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# PYTHIA 8

### Welcome to PYTHIA - The Lund Monte Carlo!

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 `C++98` to `C++11`, but the transition from 8.2 should largely be transparent to users. Also several new physics features are made available.

### Documentation

On these webpages you will find the manual for PYTHIA 8.3. The physics descriptions may in places still 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. What is lacking on these webpages is an overview, on the one hand, and an in-depth physics description, on the other.

The overview can be found in the attached PDF file
An Introduction to PYTHIA 8.2
T. Sjöstrand et al, Comput. Phys.Commun. 191 (2015) 159 [arXiv:1410.3012 [hep-ph]].
You are strongly recommended to read this summary when you start out to learn how to use PYTHIA 8.2.

For the physics description we refer to the complete
PYTHIA 6.4 Physics and Manual
T. Sjöstrand, S. Mrenna and P. Skands, JHEP05 (2006) 026,
which in detail describes the physics (largely) implemented also in PYTHIA 8, and also provides a more extensive bibliography than found here. When you use PYTHIA 8.2, you should therefore cite both.

Furthermore, a separate
PYTHIA 8.2 Worksheet,
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.

### Authors

Christian Bierlich
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
and Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
e-mail: christian.bierlich@thep.lu.se

Nishita Desai
Department of Theoretical Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
e-mail: desai@theory.tifr.res.in

Leif Gellersen
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: leif.gellersen@thep.lu.se

Ilkka Helenius
Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
e-mail: ilkka.m.helenius@jyu.fi

Philip Ilten
Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
and School of Physics and Astronomy, University of Birmingham, Birmingham, B152 2TT, UK
e-mail: philten@cern.ch

Leif Lönnblad
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: leif.lonnblad@thep.lu.se

Stephen Mrenna
Computing Division, Simulations Group, Fermi National Accelerator Laboratory, MS 234, Batavia, IL 60510, USA
e-mail: mrenna@fnal.gov

Stefan Prestel
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: stefan.prestel@thep.lu.se

Christine O. Rasmussen
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: christine.rasmussen@thep.lu.se

Torbjörn Sjöstrand
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: torbjorn@thep.lu.se

Peter Skands
School of Physics and Astronomy, Monash University, PO Box 27, 3800 Melbourne, Australia
e-mail: peter.skands@monash.edu

Marius Utheim
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
e-mail: marius.utheim@thep.lu.se

### Former authors

Stefan Ask

Jesper Roy Christiansen

Richard Corke

Nadine Fischer

Christine O. Rasmussen

### Further contributions

Makefiles, configure scripts and HepMC interface by Mikhail Kirsanov.
Conversion of XML files to PHP ones by Ben Lloyd.
Simple Makefile for Win32/NMAKE by Bertrand Bellenot.
Extended Higgs sector partly implemented by Marc Montull.
Parts of charm and bottom decay tables courtesy DELPHI and LHCb collaborations.
Tunes and comparisons with data, based on Rivet and Professor, by Hendrik Hoeth.
Text and code on the use of ROOT in conjunction with PYTHIA by Rene Brun, Andreas Morsch and Axel Naumann.
Code and data for MRST/MSTW PDFs by Robert Thorne and Graeme Watt.
Code and data for the CTEQ/CT PDFs by Joey Huston and colleagues.
Help with implementing new proton PDFs by Tomas Kasemets.
Code and data for Pomeron PDFs by H1 collaboration and especially Paul Newman.
Help with implementing new Pomeron fluxes and PDFs by Sparsh Navin.
The new Hidden Valley code developed together with Lisa Carloni.
Code for a Kaluza-Klein electroweak gauge boson provided by Noam Hod and Mark Sutton.
Code for equivalent photon flux around an unresolved proton by Oystein Alvestad.
The MBR diffractive model and central diffraction by Robert Ciesielski.
2012 branching ratios for most light hadrons, and the tau lepton, by Anil Pratap Singh.
The pythia8-config script has been contributed by Andy Buckley, along with many other helpful suggestions.
Code and data for several of the NNPDF2.3 QCD+QED sets, and further later ones, provided by Juan Rojo and Stefano Carrazza.
The fjcore code from FastJet provided by Matteo Cacciari, Gavin Salam and Gregory Soyez.
The initial-final dipole approach has been developed and implemented by Baptiste Cabouat.
The MixMax random number generator has been contributed by Konstantin Savvidy and George Savvidy.
Space-time hadronic production points in string fragmentation have been studied and implemented by Silvia Ferreres-Solé.
The code for deuteron production was tested by Sophie Baker.
C++ interface to MG5 matrix elements by V. Hirschi with additional work by O. Mattelaer.
Mass corrections and initial-state showers in Vincia by M. Ritzmann.
Final-state sector showers in Vincia by J. Lopez-Villarejo.
Helicity-dependence in Vincia by A. Larkoski.
QED and Electroweak showers in Vincia by R. Verheyen.
Interleaved resonance showers and work on sector merging and QED+EW showers in Vincia by H. Brooks.
Note: 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.

### Licence

PYTHIA 8 is licensed under the GNU General Public Licence version 2 or later.
Please respect the MCnet Guidelines for Event Generator Authors and Users.

The program and the documentation is Copyright © 2019 Torbjörn Sjöstrand