<chapter name="Front"> 
 
<h1>PYTHIA 8</h1> 
 
<h3>Welcome to PYTHIA - The Lund Monte Carlo!</h3> 
 
<p/> 
PYTHIA is a general-purpose event generator for high-energy particle 
collisions. PYTHIA 8 is the successor to PYTHIA 6, rewritten from 
scratch in C++. It includes many new features, but rests on the same 
historical physics base, including concepts such as soft and hard 
processes, parton showers, multiparton interactions and string 
fragmentation. With the current PYTHIA 8.3 version the step has been 
taken from <code>C++98</code> to <code>C++11</code>, but the 
transition from 8.2 should largely be transparent to users. Also 
several new physics features are made available. 
 
<h3>Documentation</h3> 
 
On these webpages you will find the HTML manual for PYTHIA 8.3. The physics 
descriptions may in places need to be updated, but the 
documentation of available settings and other practical details should 
be fully up-to-date. Use the left-hand index to navigate this 
documentation of program elements, especially of all possible program 
settings. All parameters are provided with sensible default values, 
however, so you need only change those of relevance to your particular 
study, such as choice of beams, processes and phase space cuts. The 
pages also contain a fairly extensive survey of all methods available 
to the user, e.g. to study the produced events. 
 
<p/> 
What these pages are not very well suited to give you, is a comprehensive 
explanation of choices made and the physics behind them. For that, we 
refer the reader to: 
<br/><a href="../pdfdoc/pythia8300.pdf" target="page"> 
<b>A comprehensive guide to the physics and usage of PYTHIA 8.3</b></a> 
<br/>C. Bierlich et al, SciPost Phys. Codebases 8-r8.3 (2022) 
[arXiv:2203.11601 [hep-ph]]. 
<br/> 
 
<p/> 
We ask that you cite this publication, when using PYTHIA 8.3. Older 
documentation still exists, and could be useful in particular from 
users transitioning from older versions of PYTHIA. An overview of 
PYTHIA 8.2 can be found in the attached PDF file 
<br/><a href="../pdfdoc/pythia8200.pdf" target="page"> <b>An Introduction to 
PYTHIA 8.2</b></a> 
<br/>T. Sj&ouml;strand et al, Comput. Phys.Commun. 191 (2015) 159 
[arXiv:1410.3012 [hep-ph]]. 
<br/> 
 
<p/> 
The even older 
<br/><b>PYTHIA 6.4 Physics and Manual</b> 
<br/>T. Sj&ouml;strand, S. Mrenna and P. Skands, JHEP05 (2006) 026, 
<br/>describes the physics of PYTHIA 6 in great detail, but is now 
superseeded by the PYTHIA 8.3 manual. 
 
<p/> 
Furthermore, a separate 
<br/><a href="../pdfdoc/worksheet8312.pdf" target="page"> 
<b>PYTHIA 8.3 Worksheet</b></a>, 
<br/>also an attached PDF file, offers a practical introduction to 
using the generator. It has been developed for and used at a few 
summer schools, with minor variations, but is also suited for 
self-study. 
 
<h3>Authors</h3> 
 
<p/> 
<b>Javira Altmann</b><br/> 
School of Physics and Astronomy, Monash University, PO Box 27, 3800 Melbourne, 
Australia<br/> 
e-mail: javira.altmann@monash.edu 
 
<p/> 
<b>Christian Bierlich</b><br/> 
Department of Physics, Lund University, 
Box 118, SE-221 00 Lund, Sweden<br/> 
e-mail: christian.bierlich@fysik.lu.se 
 
<p/> 
<b>Naomi Cooke</b><br/> 
School of Physics and Astronomy, University of Glasgow, 
Glasgow G12 8QQ, Scotland, United Kingdom<br/> 
e-mail: naomi.cooke@cern.ch 
 
<p/> 
<b>Nishita Desai</b><br/> 
Department of Theoretical Physics, Tata Institute of Fundamental Research, 
Homi Bhabha Road, Mumbai 400005, India<br/> 
e-mail: desai@theory.tifr.res.in 
 
<p/> 
<b>Ilkka Helenius</b><br/> 
Department of Physics, University of Jyv&auml;skyl&auml;, P.O. Box 35, 
FI-40014 University of Jyv&auml;skyl&auml;, Finland<br/> 
e-mail: ilkka.m.helenius@jyu.fi 
 
<p/> 
<b>Philip Ilten</b><br/> 
Department of Physics, University of Cincinnati, 
Cincinnati, OH 45221, USA<br/> 
e-mail: philten@cern.ch 
 
<p/> 
<b>Joni Laulainen</b><br/> 
Department of Physics, University of Jyv&auml;skyl&auml;, P.O. Box 35, 
FI-40014 University of Jyv&auml;skyl&auml;, Finland<br/> 
e-mail: joni.o.laulainen@jyu.fi 
 
<p/> 
<b>Leif L&ouml;nnblad</b><br/> 
Department of Physics, Lund University, 
Box 118, SE-221 00 Lund, Sweden<br/> 
e-mail: leif.lonnblad@fysik.lu.se 
 
<p/> 
<b>Stephen Mrenna</b><br/> 
Computing Division, Simulations Group, 
Fermi National Accelerator Laboratory, 
MS 234, Batavia, IL 60510, USA<br/> 
e-mail: mrenna@fnal.gov 
 
<p/> 
<b>Christian Preuss</b><br/> 
Institute for Theoretical Physics, University of G&ouml;ttingen,, 
Friedrich-Hund-Platz 1, 37077 G&ouml;ttingen, Germany<br/> 
e-mail: christian.preuss@uni-goettingen.de 
 
<p/> 
<b>Torbj&ouml;rn Sj&ouml;strand</b><br/> 
Department of Physics, Lund University, 
Box 118, SE-221 00 Lund, Sweden<br/> 
e-mail: torbjorn.sjostrand@fysik.lu.se 
 
<p/> 
<b>Peter Skands</b><br/> 
School of Physics and Astronomy, Monash University, PO Box 27, 3800 Melbourne, 
Australia<br/> 
e-mail: peter.skands@monash.edu 
 
<h3>Former authors</h3> 
 
<p/><b>Stefan Ask</b> 
 
<p/><b>Jesper Roy Christiansen</b> 
 
<p/><b>Richard Corke</b> 
 
<p/><b>Nadine Fischer</b> 
 
<p/><b>Leif Gellersen</b> 
 
<p/><b>Stefan Prestel</b> 
 
<p/><b>Christine O. Rasmussen</b> 
 
<p/><b>Marius Utheim</b> 
 
<p/><b>Rob Verheyen</b> 
 
<h3>Further contributions</h3> 
 
HepMC interface by <b>Mikhail Kirsanov</b>. 
<br/>Conversion of XML files to PHP ones by <b>Ben Lloyd</b>. 
<br/>Simple Makefile for Win32/NMAKE by <b>Bertrand Bellenot</b>. 
<br/>Extended Higgs sector partly implemented by <b>Marc Montull</b>. 
<br/>Parts of charm and bottom decay tables courtesy <b>DELPHI</b> and 
<b>LHCb</b> collaborations. 
<br/>Tunes and comparisons with data, based on Rivet and Professor, 
by <b>Hendrik Hoeth</b>. 
<br/>Text and code on the use of ROOT in conjunction with PYTHIA 
by <b>Rene Brun</b>, <b>Andreas Morsch</b> and <b>Axel Naumann</b>. 
<br/>Code and data for MRST/MSTW PDFs by <b>Robert Thorne</b> and 
<b>Graeme Watt</b>. 
<br/>Code and data for the CTEQ/CT PDFs by <b>Joey Huston</b> 
and colleagues. 
<br/>Help with implementing new proton PDFs by <b>Tomas Kasemets</b>. 
<br/>Code and data for Pomeron PDFs by <b>H1</b> collaboration and 
especially <b>Paul Newman</b>. 
<br/>Help with implementing new Pomeron fluxes and PDFs by 
<b>Sparsh Navin</b>. 
<br/>The new Hidden Valley code developed together with <b>Lisa Carloni</b>. 
<br/>Code for a Kaluza-Klein electroweak gauge boson provided by 
<b>Noam Hod</b> and <b>Mark Sutton</b>. 
<br/>Code for equivalent photon flux around an unresolved proton by 
<b>Oystein Alvestad</b>. 
<br/>The MBR diffractive model and central diffraction by 
<b>Robert Ciesielski</b>. 
<br/>2012 branching ratios for most light hadrons, and the tau lepton, 
by <b>Anil Pratap Singh</b>. 
<br/>The pythia8-config script has been contributed by 
<b>Andy Buckley</b>, along with many other helpful suggestions. 
<br/>Code and data for several of the NNPDF2.3 QCD+QED sets, and further 
later ones, provided by <b>Juan Rojo</b> and <b>Stefano Carrazza</b>. 
<br/>The fjcore code from FastJet provided by <b>Matteo Cacciari</b>, 
<b>Gavin Salam</b> and <b>Gregory Soyez</b>. 
<br/>The initial-final dipole approach has been developed and 
implemented by <b>Baptiste Cabouat</b>. 
<br/>The MixMax random number generator has been contributed by 
<b>Konstantin Savvidy</b> and <b>George Savvidy</b>. 
<br/>Space-time hadronic production points in string fragmentation have 
been studied and implemented by <b>Silvia Ferreres-Sol&eacute;</b>. 
<br/>The code for deuteron production was tested by <b>Sophie Baker</b>. 
<br/>C++ interface to MG5 matrix elements by <b>Valentin Hirschi</b> with 
additional work by <b>Olivier Mattelaer</b>. 
<br/>Mass corrections and initial-state showers in Vincia by 
<b>Mathias Ritzmann</b>. 
<br/>Final-state sector showers in Vincia by <b>Juan Lopez-Villarejo</b>. 
<br/>Helicity-dependence in Vincia by <b>Andrew Larkoski</b>. 
<br/>Interleaved resonance showers and work on 
MESS merging and QED+EW showers in Vincia by <b>Helen Brooks</b>. 
<br/>Plugin and hadronization reweighting framework by the <b><a 
href="https://uchep.gitlab.io/mlhad-docs/">MLhad team</a></b>. 
<br/>Mini-junction string improvements and additions to the QCD-based colour 
reconnection model by <b>Harsh Shah</b>. 
<br/>Python batch interface using Awkward arrays by <b>Aartem Havryliuk</b> 
and <b>Jim Pivarski</b>. 
<br/> 
 
<br/><b>Note</b>: in several cases modifications have been made to 
the original code, in order to integrate it with PYTHIA. In these cases 
the blame for any mistakes has to rest with the regular authors. 
 
<h3>Contact</h3> 
 
<h4>New version alerts</h4> 
 
If you want to be alerted whenever a new public version is released, 
you can subscribe on 
<a href="https://lists.hepforge.org/mailman3/lists/pythia8-announce.projects.hepforge.org/"> 
  pythia8-announce</a>. Expect a 
handful of such messages per year. There is no fixed schedule; new 
versions can appear either because a sufficient amount of changes have 
been accumulated over time, or because some crucial new feature or bug 
fix prompts action. 
 
<h4>Bug reports and physics questions</h4> 
 
No code is perfect, and we rely on PYTHIA users to report suspect 
behaviour to us. We are starting to collect common questions and 
problems with our <a 
href="https://gitlab.com/Pythia8/releases/-/issues" 
target="_blank">issue tracker</a>. Please check this resource first for 
answered questions. New questions and issues can be either opened with 
a freely available <a href="https://gitlab.com/users/sign_up" 
target="_blank">gitlab.com account</a> or emailing us at 
<a href="mailto:authors@pythia.org">authors@pythia.org</a> where no 
account is required. To help us in the subsequent debugging efforts, 
however, it is helpful if you can present your case in as clear terms 
as possible. A convenient path is the following. 
<ul> 
<li>Users often combine PYTHIA with a set of other libraries in their 
studies, and then problems originally attributed to PYTHIA usually 
turn out to be located somewhere else. Therefore, check that the 
problem remains if no other libraries are being used. If you want to 
file a report that involves other libraries please have a <b>very</b> 
convincing case that the problem is in PYTHIA to be taken 
seriously.</li> 
<li>Make sure you use a clean copy of PYTHIA, not one that has 
been subject to "local modifications" by parties unknown. Also 
make sure it is a recent one - you may have encountered a true bug, 
but one already solved in the current version. If uncertain, 
pick up a new copy directly from the PYTHIA webpage, and create a 
new cleanly compiled PYTHIA library code.</li> 
<li>Write a simple standalone main program <code>mymainNN.cc</code>, 
with <code>NN</code> between 00 and 99, that demonstrates the 
claimed bug. Feel free to use some of the existing sample main programs 
<code>share/Pythia8/examples/mainNNN.cc</code> as templates.</li> 
<li>Put the <code>mymainNN.cc</code> program in the 
<code>share/Pythia8/examples</code> subdirectory and run it from there 
(<code>make mymainNN</code> followed by <code>./mymainNN</code>).</li> 
<li>Now, systematically peel away all irrelevant code in the main 
program, so that only code really needed to reproduce and document 
the bug remains. Where explanations in the code would be helpful, 
do add comments in English.</li> 
<li>Send the main program, with a description what is the problem 
and whatever guesses you may have about a probable cause.</li> 
</ul> 
 
<p/> 
Note, the email <a 
href="mailto:authors@pythia.org">authors@pythia.org</a> should always 
be used when contacting the authors as it allows us to coordinate 
replies. You may also include the individual addresses of authors that 
are relevant for a specific issue. Never contact several persons 
independently on the same topic, thereby potentially leading to 
doublework. Abuse will have consequences. 
 
<p/> 
When responding to emails from us at 
<a href="mailto:authors@pythia.org">authors@pythia.org</a>, reply 
directly to the email, which has a unique return address of the form 
<code>incoming+&lt;hash&gt;@incoming.gitlab.com</code>, and not 
<a href="mailto:authors@pythia.org">authors@pythia.org</a>. Please 
consider allowing your correspondence to be made publicly available to 
other users via our <a 
href="https://gitlab.com/Pythia8/releases/-/issues" 
target="_blank">issue tracker</a>, by replying at any point with your 
express permission, e.g. "this correspondence can be made public". 
 
<p/> 
Do note that the major collaborations, such as ATLAS and CMS, have 
their own Monte Carlo support groups, with a lot of experience in 
solving typical issues, many of which are related to the setups and 
interfaces created inside the collaborations. If you are a member of a 
major collaboration you please turn to these groups in the first 
place, and only turn to us when it has been confirmed as a true 
problem going beyond the local installation. 
 
<h3>Licence</h3> 
 
PYTHIA 8 is licensed under the 
<a href="COPYING" target="page"><b>GNU General Public Licence 
version 2</b></a> or later. 
<br/>Please respect the 
<a href="GUIDELINES" target="page"><b>MCnet Guidelines</b></a> 
for Event Generator Authors and Users. 
 
<p/> 
The program and the documentation is 
Copyright &copy; 2026 Torbj&ouml;rn Sj&ouml;strand 
 
</chapter> 
 
<!-- Copyright (C) 2026 Torbjorn Sjostrand --> 
