# PYTHIA 8

## Welcome to PYTHIA - The Lund Monte Carlo!

PYTHIA 8 is the successor to PYTHIA 6, rewritten from scratch in C++. With the release of PYTHIA 8.1 it now becomes the official "current" PYTHIA version, although PYTHIA 6.4 will be supported in parallel with it for some time to come. Specifically, the new version has not yet been enough tested and tuned for it to have reached the same level of reliability as the older one. This testing will only happen if people begin to work with the program, however, which is why we encourage a gradual transition to the new version, starting now. There are some new physics features in PYTHIA 8.1, that would make use of it more attractive, but also some topics still missing, where 6.4 would have to be used. Further, many obsolete features will not be carried over, so for some backwards compatibility studies again 6.4 would be the choice.

## Documentation

On these webpages you will find the up-to-date manual for PYTHIA 8.1. 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
A Brief Introduction to PYTHIA 8.1
T. Sjöstrand, S. Mrenna and P. Skands, Comput. Phys. Comm. 178 (2008) 852 [arXiv:0710.3820].
You are strongly recommended to read this summary when you start out to learn how to use PYTHIA 8.1. Note that some details have changed since the 8.100 version described there.

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 8.1, you should therefore cite both, e.g. like
T. Sjöstrand, S. Mrenna and P. Skands, JHEP05 (2006) 026, Comput. Phys. Comm. 178 (2008) 852.

Furthermore, a separate
PYTHIA 8 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

Torbjörn Sjöstrand
Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 14A, SE-223 62 Lund, Sweden
phone: + 46 - 46 - 222 48 16, e-mail: torbjorn@thep.lu.se

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

Nishita Desai
Institut für Theoretische Physik, Universität Heidelberg, Philosophenweg 16, D-69120 Heidelberg, Germany
phone: +49 - 6221 54 9424, e-mail: Nishita.Desai@cern.ch

Philip Ilten
Massachusetts Institute of Technology, stationed at CERN, CH-1211 Geneva 23, Switzerland
e-mail: philten@cern.ch

Stephen Mrenna
Computing Division, Simulations Group, Fermi National Accelerator Laboratory, MS 234, Batavia, IL 60510, USA
phone: + 1 - 630 - 840 - 2556, e-mail: mrenna@fnal.gov

Stefan Prestel
Theory Group, DESY, Notkestrasse 85, D-22607 Hamburg, Germany
phone: + 49 - 40 - 8998-4250, e-mail: stefan.prestel@thep.lu.se

Peter Skands
Theoretical Physics, CERN, CH-1211 Geneva 23, Switzerland
phone: + 41 - 22 - 767 2447, e-mail: peter.skands@cern.ch

## Former authors

Stefan Ask, e-mail: ask.stefan@gmail.com

Richard Corke, e-mail: r.corke@errno.net

## 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 provided by Juan Rojo and Stefano Carrazza.
The fjcore code from FastJet provided by Matteo Cacciari, Gavin Salam and Gregory Soyez.
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.
Please respect the MCnet Guidelines for Event Generator Authors and Users.

The program and the documentation is Copyright © 2014 Torbjörn Sjöstrand