Update History
 
  - 8.314: 11 April 2025
- 8.313: 10 January 2025
- 8.312: 23 May 2024
- 8.311: 08 March 2024
- 8.310: 25 July 2023
- 8.309: 16 February 2023
- 8.308: 16 November 2022
- 8.307: 25 February 2022
- 8.306: 28 June 2021
- 8.305: 03 May 2021
- 8.304: 09 April 2021
- 8.303: 01 September 2020
- 8.302: 24 April 2020
- 8.301: 30 October 2019
These update notes describe major updates relative to the 
PYTHIA 8.243 version, which was the starting point for PYTHIA 
8.3. The step from 8.2 to 8.3 gave an occasion to break backwards 
compatibility. The main change is that C++11 is now assumed. 
The impact on existing used code should be mild, however.8.314: 11 April 2025
 
 
 
- Both authors of the DIRE parton shower code have now left the 
Pythia team, and the field of particle physics. Since there is 
currently not sufficient person power to properly maintain the code, 
we are considering removing DIRE in a future release. Users of DIRE 
who are interested in maintaining the code in the future should 
contact authors@pythia.org.
- A batch generation interface in Python is now available using 
Awkward 
arrays. Thanks to Aartem Havryliuk and Jim Pivarski for providing 
this interface, as well as IRIS-HEP for funding this work.
- Introduced new options to treat the aLundandbLundparameters in the Lund symmetric 
fragmentation function as derived quantities. Instead, the 
user can specify a desired mean and RMS width of the 
Lund FF. Options to allowaExtraDiquarkandaExtraSQuarkto be derived from corresponding 
multiplicative factors are also made available.
- New option in SimpleTimeShowerto interpolate 
smoothly between applying and not applying the RF eikonal reweighting 
factor forrecoilStrategyRF = 0, in decays of coloured 
resonances such as t→bW, via the new parameterweightRF. New default behaviour is to apply the RF 
reweighting factor at full strength (weightRF = 1.). 
Does not affect the first emission in top decay (which is anyway ME 
corrected) but will affect second and subsequent emissions. Thanks to 
D. Hirschbuehl and E. Schmidt for suggesting and validating this 
update.
- The example main144.cchas been updated so that it 
can be optionally run with ROOT or Rivet. The ROOT streamer dictionary 
formain144has also been updated so that it has no 
Pythia dependencies and is generated with CLING rather than 
CINT. Finally, a ROOT macromain144.Chas been added as 
an example on how to read the generated ROOT file.
- Reintroduced the zHadmember to theStringEndclass for access viaUserHooks.
- The parsed user supplied configuration command is now written to 
Makefile.inc. Thanks to Andy Buckley for the 
suggestion.
- Minor documentation updates for the string Close Packing 
model.
- The Glauber statistics in Angantyr are now reset any time the 
beams are changed.
- Internally, beam particles are now stored and passed as shared 
pointers. This change should have no effect on public facing 
interfaces such as UserHooks.
- Fixed bug in VINCIA that caused Print:verbosityto be 
increased by one. Also added more detailed warning message when 
encounteringPaccept > 1in the VINCIA ISR 
shower. Thanks to B. Tuncay.
- Fixed incorrect verbosity setting in VINCIA's massive final-final 
branching kinematics introduced in 8.313 that resulted in incorrect 
warnings about failed kinematics.
- Fixed typo in main487.cc, mildly affecting 
results.
- Fixed bug in Hidden Valley fragmentation, whereby the lowest 
vector meson mass could be incorrectly identified. Thanks to Matt 
Strassler, Rabia Husain and Junyi Cheng.
- Explicitly declared inline methods in Pythia.hfor 
C++17 compatability. Thanks to Jonas Rembser.
- Fixed bug where, if an initialisation file is read in, the beam A 
id in MultipartonInteractionswas set to the first beam 
in theMultipartonInteractions::beamAList, rather than 
keeping the one requested inBeams:idA.
- Updated links to download the EPPS16 PDF sets. Thanks to Wilke van 
der Schee.
- Fixed issue with a high failure rate in fixed target Angantyr 
events at very high energies due to momentum non-conservation. Thanks 
to E. Elén.
- Fixed issue when using Tune:preferLHAPDF > 0which 
resulted in an incorrect PDF set, thanks to Markus Seidel.
- Added the constructor LHEF3FromPythia8(Pythia*)to 
allow for writing of LHEF files via the Python interface. Thanks to 
Andre Sznajder for bringing this to our attention.
- Debug feature for catching floating point exceptions extended. 
Enabled with compilation flag -DGCCFPDEBUG. This is only 
available withgcconx86/x86_64platforms.
- Fixed floating point exceptions in 
MultipartonInteractions.cc,HISubCollisionModel.handHISubCollisionModel.cc. Generallly, these exceptions 
should not have effected the output of Pythia.
- Fixed issue with RIVET linking in the examples.
- Corrected a bug in the Vincia QED spin-dependent RF antennae where 
scalars were previously treated the same as fermions. Thanks to 
G. Morgante.
- More verbose error message added when the XML directory is not 
found. Thanks to Bjarne Stugy.
- Corrected file name for the ATLAS A14 NNPDF2.3 LO tune. No physics 
results should change as it was only the file name itself that was 
incorrect. Thanks to A. Buckley.
8.313: 10 January 2025
 
 
 
- Leif Gellersen leaves as an active author.
- Marius Utheim leaves as an active author.
- Rob Verheyen leaves as an active author.
- A new merging setup for DIS processes has been implemented, see 
[Hel24]. The flag Merging:doDynamicMergingcan 
now be used to set the merging scale dynamically according to a 
function that interpolates between the regions of large virtuality and 
large transverse momentum. The new merging scheme is available only 
for VINCIA and showcased in themain164dis.cmndcommand 
file for themain164.ccexample.
- A new parameter Diffraction:sigmaRefPomPomis 
introduced to describe the effective pomeron-pomeron total cross 
section, used to normalize the multiparton interactions machinery 
in central diffraction. PreviouslyDiffraction:sigmaRefPomPwas used also for this purpose, 
but now it is only used for single and double diffraction.
- Central diffraction initialization of multiparton interactions 
is now not performed when the central diffractive cross section 
is vanishing, specifically for the default values 
SigmaDiffractive:mode = 1in combination withSigmaTotal:zeroAXB = on.
- Several minor changes made to Angantyr. 
 
- A modified beam remnant treatment has been introduced.
- Accessors for Glauber statistics has been fixed for bugs giving 
faulty output for pA configurations, thanks to Antonio Ortiz Velasquez.
- Accessors for Glauber level cross sections were renamed and 
expanded and an accessor for the elastic amplitude was added.
- HepMC2 and 3 output were updated accordingly.
- Also the output of Pythia::stat()has been modified 
with a clearer description of the cross sections listed.
- Unit conversions in Angantyr changed from Angantyr specific ones, 
to use the same as the rest of Pythia.
- A new option HeavyIon:forceUnitWeightwas introduced 
to change the Angantyr impact parameter sampler to give unit weights 
added.
- Finally, for stability reasons when going to very small energies, 
the default Angantyr:CollisionModelhas been changed 
from1to2.
 
- Cleanup of several main programs related to heavy ions and cosmic 
rays, notably to take into account new features in Angantyr. 
 
- The main426was added to illustrate how to use Glauber 
statistics and gather cross sections.
- The new main427shows how to change the Angantyr 
impact parameter sampler to unit weights.
- The new main487compares a hadronic cascade 
simulated either withPythiaCascadeor with Angantyr, 
in a mixed atmosphere of nitrogen, oxygen and argon.
 
- Updated handling of fragmentation parameters in the 
Hidden Valley. 
Three levels of user control are introduced by the new 
HiddenValley:setabsigmaswitch, accessing default 
values updated to be based on the Monash tune, with explicit 
prescriptions for scaling rules from the corresponding QCD parameters 
to the Hidden Valley ones. Some parameter names and behaviours are 
changed, fixing previous inconsistencies. Newmain511.ccexample illustrates scaling from QCD to HV. Also note, in case you 
missed it, that the Hidden Valley parton shower parameters were partly 
changed in version 8.309, see notablyHiddenValley:alphaOrderandHiddenValley:setLambda. Thanks to Matt Strassler 
and Suchita Kulkarni for valuable input.
-  The logic for modifying string fragmentation with 
UserHookshas changed.canChangeFragPar()will now only enabledoChangeFragPar(), while the new 
functioncanChangeVetoFragmentation()enables thedoChangeVetoFragmentation()functions and the newdoChangeVetoFnalTwo()function.  See 
User Hooks for details.
- An experimental feature has been introduced where the 
fragmentation sequence of Pythia can be modified by the user through 
the  
set/add/insertFragmentationPtrmethods. Details are 
given in in the implement new 
fragmentation section on how to create a custom fragmentation 
model via theFragmentationModelbase class. Some further 
modifications have also been made to enable more control of including 
plugins. 
- Vectors of external pointers in Pythia where the method 
addis allowed can also now be set with aninsert(i)method, whereiis the vector 
insertion index.
- The insert(i)statement can also be used 
when loading plugins with theInit:pluginssetting.
 
- The weighting of the fragmentation can now be determined for 
arbitrary flavor parameter variations after an event has been 
generated by setting VariationFrag:flav = on, see 
Hadronization Variations, 
and then using theFlavorVariationsclass provided ininclude/Pythia8PluginsFlavorVariations.h. Seemain264.ccfor an example. This is an experimental 
feature.
- The tune settings have been migrated to command files under 
share/Pythia8/settings/tune. Under the hood, the tunes 
are now loaded using these settings, but there should be no difference 
to users. In addition to the previousTune:eeandTune:ppsettings, it is now possible (and preferred) to set 
tunes with the commandinclude = tune/settings.cmndwheresettings.cmndis the tune settings file. The following 
additional changes have also been made to enable this new tune setup. 
- The reading of command files has been migrated from the 
Pythiaclass to theSettingsclass for 
technical reasons. This should not directly impact users.
- The syntax Setting = DEFAULTis now available to 
reset a setting to its default value.
- The default for Tune:preferLHAPDFhas been updated to0so that the internally available PDF is used by 
default. An additional option,3, has been added to also 
allow for the use of LHAPDF 6 files via PYTHIA's internal LHAGrid 
interpolation, which does not require the LHAPDF 6 library.
- The default for Vincia:tunehas been changed from0to-1. Users must now explicitly switch on 
the VINCIA tune, either withVincia:tune = 0orinclude = tune/VinciaDefault.cmnd.
 
- The format and procedure for reusing initialization data has been 
changed. The options 1,2, and3forMultipartonInteractions:reuseInitand 
corresponding modes in Angantyr works as before, but now internally 
use settings in the formInit:reuseXXXto store the 
initialisation data, and the file format changed so that the files can 
be read in as normal settings. In the future theXXX:reuseInitsettings will be removed and the 
initialisation will always use data inInit:reuseXXXif 
present and consistent, and will always store the initialisation there 
for the user to write to a file and use in another run if needed.
- The number of pT bins used in the (differential) MPI 
cross section initialization has been reduced from 100 to 50, and 
is now hardcoded in NSUDPTS. TheMultipartonInteractions:nSamplenow represents the total 
number of phase space samplings, rather than that per bin.
- Removal of the pearl fragmentation procedure due to both infrequency 
of usage and in order to simplify the fragmentation code. Should this 
pearl fragmentation need revisiting, the version of the code 
remains in 8.312.
- Closepacking parameters have been reparameterised and 
renamed.
- The SimpleTimeShower::list()has been expanded 
to show a few more properties of the current dipole ends.
- When Print:verbosityis set to 3 (report mode), 
errors and warnings are printed every time they occur instead of just 
the first time.
- Documentation updated to reflect previous changes in settings 
scheme, where a parameter set outside limits without using the 
keyword force, will make PYTHIA fail to initialize. 
Thanks to Giancarlo Panizzo, who pointed out that the documentation 
was lacking.
- Updated the HTML manual description of histograms to match the 
default options for Hist::table.
- A few minor upgrades of four-vector handling, the main update being 
an improved calculation of a very small opening angle between two 
vectors.
- Method added for peeking at next random number in 
PythiaRndm. Intended for debugging.
- A convenience class for parsing command line input 
Pythia8Plugins/InputParser.hin examples was added.
- A lightweight interface to the 
YODA histogramming library has 
been introduced in include/Pythia8Plugins/Pythia8Yoda.h. 
Example user code available in the new examplemain114.
- Several improvements to the HepMC2/3 interfaces from Andrii 
Vertbytskyi have been included.
- Example programs optionally using RIVET are now working correctly, 
as a missing flag in the makefilein theexamplesfolder has been added.
- New possibility to smear the mass of a hadron provided in 
Les Houches input, see LesHouches:smearHadronMasson the Les Houches Event Files page. 
The reason could be that a rho meson, say, has been 
generated with a fixed input mass, which then distinguishes 
such a rho meson from ones generated inside PYTHIA. 
Thanks to Andy Buckley for the suggestion.
- The full Python interface, generated with ./generate 
--fullin theplugins/pythondirectory, now 
includes classes frominclude/Pythia8Pluginswith no 
dependencies. Now,main295.pyshould work when the full 
Python interface is used. Thanks to Matt LeBlanc for pointing this 
out.
- The full Python interface is now packaged with the Pythia release 
by default, to switch back to the simplified interface, it can be 
generated with ./generate --partin theplugins/pythondirectory.
- An issue with sigmaGenoutput was fixed when using 
the CKKW-L scheme. Thanks to Giancarlo Panizzo for reporting 
this.
- A number of bugs have been fixed in VINCIA's merging framework 
that previously prevented the use of VINCIA's evolution variable as 
the merging-scale definition. Also, the way cross-section estimates 
are calculated in VINCIA's merging has been changed, so that "dry 
runs" without shower agree better with actual merging runs. 
- The StringEnd::kinematicsHadronmethod has been 
factorized to remove z selection. This also fixed a small bug 
where a zero momentum was returned for hard baryon fragmentation in 
beam remnants.
- Minor correction in the joining of the fragmentation from the two 
string ends towards the middle: if a popcorn diquark is produced in 
the final steps, it should not contribute with two separate diquark 
masses in the calculation of necessary remaining energy for the final 
two hadrons.
- Fixed tiny typo in StringFragmentation::extraJoin, 
found by Pi Duan. Only affects of the order of 1% of events, and then 
only mildly, but enough to break the random-number sequence.
- A bug causing the popcorn destructive interference mechanism to 
not work has been fixed.
- Fixed flavour sampling of enhanced gamma → f fbar 
splittings in the QED final-state shower. Thanks to Alexander Puck 
Neuwirth.
- A few bugs related to the TimeShower:recoilStrategyRFhave been fixed. In addition, the correct recoiler in strategy2and3is now assigned also in the case 
when the decay of the coloured resonance produces more than two 
particles.
- Bug fix in the QCD Colour-Reconnection model 
(ColourReconnection:mode = 1) to prevent infinite 
recursions when handling junction-junction connections. Thanks to 
Laurent Dufour for alerting us to this bug.
8.312: 23 May 2024
 
 
 
- The Sample Main Programs in 
the examplessubdirectory 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-digitmainNN.ccstructure 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 codesmainNNN.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, wheremain162.ccandmain164.ccprovide 
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 = 1has been sped up by over a 
factor of two, thanks to Michal Kreps.
- Variable energy beams have been added to Angantyr. See 
main424.ccfor details.
- Weighting of the fragmentation with parameter variations is now 
available. See Hadronization 
Variations for more details and main263.ccfor an 
example. This is an experimental feature.
- New parameters BeamRemnants:heavyQuarkEnhanceallows 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.ccandmain296.py.
- The option --versionis now available forpythia8-configwhich 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 
SettingsandParticleDataclasses.
- Tiny correction in configurethat fixes issue withpythia8-configon 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 
main301for 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:pearlFragmentationandStringFragmentation: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 parameterColourReconnection: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:dipoleMaxDistin 
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:tensionRatioand otherClosePackingparameters. See further on 
Close-packing.
- Include strangeness enhancement for string breaks next to a 
junction, controlled by 
StringFragmentation:strangeJunctionsandStringFragmentation:strangeJuncFactor.
- Include popcorn destructive interference via simple diquark 
suppression controlled by ClosePacking:qqFacQandClosePacking: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:SigFitDefParis changed. For the caseAngantyr: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.cchas had minor changes and 
cleanup.
- A fifteen percent uncertainty has been included for the 
bslope parameter of theHeavyIon:SigFitErrparameter 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 
readStringorreadFilewith 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 
theforcemethods in theSettingsclass.
- 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:pTdampMatchparameter. Themain202example 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 ormain164.
- New Vincia example programs for EW showers, 
main205.cc-main207.cc.main205illustrates electroweak showers in Z decays to 
neutrinos (at very high Z mass).main206illustrates EW 
showers for dijets at LHC and was previously namedmain203.main207.ccillustrates read-in of 
LHEF files for EW showering with Vincia.
- New main208example 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 pT2nextQCDwhenOniaShower:octetSplitwas set to0(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, andnameProcmethods in theInfoclass to prevent exceptions when requesting an 
invalid process ID.
- Fixed issue where if Event::clearis called before 
the particle database was initialized caused a segmentation 
fault.
- When using TauDecays:mode = 2or3the 
parameterTauDecays:tauPolarizationwas 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 SPINUPinformation and withTauDecays:externalMode = 1, the 
production matrix element for a tau produced from W 
or W' boson was being incorrectly set to that of of aZboson. This has now been fixed. Thanks to Florian Harz 
for reporting this.
- The heavy ion information, provided by the HIInfoclass and available through thehiInfomember of theInfoclass, 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 SubCollisionModelclass, where the 
functionssigSDE(),sigSDEP(), andsigSDET(), gave the wrong values. This will only affect 
the behaviour of Angantyr, in case theNaiveSubCollisionModelclass 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 BeamSetupclass 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 inmain157andmain158.
- 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 = onand using thePythia::setKinematicsmethods. In addition Angantyr can 
now handle allBeams:frameTypeoptions.
- MPI initialization reuse has now been enabled for heavy ions. The 
MultipartonInteractions:reuseInitandMultipartonInteractions:initFileparameters control 
whether the MBIAS Pythia object will reuse MPIs. In addition, theHeavyIon:SasdMpiReuseInitandHeavyIon:SasdMpiInitFileparameters 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:SigFitReuseInitandHeavyIon:SigFitInitFilesettings.
- The HIUserHookssource has been split intoHIInfo,HINucleusModel, andHISubCollisionModel.
- Some code is offloaded to new classes NucleusandSubCollisionSet, simplifyingHeavyIons.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 theSimpleTimeShower. See 
further the TimeShower:recoilStrategyRF 
switch, which replaces and extends the oldTimeShower:recoilToColouredflag. 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.
Introduced a new use plugin system, see 
Plugins for details. This system is currently 
experimental and feedback is welcome. As part of this new system, a 
few changes have been made to how user provided classes are passed to 
aPythia> object for consistency. 
 
- The arguments to the setSigmaPtrmethod are now 
shared pointers. Additionally,addSigmaPtrhas been 
introduced with the previous behaviour ofsetSigmaPtr, 
and the newsetSigmaPtroverrides all previously added 
pointers.
- A similar change has been made for the 
setResonancePtrmethod with the addition of theaddResonancePtrmethod.
The newLogger class now handles printing such as 
initialization information and diagnostic messages. For more information 
about this class, see the Logging page. 
 
The new BeamSetup class has been introduced to handle the 
beam configuration, including initializing and changing beam identities, 
kinematics and parton distributions. This is a structural change that 
simplifies the code in the Pythia class, but does not 
introduce any functional changes. 
 
Modified interface to LHAPDF6 to allow for 
multi-threading. Thanks to Christopher Jones for the report. 
 
Updated the Python interface to use pybind11 version 
2.10.4 which is compatible with Python 3.11. Thanks to Mattias Ellert 
and David Runge for reporting the incompatibility and maintaining the 
Fedora and Arch Linux packages. Note, this version is not compatible 
with Python versions 3.5 and below, and so version 2.9.2 of 
pybind11 is used automatically for these versions. 
 
Fix for the Python interface working with gcc13, thanks 
to Dmitri Konstantinov. 
 
Fixed a bug in the Python interface which prevented 
main162.py from running correctly when passing a custom 
function for initialization. 
 
Updated the Lambda_b0 lifetime to c*tau = 0.44 
mm. The value was rounded to two significant digits since PDG 
and HFLAV do not quite agree to three digits. Thanks to A. Rossi for 
pointing out the previous default value (0.369 mm) was 
outdated. 
 
Included several small fixes in response to running an LLVM static 
code analysis, thanks to Michal Kreps. 
 
Fixed out-of-range access when calling Hist::getYMin 
and Hist::getYMax for a histogram that has not been 
booked. 
 
Fixed BeamRemnants:unresolvedHadron option that was 
broken and had no effect since 8.307 
 
Removed use of simple-template-ids in 
SusyLesHouches.h to make compliant with 
C++20. Thanks to Jonas Hahnfeld. 
 
Fixed dipole sorting behaviour in ColourReconnection 
which resulted in an ordering that depended on the current memory 
state of the program. 
 
Changed the default for Parallelism:balanceLoad to 
on and updated the documentation. This ensures each 
thread will always generate the same number of events. 
 
Added the += notation for settings vectors, as well 
as the include = fileName directive, which allows for 
additional settings files to be directly read in. 
 
Fixed a bug in the bookkeeping the number of hadrons in the string 
fragmentation. 
 
 
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:runtimeAMCATNLOInterfaceon 
Merging page for further info and 
references.
- The PowhegHooksnow work with all three shower models 
and automatically switch between the simple shower, Vincia, and Dire 
based on thePartonShowers:modelmode. 
In addition, the option to setPOWHEG:nFinal = -1has 
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 AlphaSUNclass 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 fjcoreto 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 PythiaParallelframework can now be used in the 
Python interface, seemain162. This required some small 
changes toPythiaParallel, where callback functions now 
pass aPythiainstance 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 = 2gives 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 HepMC3header to avoid compiler warnings 
with GCC 12.2.
8.308: 16 November 2022
 
 
 
- The HIInfoclass now has information about the impact 
parameter angle between colliding nuclei in thephi()function. In addition the impact parameter positions of the incoming 
nuclei are now set properly inParticle::vProd().
- A new PythiaCascadewrapper class (ininclude/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 
newmain184.ccexample shows how to usePythiaCascadeto track an atmospheric cascade, equivalent 
to the standalonemain183.cc. An even simpler new example 
is found inmain185.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:recoilToColouredswitch is extended also to apply to the HV colours.
- Update to decay method of ParticleDecaysclass 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.ccandmain95.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 
Histclass, with error propagation in the various 
histogram operations and friend methods, for both unweighted and 
weighted events. UpdatedHistcompatibility 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 antiNameandchargeTypeproperties for particle ID codes for 
diffractive states (e.g., 9904230) inParticleData.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::getWeightNamesto 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::getSigmaTotalandPythia::getSigmaPartialmethods give the user direct 
access to these cross sections. These extend and replace thePythia::getLowEnergySigmamethods. 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:maxValQuarkhas been updated to allow also 
charm and bottom hadrons as incoming beam particles.
- The framework for changing the beam kinematics between subsequent 
Pythia::nextcalls when using variable collision energies 
has been expanded. ThePythia::setKinematicsmethods 
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 
SoftQCDandLowEnergyQCDprocesses 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:spinFvdefault 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 PythiaParallelfor 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:reuseInitandMultipartonInteractions:initFile. Also option to 
symmetrize the MPI rate between e.g. pi^+ p and pi^- p, 
seeMultipartonInteractions: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:singleDiffractiveXBandSoftQCD:singleDiffractiveAXallow 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 ShowerMEstoExternalMEsandShowerMEsMadgraphtoExternalMEsMadgraph. 
Introduced newHelicitySamplerclass to sample helicities 
using external matrix elements.
- New options to get and set the state of the random number generator, 
Rndm::getStateandRndm::setState, as a 
direct-transfer option to the file-baseddumpState/readStatemethods.
- New methods Hist::fillTableto read back in a table 
of histogram data, specifically as written by theHist::tablemethods.
- Some minor rearrangements in HadronLevelto better 
structure the interleaving of hadronic rescattering and particle 
decays. New methodPythia::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:xSymmetricparameters 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 ColourReconnectionclass.
- Fixed some recent compiler warnings and errors; thanks to Mattias 
Ellert.
- Removed the deprecated SubCollisionModelcalledMultiRadialfrom 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 
ResonanceWidthsandParticleDataEntrywhich 
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 Pythiaclass.
- 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 assertstatements 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.ccifweightsShowerPtr == 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 
  VinciaConstantsfrom headers to source files and movedenum antFunTypeinside 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:unresolvedHadronwhich 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 Histclass.
- 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 stopMassNowinStringFragmentation.ccis 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 ResonanceDecayFilterHookand example 
main103.ccto 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 rapandetamethods inBasics.h, andyinEvent.cc.
- Fixed pointer logic in Dire getXPDF.
- Migrated from log(1 + x)tolog1p(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 
dsigmaElmethod in the classes derived fromSigmaTotAux.
- Flawed and unused LowEnergySigma::sigmaPartialmethod removed.
- Fix inconsistent type for varWidthIninParticleDataEntryclass.
- 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 
usingswitch()instead ofifto 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 programsmain151.ccthroughmain156.ccfor 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 LowEnergySigmahas 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 usingPythia::getLowEnergySigma.
- Several changes in the LowEnergyProcessandMiniStringFragmentationframeworks, in particular with 
several new approaches to produce nondiffractive inelastic events as 
close as possible to the kinematical threshold. New methodStringFlav::combineToLightestattempts 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:maxSmearparameter. The newHadronVertex:maxTaugives 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:beamA2gammaandPDF:beamB2gamma, which 
supersede the previousPDF:leptonA2gammaoption. 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.ccto generate 
elastic, single- and double-dissociative photon-induced processes in 
proton-proton collisions, using optionsPDF:beamA2gammaandPDF:beamB2gamma. Also the sample programsmain69.ccandmain70.cchave 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 
SimpleTimeShowerfor 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:primordialKThardparameter, ignorningBeamRemnants: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-mg5mesflag during configuration and 
with theDire:MEpluginandVincia:MEpluginsettings.
- 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:keepSMas it duplicates the 
functionality provided bySLHA:minMassSM.
- Some new methods in the HistandHistPlotclasses.Hist::plotFunccreates a 
histogram from the function provided as first argument, by evaluating 
its value in the middle of each histogram 
bin.Hist::normalizecan rescale a histogram to an 
intended sum of bin contents. Methods to get x limits and 
lowest and highest y value.Hist::smallestAbsValuerenamedHist::getYAbsMinfor consistency.HistPlot::addFilecan add a file of data points, with or 
without errors, on to an existing histogram, for comparison purposes. 
A new variant ofHistPlot::plotcan set the frame 
borders.
- New LinearInterpolatorclass inMathTools.
- New RotBstMatrixmethodstoSameVframeandfromSameVframesets 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 apair<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 -DGCCFPDEBUGenables 
floating point exceptions on a gcc compiler.
- New index()method in thePy8Particleclass 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 ASecondHardProcessandSecondHardProcesslabels resolved. Thanks to Anton 
Karneyeu.
- Number of weights in inforeset to 1 for each newinitcall.
- 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 dlsymfor 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.ccthe 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 tophaseSpacePtr->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 hasOwnEventAttributesinInfo.his necessary.
- Protected against explicit evaluation of alphaS(LambdaQCD) in 
VinciaISR. Also added explicit member-variable 
initialisations for theQEDemitElementalclass inVinciaQED.
- Protected against floating point exceptions caused by dividing by 
zero in PhaseSpacefor 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 fabsbyabs.
- 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, excludingmain44.ccandmain93.ccwhich 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 ofInfo::getGroupWeight()has been made more transparent. 
Themain121.ccexample has been modified 
accordingly. Note that the bug fix forlhaStrategy = 
±4parallels 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 tauCalcallows 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 BeamParticleclass.
- Optimization of array handling in LHAGrid1give 
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 
LHAGrid1implementation. The NNPDF 2.3 sets therefore 
have been moved to the latter framework.
 
- New base class ShowerModelintroduced, as a 
top-level handler of parton-shower models, interfaced via a shared 
pointer inPythia.cc. 
Changes to Pythia's default showers 
and to the Vincia and Dire shower models to inherit fromShowerModel. New public methodPythia::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()byabs()in several places. Although default C++abs()only takes 
integer arguments,PythiaStdLib.hcontains the 
declarationusing std::abs(via thecmathheader), 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/tunesdirectory, 
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++11flag.
- 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:modelallows 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-mg5messhould 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.ccprovides a simple test of final-state 
shower components, in a LEP-like setup.main201.cccompares two different shower models on dijet events. The two runs 
are executed in parallel if Pythia has been configured to link to 
OpenMP.main202.ccmeasures the running time for ttbar 
events and outputs a few test distributions.main300.cccan be linked against both HepMC2 and OpenMP, if Pythia has been 
configured correspondingly. It is capable of command-line 
instructions.
- A new plugin header, Visualisationis available and 
used in the example programmain300.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 
SLHAfile is used to set particle properties, thenSLHA:allowUserOverride = trueallows 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/phpdocdirectory has been removed.
- The Python interface has been completely redesigned to handle 
C++11by migrating from aSWIGgenerated 
interface to aPyBind11one. It is now possible for 
users to generate their own interface. Full details are given on the 
Python Interface page.
- The field Pythia::infoof typeInfohas 
been made a constant reference, so that it cannot be externally 
manipulated outside of the internal classes. This constant reference 
points back toPythia::infoPrivateinstance, which is 
for internal use only.  This instance now also contains pointers to 
several other commonly used classes.
- A new base class PhysicsBasehas been created, from 
which several of the "physics" classes derive. This class standardizes 
the setup of pointers to commonly used classes, via theInfoclass. 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 enumStatusis 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 usedSettingsmethods is streamlined.
- Most of the pointers in the Pythiaclass are now 
shared pointers, such that there is no need forPythiato 
keep track ofnewanddeleteobligations. New headerSharedPointers.h.
- The CoupSMandCoupSUSYcouplings 
classes are now separated, rather than the latter being derived from 
the former.  This avoids the problem of the oldCouplingspointer potentially changing address, and some pointer casting.
- Mathematical functions and methods have been extracted from the 
PythiaStdlib.h/.ccfiles and put in newMathTools.h/.ccfiles. TherebyPythiaStdlibis more clearly reserved for declarations related to the Stdlib 
library, to some simple inline expressions (nowconstexprwhere feasible) and to an expanded list of conversion constants.
- A new method Rndm::phaseSpace2has 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/pdfdatadirectory, 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 LowEnergyProcessfor 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 StringInteractionsbase class to provide common 
administrative framework for string shoving, rope hadronization, 
swing, (some) colour reconnection models, and more.
- The headers in include/Pythia8Pluginshave been 
updated to no longer containusing 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 ProMCexample and configuration option has been 
dropped.
- Optional package configurations for RIVETandYODAhave been added.
- Support for BOOSThas been dropped and onlyLHAPDF6with versions greater than6.2are 
supported.
- The flag --openmphas been added. Currently, OpenMP 
is used in some of the examples mentioned above, and inDireWeightContainer.cc.
- Bug fix so the option UncertaintyBands:ISRpTmin2Facis correctly used inSimpleSpaceShower.cc.
- Bug fix stopping growth of number weights when multiple 
initcalls are made inInfo.
- Bug fix for SusyWidthFunctions.ccincorrectly callingintegrateGauss, 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.