[Pythia8-announce] Pythia 8.308 now available

[Peter Skands]

Dear PYTHIA subscriber,

PYTHIA 8.308 has been released, and can be downloaded from

• https://pythia.org/

Main physics updates include new functionality geared towards cosmic-ray studies, several updates for Hidden-Valley scenarios, and an update to FxFx merging (see list below for details, and/or the update notes online). The default value of the top quark mass has also been updated, from 171 GeV to 173 GeV.

Technical updates include new methods in Pythia’s Hist class to compute statistical uncertainties, higher moments, and more, and a couple of new ROOT-based example programs to visualise event and jet structure in (eta, phi) space provided by D. Gillberg, in addition to the usual minor improvements and bug fixes (see full list below).

This release coincides with the journal publication of the PYTHIA 8.3 manual. Please update relevant bibtex reference databases to include the full journal reference:

• C. Bierlich, S. Chakraborty, N. Desai, L. Gellersen, I. Helenius, P. Ilten, L. Lönnblad, S. Mrenna, S. Prestel, C. T. Preuss, T. Sjöstrand, P. Skands, M. Utheim, R. Verheyen, A comprehensive guide to the physics and usage of PYTHIA 8.3, SciPost Physics Codebases 8 (2022), doi:10.21468/SciPostPhysCodeb.8.

Also note that there is a GitLab issue desk for PYTHIA 8.3, where already hundreds of questions have been posted (and most of them answered). Most of these are publicly visible and can be searched and filtered, to see if a question you have has already been discussed:

• https://gitlab.com/Pythia8/releases/-/issues/?sort=updated_desc&state=all&first_page_size=50


A brief summary of all changes is included below and in the Update History part of the HTML manual.

Wishing you an enjoyable experience with PYTHIA 8.308.

Sincerely,
The Pythia 8 Collaboration
(Christian Bierlich, Nishita Desai, Leif Gellersen, Ilkka Helenius, Philip Ilten, Leif Lönnblad, Stephen Mrenna, Stefan Prestel, Christian Preuss, Torbjörn Sjöstrand, Peter Skands, Marius Utheim, Rob Verheyen)

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Summary list of changes for 8.308:

• 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. Notaby 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 intialization.
• 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.

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