For the case Angantyr:CollisionModel=1,2, the order of the parameters has changed as well. This means that saved parameters from previous releases cannot be used, and a refitting of parameters is recommended.
• The example main200.cc has had minor changes and cleanup.
• A fifteen percent uncertainty has been included for the b slope parameter of the HeavyIon:SigFitErr parameter vector used in fitting the cross sections for heavy ion collisions. This helps stabilize the fit at low collision energies.
• When parameters or modes are configured by the user via readString or readFile with out-of-range values, Pythia will now abort upon initialization, similar to when invalid user settings or particle data are provided, rather than the previous behaviour where the value would be set to the limit. Out-of-range parameters can still be used by directly calling the force methods in the Settings class.
• Parton shower enhancements in the simple shower are now available for the individual splittings g → c + cbar and g → b + bbar.
• Added capability to over sample emissions in combination with enhancing emissions so as to reduce large weight fluctuations.
• The documentation for POWHEG Matching has been updated with new recommendations and defaults.
• An option for suppressed power showers is now implemented also for the Vincia shower model, via the Vincia:pTdampMatch parameter. The main202 example updated to illustrate this for ttbar.
• Entries in the event record may now be skipped when requesting matrix element corrections in the parton shower for a given event.
• Added new capability to disable matrix element corrections in the Timelike Shower for the first emission (only) of the hard process and/or resonance decays. The choice(s) of resonance(s) is customizable.
• Added example UserHook to modify the decay products in LHE files containing resonance decays.
• Updated RIVET examples to handle support for RIVET 4, while maintaining backwards compatibility.
• A new option for parallelism, Parallel:doNext, makes it possible for the user to run custom code before each event is generated, such as changing the energy on an event-by-event basis. For more information, see Parallelism or main164.
• New Vincia example programs for EW showers, main205.cc - main207.cc. main205 illustrates electroweak showers in Z decays to neutrinos (at very high Z mass). main206 illustrates EW showers for dijets at LHC and was previously named main203. main207.cc illustrates read-in of LHEF files for EW showering with Vincia.
• New main208 example program illustrates Vincia QED showering in photon-initiated processes, specifically photon-photon → mu+ mu- production at LHC.
• Minor bug fix in Vincia's setup for helicity showers, to ignore beam particles when deciding whether helicity assignments need to be generated for the hard system. Thanks to M. di Mauro for help to identify this bug.
• Ensured that displaced-vertex information is propagated in the VINCIA QED and QCD FSR showers. This should correct a problem with displaced taus showing up with zero vertex information in HepMC events produced with VINCIA. Thanks to D. Hirscbuehl and S. Tuncay.
• Fixed issue where onia were showered with pT2nextQCD when OniaShower:octetSplit was set to 0 (do not allow the octet states to radiate). Thanks to Jake Pfaller for reporting this issue.
• Fixed issue where string systems were not updated when eliminating small string regions.
• Added protection when calling the nTried, nSelected, nAccepted, sigmaGen, sigmaErr, and nameProc methods in the Info class to prevent exceptions when requesting an invalid process ID.
• Fixed issue where if Event::clear is called before the particle database was initialized caused a segmentation fault.
• When using TauDecays:mode = 2 or 3 the parameter TauDecays:tauPolarization was being read in as an integer rather than a real number. This has now been fixed. Thanks to Florian Harz for reporting this.
• When reading in external events without SPINUP information and with TauDecays:externalMode = 1, the production matrix element for a tau produced from W or W' boson was being incorrectly set to that of of a Z boson. This has now been fixed. Thanks to Florian Harz for reporting this.
• The heavy ion information, provided by the HIInfo class and available through the hiInfo member of the Info class, is now available by default in the simplified Python interface.
• Introduced direct access to the particles of an event by the new Event.particles() method. Thanks to H. Dembinski for the sugggestion.
• Fixed issue where the Python interface does not compile under 32-bit. Thanks to Mattias Ellert for reporting the issue.
• Introduced an experimental random number sequence debugging tool for advanced debugging usage.
• Some memory management issues were corrected in Vincia.
• Further minor updates in Vincia to streamline Pythia coding style and improve naming of antenna functions.
• When using subruns, each with a different spectrum from an SLHA file for each initialization, the mass for particles can be incorrectly generated. A fix for this has now been introduced.
• Fixed bug in the SubCollisionModel class, where the functions sigSDE(), sigSDEP(), and sigSDET(), gave the wrong values. This will only affect the behaviour of Angantyr, in case the NaiveSubCollisionModel class is used in the Glauber calculation (Angantyr:CollisionModel=0).
• A bug in the merging framework has been fixed that was introduced by the changes for aMC@NLO-Delta in 8.309. Thanks to Marjorie Shapiro for the report.
• The introduction of the new BeamSetup class broke parts of the photoproduction initialization, which has now been fixed.
• Fixed memory leak in main10.
• Fixed possible access of unitialized variables in PhaseSpace::setupSampling123.

8.310: 25 July 2023

• Naomi Cooke joins the author team.
• Production of quarkonia is now available via the simple timelike shower, see Onia Showers for details.
• Added a new feature that allows producing user-defined resonances in hadronic rescattering. Newly added particles can be produced as resonances if they set varWidth = on. Examples of this are shown in main157 and main158.
• A number of changes have been made to the heavy ion machinery.
• Variable energies can now be used in Angantyr. This is done by setting Beams:allowVariableEnergy = on and using the Pythia::setKinematics methods. In addition Angantyr can now handle all Beams:frameType options.
• MPI initialization reuse has now been enabled for heavy ions. The MultipartonInteractions:reuseInit and MultipartonInteractions:initFile parameters control whether the MBIAS Pythia object will reuse MPIs. In addition, the HeavyIon:SasdMpiReuseInit and HeavyIon:SasdMpiInitFile parameters do the same thing for the SASD Pythia object.
• The parameters calculated from the evolutionary algorithm can also be saved to file. This is controlled with the HeavyIon:SigFitReuseInit and HeavyIon:SigFitInitFile settings.
• The HIUserHooks source has been split into HIInfo, HINucleusModel, and HISubCollisionModel.
• Some code is offloaded to new classes Nucleus and SubCollisionSet, simplifying HeavyIons.cc.
• Simplified the output for the Angantyr procedure for fitting nucleon cross sections.
• Included possibility to sample intermediate photon virtuality also for externally provided events using built-in photon flux for protons.
• Included possibility to generate DIS events with a (anti-)neutrino beam.
• The TopRecoilHook, formerly available separately, has now been integrated into the SimpleTimeShower. See further the TimeShower:recoilStrategyRF switch, which replaces and extends the old TimeShower:recoilToColoured flag. In top decays the radiation pattern in the new option is reweighted to have top as recoiler in case the relevant colour line connects to it.

• The arguments to the setSigmaPtr method are now shared pointers. Additionally, addSigmaPtr has been introduced with the previous behaviour of setSigmaPtr, and the new setSigmaPtr overrides all previously added pointers.
• A similar change has been made for the setResonancePtr method with the addition of the addResonancePtr method.

8.309: 16 February 2023

• New functionality for aMC@NLO-Delta matching with a reduced number of negative-weight events is introduced. This includes the possibility to directly call shower Sudakov factors from a runtime interface to Madgraph5_aMC@NLO. See documentation of new flag Merging:runtimeAMCATNLOInterface on Merging page for further info and references.
• The PowhegHooks now work with all three shower models and automatically switch between the simple shower, Vincia, and Dire based on the PartonShowers:model mode. In addition, the option to set POWHEG:nFinal = -1 has been ported from 8.244 to allow the use of event files with a variable number of real emissions.
• The Angantyr model for ion collisions has been updated with several possible nuclear geometries, suitable for light nuclei. It is now possible to select a harmonic oscillator shell model for nuclei with A <= 16, and a Hulthen potential for deuterons. It is also possible to read a nuclear configuration from an external file, format given in the manual.
• The documentation for ion collisions at Heavy Ions in the HTML manual has been extended.
• Allow scattered lepton to appear in different positions of the event record for remnant handling in externally generated DIS processes.
• Enable generation of externally generated hard processes for direct photoproduction.
• The Hidden Valley scenarios have now been updated with options that allow a second- or third-order running coupling in the shower, see further Hidden Valley Processes. A new AlphaSUN class implements the running for an arbitrary SU(N) gauge group, neglecting flavour thresholds. Thanks to Joshua Lockyer and Suchita Kulkarni for code contributions.
• Equation (9.5) in the 2006 RPP [Yao06] has erroneously been taken as a second-order alpha_strong expression, while actually it is a third-order one. This has been corrected, with separate second- and third-order options now available for matrix elements and showers. In practice the difference between second- and third-order running is minor, and furthermore first-order is default throughout.
• Updated fjcore to version v3.4.0 with minor modifications to remove compiler warnings. This version is compiled with limited thread safety, which is sufficient if "each thread is independently processing an event" and should have "essentially no speed overhead". For more details see the fjcore documentation. Thanks to Matteo Cacciari, Gavin Salam and Gregory Soyez.
• The PythiaParallel framework can now be used in the Python interface, see main162. This required some small changes to PythiaParallel, where callback functions now pass a Pythia instance by pointer rather than reference.
• The Worksheet has been updated and somewhat expanded.
• Minor fix in the new color reconnection to protect against out-of-bounds memory access.
• Fixed a bug where changing beam energies with Beams:frameType = 2 gives wrong energies.
• A fix for overestimate of photon flux from nuclei when applying PDF:beam2gammaApprox = 2 to make sure that the two applied approximations separated at PDF:gammaFluxApprox2xCut match.
• Fixed a bug related to boosts for initial-final dipoles when performing parton showers.
• Fixed an issue with tau decays interacting with the weak shower. Thanks to Mattia Di Mauro for reporting the issue.
• A templated method to shuffle contents of a vector, called shuffle, has been added to the Random numbers machinery.
• Small bug fix to avoid segmentation fault when parsing ALPGEN input.
• Tiny fix in HepMC3 header to avoid compiler warnings with GCC 12.2.

8.308: 16 November 2022

• The HIInfo class now has information about the impact parameter angle between colliding nuclei in the phi() function. In addition the impact parameter positions of the incoming nuclei are now set properly in Particle::vProd().
• A new PythiaCascade wrapper class (in include/Pythia8Plugins) implements the functionality to perform fixed-target hadron-nucleus collisions and decays, as useful for the tracking of cascades is the atmosphere or in solid materials. The code in this class now is set up to handle any target nucleus. The new main184.cc example shows how to use PythiaCascade to track an atmospheric cascade, equivalent to the standalone main183.cc. An even simpler new example is found in main185.cc.
• The option to completely forbid popcorn baryon production from the beam remnant has been replaced by the possibility of a continuous suppression, see BeamRemnants:dampPopcorn.
• A new option for Pomeron flux has been added to provide more flexibility to modify the shape of the flux.
• Modified handling for resolved-lepton remnants so that also lepton-lepton collisions with unequal masses are correctly dealt with.
• Explicit bookkeeping of Hidden Valley (HV) colours in SU(N) scenarios has been introduced. These colours can be accessed in the event record, but are not shown by default. This allows for a more robust tracking of HV colours in showers. Notably a Higgs decay to a pair of HV gluons can now be handled, which used to be impossible. The functionality of the TimeShower:recoilToColoured switch is extended also to apply to the HV colours.
• Update to decay method of ParticleDecays class allows for proper showering of leptons produced in Hidden Valley particle decays.
• A simple example of Hidden Valley production mechanisms introduced with main171.cc.
• Fixed a bug that could block the radiation of HV photons in a broken U(1) scenario, i.e. where the HV photons acquire mass. Affected was radiation in the decay of Fv states, i.e. states with a charge both in the normal and in the HV sector.
• Two new ROOT-based example programs, main94.cc and main95.cc, illustrating how to visualize events and jets, respectively, in (y, phi)-space. Thanks to D. Gillberg for contributing these examples.
• Added handling of statistical uncertainties to the Hist class, with error propagation in the various histogram operations and friend methods, for both unweighted and weighted events. Updated Hist compatibility for current Matplotlib.
• Updated JetMatching to support improved treatment of non-enhanced jets in FxFx merging, see Jet Merging for more details. The updated treatment also fixes a bug that led to an asymmetric treatment of heavy quarks and the corresponding anti-quarks. Thanks to Rikkert Frederix.
• Vincia's tune initialisation has been changed to not override initialization, event-generation, statistics, and main-program settings.
• Included particle ID code in error message when a workable decay channel cannot be found in ParticleDecays::decay(). Thanks to K. Pedro for suggesting this.
• Added additional protection for Pythia initialization.
• Fixed an issue in the tau decays which caused segmentation faults when a tau nu final state is present in the event record without a mother W. Thanks to S. Jeon for reporting the problem.
• Corrected several typos in antiName and chargeType properties for particle ID codes for diffractive states (e.g., 9904230) in ParticleData.xml; thanks to A. Verbytskyi, G. Panizzo, and D. Konstantinov.
• Updated default top quark mass to 173 GeV (up from 171 GeV) in line with current PDG average (rounded to nearest GeV). Added particle data for a few more nuclei.
• Corrected a bug that affected the read-in of RPV couplings via SLHA files. In the erroneous version, a coupling for the third generation in the first index (i.e. 3,i,j) could not be set.
• Fixed bug preventing the generation of Python bindings in MacOS. Also fixed external library issue with Makefiles for MacOS.
• Updated WeightsBase::getWeightNames to provide RIVET compatible names.
• Updating HTML manual and program banner with new manual reference.
• Updating affiliation for Christian Bierlich.

8.307: 25 February 2022

• New option with interpolation between the low-energy hadronic cross sections, used for rescattering, and the more traditional SaS/DL cross sections, used at high energies. The latter framework has also been extended to include many more colliding hadron pairs. The Pythia::getSigmaTotal and Pythia::getSigmaPartial methods give the user direct access to these cross sections. These extend and replace the Pythia::getLowEnergySigma methods. See further on Total Cross Sections.
• New "SU21" PDFs for some 20 mesons and baryons, to be used e.g. in the MPI scenarios in hadron-hadron collisions, e.g. in a cascade in the atmosphere. The pi+ default PDF has been upgraded from GRV92 LO to GRS99 LO. The GJR07 LO set has been included as an option for the proton. The default value of BeamRemnants:maxValQuark has been updated to allow also charm and bottom hadrons as incoming beam particles.
• The framework for changing the beam kinematics between subsequent Pythia::next calls when using variable collision energies has been expanded. The Pythia::setKinematics methods mirror the frame type options for the beams. See further on Program Flow and Beam Parameters.
• As a further extension of the above, a new option exists wherein the first beam particle is initialized so it can be switched event-by-event between essentially any incoming hadron, while the second beam is restricted to be a proton or neutron. Again only SoftQCD and LowEnergyQCD processes are possible. To handle this feature, the code has been extended so that some 20 different PDFs are initialized, each with a corresponding MPI initialization and storage of required coefficients. See further Beam Parameters.
• Extended and improved handling of the Hidden Valley (HV) scenario.
  • Improved tracing of hidden-valley colours through a hidden-valley shower evolution history, thereby avoiding problems for some topologies.
  • Introduced the possibility to bookkeep individual HV hadron types, in particular HV-mesons, and thereby manage masses and decay properties. The default remains the simpler scheme, where only a smaller set of HV-hadron properties need to be defined. The detailed scenario also involves a mass-dependent handling of the joining of hadronization chains, or the collapse into a single hadron.
  • The list of Zv decays has been expanded to include further HV-quarks. Some default masses have been modified.
  • It is possible to set different production rates for HV-quarks during hadronization, and for HV-diquarks leading to HV-baryon production.
  • Introduced an option to suppress production of one flavour-diagonal pseudoscalar state, representing the eta_1 flavour singlet.
  • The HiddenValley:spinFv default value has been changed from 1 to 0, to make it transparent that HV-quarks have spin 1/2 by default.
  • Expanded the list of invisible particles accordingly.
  • A few minor bug fixes.
  • A significantly expanded description in Hidden Valley Processes.
  Thanks to Matt Strassler and Suchita Kulkarni for useful input.
• Added a new class PythiaParallel for easily making runs in parallel. See main161, main162 and main163 for usage examples. NOTE: this is an experimental feature. Feedback and bug reports should be sent to authors@pythia.org.
• New option introduced, where MPI initialization data can be stored on file and reused in a later run, to save time. See further on MultipartonInteractions:reuseInit and MultipartonInteractions:initFile. Also option to symmetrize the MPI rate between e.g. pi^+ p and pi^- p, see MultipartonInteractions:setAntiSame.
• New options allow a harder fragmentation function for a beam-remnant diquark, by a separate choice of the a and b parameters, and also forbid the popcorn mechanism for it. See further Beam Remnants.
• New diffractive states introduced, to cover all five-flavour mesonic states, and all baryonic ones with at most one c or b quarks. Furthermore, nitrogen and oxygen have been added to the set of nuclei
• New processes SoftQCD:singleDiffractiveXB and SoftQCD:singleDiffractiveAX allow to set single diffraction to occur only by A B → X B or only by A B → A X.
• Reorganised and consolidated the interface to external matrix element generators. Renamed ShowerMEs to ExternalMEs and ShowerMEsMadgraph to ExternalMEsMadgraph. Introduced new HelicitySampler class to sample helicities using external matrix elements.
• New options to get and set the state of the random number generator, Rndm::getState and Rndm::setState, as a direct-transfer option to the file-based dumpState/readState methods.
• New methods Hist::fillTable to read back in a table of histogram data, specifically as written by the Hist::table methods.
• Some minor rearrangements in HadronLevel to better structure the interleaving of hadronic rescattering and particle decays. New method Pythia::moreDecays(int i) to perform the decay of a single specified particle.
• Switched tunes to by default prefer LHAPDF 6 PDFs over LHAPDF 5 ones, via the setting Tune:preferLHAPDF.
• New LHAPDF:xSymmetric parameters allow the user to choose to use symmetric PDFs for s, c, and b quarks when using LHAPDF for a performance boost. By default, c and b are taken as symmetric (this matches the previous behaviour).
• Info::pTnow() can now also be used for showers during resonance decays, and when using the shower() method. The latter so far only for the Simple and Vincia shower models.
• Corrected a bug that mainly affects beam-remnant fragmentation in the QCD CR model. Previously, diquark-antidiquark systems with invariant masses below the threshold for baryon-antibaryon production would be combined into tetraquark-like states that do not exist in PYTHIA, resulting in errors with unknown particle codes and charge non-conservation. Now, MiniStringFragmentation::ministring2two() instead reconnects such topologies into two mesons, noting that this is consistent with the colour structure. Further motivation comes from studies of low-mass junction-antijunction systems in [Sjo03].
• Introduced fix to prevent the central description of Les Houches events showered with global recoil from shifting when shower variations are enabled.
• Fixed a few issues in Vincia's sector merging. Specifically, while the merging itself still only handles QCD corrections, Vincia's QED showers can now be left on during merging. Also corrected a bug leading to incomplete histories replacing complete ones with smaller probability, and fixed an inconsistent treatment of trial showers in systems with coloured resonances.
• When running Angantyr at very low energies, initialization would abort due to an incorrect initialization of the high-mass diffraction MPI machinery. Logic has been introduced to prevent this initializatoin.
• Fixed memory leaks in the ColourReconnection class.
• Fixed some recent compiler warnings and errors; thanks to Mattias Ellert.
• Removed the deprecated SubCollisionModel called MultiRadial from the heavy ion machinery.
• Added an abort in Pythia initialization to prevent using a low energy process with a hard process. Thanks to Jenna Bacon.
• Broke a circular shared pointer reference in ResonanceWidths and ParticleDataEntry which caused a small memory leak. Thanks to Gabriele Inghirami.
• Minor fix to avoid unnecessary (attempted) MPI initialization.
• Fixed issue with Dire getting stuck when showering soft QCD events. Note, Dire has not been validated for these types of events. Thanks to Giancarlo Panizzo and Marvin Flores for reporting the issue.

8.306: 28 June 2021

• Introduced ability to write intermediate output with RIVET, as demonstrated in the updated main93.cc.
• Fixed the incorrect scale being set when applying the POWHEG veto, i.e. main31.cc. Note that the number of emissions is correct in 8.303, but is incorrect in 8.304 and 8.305. Thanks to Mikhail Kirsanov.
• Fixed repeated initialization causing an ever expanding physics pointer list in the main Pythia class.
• Fixed issue for HepMC output from Vincia, which would previously issue warnings about inconsistent mother/daughter relationships, caused by Vincia's antenna-style bookkeeping by which emitted partons have two mothers instead of one. For status codes 43, 51, and 53, the HepMC interface now ignores the second parent, always using just the first one to define the vertex structure. Minor modifications to Vincia's QCD shower to ensure that the first mother is the one that changed colour and hence would be identified with the "radiator" in a collinear context. Analogous modifications in the QED module so the most collinear parent is the first mother.
• Removed assert statements from Angantyr.
• Shortened Pythia constructor header.

8.305: 03 May 2021

• Fixed issue that otherwise caused the weights of the Dire parton shower to be erroneously discarded. The fix will impact the results produced with the Dire parton shower model. The results of the previous version should be considered incorrect.
• Fixed potential for segfault in Weights.cc if weightsShowerPtr == nullptr.
• A number of fixes related to Vincia have been included.
  • Bug fixes in Vincia's merging, addressing the issue that occasionally incomplete histories were accepted while a complete one was available.
  • Fixed some small bugs in Vincia's QED shower that could cause issues when running at very high energies.
  • Minor namespace fixes. Moved using namespace VinciaConstants from headers to source files and moved enum antFunType inside the Pythia8 namespace.
  • Added default initialisers for data members in several Vincia classes, to avoid potentially undefined behaviour. Thanks to V. Zecca.
• Fixed unintentional initial-state radiation from unresolved hadron beams when set with option BeamRemnants:unresolvedHadron which would break up the beam. Minor fix also for cases where two unresolved hadrons.
• Added threshold check on low energy cross sections. This fixes a small bug reported by Vittorio Zecca.
• Corrected bug in normalization of histograms in main07.cc. Thanks to M. Baker and A. Thamm.

8.304: 09 April 2021

• Christian Preuss and Rob Verheyen join the author team.
• Christine O. Rasmussen leaves as an active author.
• New "sector" antenna showers implemented and made default for the Vincia antenna shower model.
• Minor changes to a few of the default Vincia hadronization parameters, reflecting the move to sector showers as default.
• New dedicated LO merging scheme for the Vincia sector antenna showers. The scheme is similar to CKKWL in spirit but exploits the bijective nature of the sector shower to vastly reduce the number of histories that need to be taken into account.
• New full-fledged electroweak shower option implemented for the Vincia shower model. The splitting kernels are based on collinear limits of corresponding helicity matrix elements so this EW shower model requires Born partons with assigned helicities. Currently these must either be input via LHEF or (for experts) runtime via Vincia's interface to MG5 matrix elements.
• Introduced tools to automatically generate MadGraph plugin libraries.
• New options for interleaved resonance decays implemented for both Pythia's default (simple) shower model, and for Vincia's antenna-shower model. Made default for the Vincia shower model, while Pythia's shower keeps its previous (non-interleaved) default setting.
• Revised internal handling of shower variations and splitting enhancements, both for regular and trial emissions. Enhancements of rare shower splittings are now steered by settings instead of UserHooks, and the necessary weights are automatically included in the shower and merging weights.
• A number of updates have been made to sophisticated tau decays.
  • External SPINUP is now always interpreted as a helicity state in the lab frame to ensure consistent treatment between tau decays and helicity showers.
  • The default external mode for tau decays has been changed to handle helicity showers.
  • gamma gamma → tau^+ tau^- processes are now handled.
  • Helicity states for initial beams can be specified and used in tau decays.
• Added HDF5 support.
• Added support for lepton-photon initiated hard processes generated internally or externally. Thanks to Liu Yao-Bei.
• Minor code changes to allow hadronic rescattering in nuclear collisions (remove limit on number of rescatterings, increase maximum transverse vertex location).
• Double quarkonia production, if enabled, is now also included in both MPI and second hard processes.
• Added some new functionality to the Hist class.
• Moved the event information in Info class to an appropriate diffractive system for hard diffraction. This way also the hard-process scale is properly propagated for primordial kT sampling.
• The stopMassNow in StringFragmentation.cc is made to increase faster for charm and bottom end quarks, to allow an increased few-body fraction further above threshold. Thanks to Andy Buckley.
• Added ResonanceDecayFilterHook and example main103.cc to demonstrate how to select specific final states from resonance decays.
• Fixed vertex smearing in heavy ion events.
• Fixed remnant handling for e+e- collisions for photon-initiated processes, thanks to Murat Köksal.
• Several (mostly minor) optimisations in Vincia, thanks to D. Konstantinov.
• Minor fixes in xml/html QCD process links and text.
• Several small bug fixes based on error reports by Vittorio Zecca.
  • Fixed a bug where some elastic cross sections for pi K at low energies were set equal to the total ones.
  • Protect against too high constituent masses inside a hadron in LowEnergyProcess.cc, already at an earlier stage of the process.
  • Protect against exp(-infinity) in HadronLevel.cc.
  • Two fixes for first step in the trial emissions in StringFragmentation.cc, used primarily in the thermal model option.
  • Properly set the photon-in-lepton PDF to vanish at very large x, rather than give unphysical values there.
  • Better protection against weird results for the rap and eta methods in Basics.h, and y in Event.cc.
  • Fixed pointer logic in Dire getXPDF.
  • Migrated from log(1 + x) to log1p(x) for numerical stability.
• Several small fixes in response to a static code analysis, provided by Dmitri Konstantinov.
  • Fix inconsistency in the usage of the third argument of the dsigmaEl method in the classes derived from SigmaTotAux.
  • Flawed and unused LowEnergySigma::sigmaPartial method removed.
  • Fix inconsistent type for varWidthIn in ParticleDataEntry class.
  • Introduce minor changes, e.g. to protect against out-of-bounds addressing or insert missing initializer, even though there is no evidence that it is needed.
  • Small speedup of the Vec4 [] operator using switch() instead of if to find element to be returned.
• Bug fixed in the new possibility to let particles with narrow widths (such as onia states) obtain a simple Breit-Wigner distribution, specifically for MPIs, introduced in 8.240. This bug led to excessive rejection of low-invariant-mass interactions. Thanks to Albert De Roeck.
• Default event record checks switched off for main21.cc, to avoid issues with junction mother-daughter relations not being fully set. Also change so that showers are not called twice. Now all available cases are looped over, but easy to change back to study only one.

8.303: 01 September 2020

• Marius Utheim and Leif Gellersen join the author team.
• A new framework for Hadronic Rescattering has been developed. To enable rescattering, use HadronLevel:Rescatter = on, or see example programs main151.cc through main156.cc for use examples. Note that while the framework is ready to use, it is still being actively developed, and changes are likely in the near future. The older and much simpler rescattering models have been removed, including the related pp tunes 33 and 34, and there has been some file reorganization in code and documentation.
• A new class HadronWidths has been added to handle hadrons with mass-dependent widths. This has been done primarily because of its importance in resonance formation in rescattering, but the class is also used for decays of hadrons produced in other processes.
• A new class LowEnergySigma has been added to calculate cross sections of hadron-hadron interactions at low energies. These are used in rescattering and when performing primary collisions at low energies. Cross sections can be accessed using Pythia::getLowEnergySigma.
• Several changes in the LowEnergyProcess and MiniStringFragmentation frameworks, in particular with several new approaches to produce nondiffractive inelastic events as close as possible to the kinematical threshold. New method StringFlav::combineToLightest attempts to form the lightest hadron for a given flavour content. Also new possibility to disable popcorn baryon production, for use in small systems.
• The "decay"/mixing time of K^0/K^0bar → K^0_S/K^0_L has been changed from 1e-15 to 1e-6 mm, such that it only happens after hadronic rescattering, but still well before K^0_S/K^0_L decays.
• The Parton Vertex impact-parameter-plane assignment machinery has been completely rewritten and expanded with new options and capabilities. The default proton radius has been change from 0.7 to 0.85 fm.
• Limit the amount by which transverse smearing of the space-time quark-antiquark string breakup vertices can give large shifts, see further the new HadronVertex:maxSmear parameter. The new HadronVertex:maxTau gives the possibility to reject hadrons of suspected misreconstructed systems and try again. The introduction of MPI impact parameters is compensated by a reduced transverse hadronization smearing. Also some other related changes.
• The internal generation of photon-induced processes in hadronic collisions has been extended. This can be enabled with the new options PDF:beamA2gamma and PDF:beamB2gamma, which supersede the previous PDF:leptonA2gamma option. Some minor updates also for other related options, see PDF Selection for details.
• New photon fluxes from hadrons implemented, see PDF Selection for details.
• New example main program main78.cc to generate elastic, single- and double-dissociative photon-induced processes in proton-proton collisions, using options PDF:beamA2gamma and PDF:beamB2gamma. Also the sample programs main69.cc and main70.cc have been updated to use the new options.
• For asymmetric beams (e.g. electron-proton collisions) in Les Houches event files, a boost to the event CM frame is now defined and applied for outgoing particles, as required for succesful event processing.
• The problem of "dangling" gluons causing changing color flows in SimpleTimeShower for top decays is now fixed. The fix is not fully validated for RPV.
• For LHE processes, the width of the primordial kT distribution is now set solely by the BeamRemnants:primordialKThard parameter, ignorning BeamRemnants:primordialKTsoft. This change is so that LHE events with low-pT extra jets (e.g., in the context of POWHEG-style merging) are given the same primordial kT as their Born-level counterparts.
• Improved HepMC3 multiweight output.
• Added automated renormalization scale variations in CKKW-L type merging schemes.
• Added UNLOPS merging scheme variations.
• Added warning messages in UNLOPS merging.
• Added flexible runtime plugin loading for Dire and Vincia matrix element corrections from Madgraph. These corrections can be enabled with the --enable-mg5mes flag during configuration and with the Dire:MEplugin and Vincia:MEplugin settings.
• Introduced common runtime plugin loading and unloading structure.
• Removed PDF set tracking for LHAPDF6 which breaks thread safety. In some cases this might cause a small penalty in memory.
• Removed SLHA:keepSM as it duplicates the functionality provided by SLHA:minMassSM.
• Some new methods in the Hist and HistPlot classes. Hist::plotFunc creates a histogram from the function provided as first argument, by evaluating its value in the middle of each histogram bin. Hist::normalize can rescale a histogram to an intended sum of bin contents. Methods to get x limits and lowest and highest y value. Hist::smallestAbsValue renamed Hist::getYAbsMin for consistency. HistPlot::addFile can add a file of data points, with or without errors, on to an existing histogram, for comparison purposes. A new variant of HistPlot::plot can set the frame borders.
• New LinearInterpolator class in MathTools.
• New RotBstMatrix methods toSameVframe and fromSameVframe sets up transformation to/from a frame where two particles move towards each other with equal-size velocities along the +-z direction.
• Some new functionality accessible via PythiaStdlib.h.
• It is now possible to use range-based for loops to iterate over particles in an event, e.g. for (Particle& p : pythia.event). Similarly, the particle data table can be iterated over, with each entry being a pair<int, ParticleDataEntry&>.
• New possibility to redirect particle data listing output.
• A configure option --obj-common='FLAGS' to add additional compiler flags to object (.o) compilation (i.e. no effect on linking).
• Compilation flag -DGCCFPDEBUG enables floating point exceptions on a gcc compiler.
• New index() method in the Py8Particle class of the Fastjet3 interface ensures that the location of the particle in the PYTHIA event record is returned correctly. Thanks to Gavin Salam and Priyotosh Bandyopadhyay.
• A minor fix for kinematics of zero-virtuality photons to retain physical kinematics.
• Protect against diffractive masses exceeding phase space bounds for hard diffraction.
• A bug for propagation of space-like partons in string shoving. Thanks to Dong Jo Kim.
• Several indexing bugs for the "guess" option used in CKKW-L merging have been fixed. However, this does not yet mean that "guess" is guaranteed to be stable for any BSM process.
• Several fixes for problems discovered in a static code analysis, including one bug fix in LowEnergyProcess.cc. Thanks to Dmitri Konstantinov.
• Mixup of ASecondHardProcess and SecondHardProcess labels resolved. Thanks to Anton Karneyeu.
• Number of weights in info reset to 1 for each new init call.
• Fixed issue where the dark matter mass was reset even after explicitly being set by the user, e.g. 57:m0 = X. This affects particle IDs 57 (DM+), 58 (DM2), and 59 (DM++).
• An indexing bug for the scattering lepton in DIS events with photon radiation allowed has been fixed.
• The redefinition of dlsym for plugin loading has now been removed as this is no longer required by modern compilers. Thanks to Chris Burr.
• Several small bug fixes based on error reports by Vittorio Zecca.
  • In ProcessContainer.cc the error calculation for Les Houches strategy +-3 should not be renewed for each file when the event sample is split across several files.
  • In ProcessContainer::trialProcess() a call to phaseSpacePtr->rescaleMomenta( sHatNew); should not be done since the momenta have not been set yet.
  • In the CT14qed PDF grid (and others?) the first two x grid values coincide, so one must not divide by their difference.
  • Default initialization of hasOwnEventAttributes in Info.h is necessary.
  • Protected against explicit evaluation of alphaS(LambdaQCD) in VinciaISR. Also added explicit member-variable initialisations for the QEDemitElemental class in VinciaQED.
  • Protected against floating point exceptions caused by dividing by zero in PhaseSpace for zero-width resonances.
  • Corrected a floating point exception in Ropewalk.cc.
  • Protected against out of range renormalization scale access for diffractive systems.
• Some further replacements of fabs by abs.
• Fix missing-pointer bug when colour reconnection is switched off for diffraction. Thanks to Valentina Zaccolo.

8.302: 24 April 2020

• A HepMC 3 interface is now included in Pythia8Plugins/HepMC3.h. All HepMC examples have been migrated from using the HepMC 2 interface to HepMC 3, excluding main44.cc and main93.cc which are kept for legacy purposes.
• Bug fix to correct units for automated shower uncertainty weights for lhaStrategy = ±4. In addition, the combination of weight variations, within the context of uncertainty bands, was not previously well documented and was therefore prone to misunderstandings. Now, all methods related to cross sections and weights are documented together on a single page, Cross Sections and Weights, the documentation on shower Variations has been improved, and the behaviour of Info::getGroupWeight() has been made more transparent. The main121.cc example has been modified accordingly. Note that the bug fix for lhaStrategy = ±4 parallels an analogous one made in Pythia 8.244 which had not been previously carried over to Pythia 8.3.
• Fix for subscripting issue for photon beams, courtesy of Vittorio Zecca.
• New particle property directive tauCalc allows for a particle's proper lifetime to be set independently of its width. This is useful for studying the effect of lifetime for a particle when the particle width is otherwise negligible.
• Significant speedup of parton distribution handling, and thereby of the whole PYTHIA execution, since PDF is such a major part of the total execution time. This has been achieved by careful work by Dmitri Konstantinov and Grigory Latyshev. In detail, the main changes are:
  • In the LHAPDF6 interface the evaluation of the x and Q^2 limits is extremely slow, such that a caching of these numbers increases execution speed by about a factor of 2 when LHAPDF is used.
  • A caching of info on the amount of x already used up helps speed up MPI, ISR and FSR execution times, giving gains of order 20%. Main changes are in the BeamParticle class.
  • Optimization of array handling in LHAGrid1 give a speedup by about 5%. This is the internal PYTHIA implementation of interpolation in LHAPDF6 grids which, still after the LHAPDF6 improvements reported above, runs more than 50% faster than using the same PDF evaluated by LHAPDF6. This streamlining is made at the cost of not offering facilities like PDF error bands.
  • Also the special internal routines for the NNPDF 2.3 series could be speeded up, but would still be slower than an equivalent LHAGrid1 implementation. The NNPDF 2.3 sets therefore have been moved to the latter framework.
• New base class ShowerModel introduced, as a top-level handler of parton-shower models, interfaced via a shared pointer in Pythia.cc. Changes to Pythia's default showers and to the Vincia and Dire shower models to inherit from ShowerModel. New public method Pythia::getShowerModelPtr() intended to provide a way to access public ShowerModel methods for diagnostics purposes.
• Extra protection against unexpected input in e+e- Les Houches events, specifically where x >= 1 by roundoff errors. Thanks to Alexander Belyaev, Alexander Pukhov and Marco Zaro.
• Bug fix in SusyLesHouches::checkSpectrum(), to ensure uniform error handling when one or more spectrum self-consistency checks fails.
• Replaced use of fabs() by abs() in several places. Although default C++ abs() only takes integer arguments, PythiaStdLib.h contains the declaration using std::abs (via the cmath header), which is suitably overloaded and obviates the need for separate methods.
• Improvements to Vincia's handling of error messages; now issued via Info::errorMsg().
• Removed share/Pythia8/tunes directory, with Vincia reverting to the same (in-code) way of setting tune presets as Dire and Pythia.

8.301: 30 October 2019

• Compilation is done with a new -std=c++11 flag.
• The Vincia and Dire parton-shower programs, which previously have been available as free-standing add-ons to Pythia, are now fully integrated into Pythia, and will henceforth be distributed as part of this package. This has significant implications for code and documentation. The new master switch PartonShowers:model allows a simple choice of which to use, with 1 being the simple old internal one, 2 being Vincia and 3 being Dire. Note that Dire always comes with weighted events, while the other two by default have unit weight events, so analysis strategies must be adjusted accordingly.
• MadGraph-generated plugins for matrix element corrections to the Vincia and Dire parton-shower programs are now available. A new plugins directory has been created, where MadGraph5 matrix elements can be stored. Note that this implementation still is preliminary, with further improvements to come. To build the available plugins, --enable-mg5mes should be specified.
• Four new main programs have been introduced in the context of illustrating the Vincia and Dire shower models. Two of them also serve as examples of multithreading if Pythia is configured to link to OpenMP. main200.cc provides a simple test of final-state shower components, in a LEP-like setup. main201.cc compares two different shower models on dijet events. The two runs are executed in parallel if Pythia has been configured to link to OpenMP. main202.cc measures the running time for ttbar events and outputs a few test distributions. main300.cc can be linked against both HepMC2 and OpenMP, if Pythia has been configured correspondingly. It is capable of command-line instructions.
• A new plugin header, Visualisation is available and used in the example program main300.cc. Events can now be visualized via output that can be passed through GraphViz to produce a postscript file.
• Introduced a compromise for the treatment of lifetime and width, so that these can be set separately in some situations. If an SLHA file is used to set particle properties, then SLHA:allowUserOverride = true allows the lifetime to be set using a command file/string. Otherwise, lifetime is calculated from particle width. The ability to have the width and lifetime as independent properties is convenient for studies of new physics.
• The HTML and XML versions of the manual have been significantly reordered, with contents grouped in an increased set of main headings, each with an expandable index of subheadings. There are several new sections, notably for the Vincia and Dire showers. Among other changes, the example main programs have been classified by keywords, and are listed and searchable by these, see here.
• The PHP version of the manual has been discontinued, since it would be difficult to maintain in conjuction with the ongoing expansion of the html manual. The include/Pythia8/phpdoc directory has been removed.
• The Python interface has been completely redesigned to handle C++11 by migrating from a SWIG generated interface to a PyBind11 one. It is now possible for users to generate their own interface. Full details are given on the Python Interface page.
• The field Pythia::info of type Info has been made a constant reference, so that it cannot be externally manipulated outside of the internal classes. This constant reference points back to Pythia::infoPrivate instance, which is for internal use only. This instance now also contains pointers to several other commonly used classes.
• A new base class PhysicsBase has been created, from which several of the "physics" classes derive. This class standardizes the setup of pointers to commonly used classes, via the Info class. It has new methods that are called at the beginning and end of the generation of each event, and at the printout of final statistics. A new enum Status is provided in the method at the end of each event, to provide information on the reason for error exits. The access of the most commonly used Settings methods is streamlined.
• Most of the pointers in the Pythia class are now shared pointers, such that there is no need for Pythia to keep track of new and delete obligations. New header SharedPointers.h.
• The CoupSM and CoupSUSY couplings classes are now separated, rather than the latter being derived from the former. This avoids the problem of the old Couplings pointer potentially changing address, and some pointer casting.
• Mathematical functions and methods have been extracted from the PythiaStdlib.h/.cc files and put in new MathTools.h/.cc files. Thereby PythiaStdlib is more clearly reserved for declarations related to the Stdlib library, to some simple inline expressions (now constexpr where feasible) and to an expanded list of conversion constants.
• A new method Rndm::phaseSpace2 has been added, and can be used to sample two-body phase space.
• The heavy ion machinery has been fixed so that particle vertex information in the event record is stored in units of mm rather than fm, while internal calculations still are in fm. Thanks to Christopher Plumberg.
• The data files related to parton distributions have been moved to a new share/pythia8/pdfdata directory, and code has been changed accordingly.
• Iterators are made available to simplify the study of the particles in the event record.
• A bug in the XML-to-HTML conversion has led to the indiscriminate removal of "more" from the HTML manual, notably in Pythia::moreDecays().
• New class LowEnergyProcess for the hadronization of low-energy collisions between two hadrons. By default there is a smooh transition between this simple nonperturbative model and the full MPI-based framework at higher energies. To be used e.g. within the modelling of hadronic rescattering in the final state of high-energy collisions. This framework is still under development.
• New StringInteractions base class to provide common administrative framework for string shoving, rope hadronization, swing, (some) colour reconnection models, and more.
• The headers in include/Pythia8Plugins have been updated to no longer contain using namespace Pythia8. Thanks to Axel Naumann for pointing out inconsistent usage of scope.
• The build system has been updated to handle external package configuration scripts.
• The ProMC example and configuration option has been dropped.
• Optional package configurations for RIVET and YODA have been added.
• Support for BOOST has been dropped and only LHAPDF6 with versions greater than 6.2 are supported.
• The flag --openmp has been added. Currently, OpenMP is used in some of the examples mentioned above, and in DireWeightContainer.cc.
• Bug fix so the option UncertaintyBands:ISRpTmin2Fac is correctly used in SimpleSpaceShower.cc.
• Bug fix stopping growth of number weights when multiple init calls are made in Info.
• Bug fix for SusyWidthFunctions.cc incorrectly calling integrateGauss, resulting in a width of zero.
• Bug fix to prevent overwriting of dark matter masses from unused model selections.
• Bug fix where instead of counting all PDG ids less than 100 as the Standard Model, masses of BSM Higges and dark matter are allowed to be set without changing minMassSM.
• All 2 to 2 SUSY processes now inherit from Sigma2SUSY.
• Bug fix for Les Houches Event File input without specified beams, such that multiparton decays of a resonance can be combined with showers, although with proper care needed so as not to doublecount emissions.
• Clarify that Q2 scale has to be set by hand for DIS processes, if so desired.