LU TP 95{20 August 1995 (revised version of CERN{TH.7112/93) (first issued December 1993) W5035/W5044 PYTHIA 5.7 and JETSET 7.4 Physics and Manual Torbj¨ornSj¨ostrand Department of Theoretical Physics, University of Lund, S¨olvegatan 14A, S-223 62 Lund, Sweden *......* *:::!!:::::::::::* *::::::!!::::::::::::::* *::::::::!!::::::::::::::::* *:::::::::!!:::::::::::::::::* *:::::::::!!:::::::::::::::::* *::::::::!!::::::::::::::::*! *::::::!!::::::::::::::* !! !! *:::!!:::::::::::* !! !! !* -><- * !! !! !! !! !! !! !! !! !! !! ep !! !! !! !! pp !! !! e+e- !! !! !! !! Important note: this is the long writeup of T. Sj¨ostrand,Computer Physics Commun. 82 (1994) 74. All references should be to the published version. Copyright Notice CERNLIB { CERN Program Library Long writeups c Copyright CERN, Geneva 1993 Copyright and any other appropriate legal protection of these com- puter programs and associated documentation reserved in all coun- tries of the world. These programs or documentation may not be reproduced by any method without prior written consent of the Director-General of CERN or his delegate. Permission for the usage of any programs described herein is granted apriori to those scientific institutes associated with the CERN experimental program or with whom CERN has concluded a scientific collaboration agreement. Requests for information should be addressed to: CERN Program Library Office CERN-CN Division CH-1211 Geneva 23 Switzerland Tel. +41 22 767 4951 Fax. +41 22 767 7155 Bitnet: CERNLIB@CERNVM DECnet: VXCERN::CERNLIB (node 22.190) Internet: [email protected] Trademark notice: All trademarks appearing in this guide are acknowledged as such. Preface The Pythia and Jetset programs are frequently used for event generation in high-energy physics. The emphasis is on multiparticle production in collisions between elementary particles. This in particular means hard interactions in e+e−, pp and ep colliders, although also other applications are envisaged. The programs are intended to generate complete events, in as much detail as experimentally observable ones, within the bounds of our current understanding of the underlying physics. Many of the components of the programs represent original research, in the sense that models have been developed and implemented for a number of aspects not covered by standard theory. Although originally conceived separately, the Pythia and Jetset programs today are so often used together that it makes sense to present them here without too much distinction. Both programs have a long history, and several manuals have come out. The former round of Pythia/Jetset program descriptions appeared in 1987. Meanwhile a large number of additions and changes have been made. Recently a new description therefore appeared in T. Sj¨ostrand,Computer Physics Commun. 82 (1994) 74. This is the one and only correct reference to the current versions of Pythia and Jetset. The long writeup that you now have before you is an (unpublished) appendix to the publication above, and need not be separately cited. Instead remember to cite the original literature on the physics topics of particular relevance for your studies. (There is no reason to omit references to good physics papers simply because some of their contents have also been made available as program code.) Event generators often have a reputation for being `black boxes'; if nothing else, this report should provide you with a glimpse of what goes on inside the programs. Some such understanding may be of special interest for new users, who have no background in the field. An attempt has been made to structure the report sufficiently well that many of the sections can be read independently of each other, so you can pick the sections that interest you. I have tried to keep together the physics and the manual sections on specific topics, where practicable, which represents a change of policy compared with previous manual versions. Any feedback on this and other aspects is welcome. A large number of persons should be thanked for their contributions. Hans-Uno Bengtsson is the originator of the Pythia program, and for many years we worked in parallel on its further development. Mats Bengtsson is the main author of the final-state parton-shower algorithm. Bo Andersson and G¨ostaGustafson are the originators of the Lund model, and strongly influenced the early development of the programs. Further comments on the programs have been obtained from users too numerous to be mentioned here, but who are all gratefully acknowledged. To write programs of this size and com- plexity would be impossible without a strong user feedback. The moral responsibility for any remaining errors clearly rests with me. However, kindly note that this is a `University World' product, distributed `as is', free of charge, without any binding guarantees. And always remember that the programs do not repre- sent a dead collection of established truths, but rather one of many possible approaches to the problem of multiparticle production in high-energy physics, at the frontline of current research. Be critical! Contents 1 Introduction 1 2 Physics Overview 9 2.1 Hard Processes and Parton Distributions . 9 2.2 Initial- and Final-State Radiation . 11 2.3 Beam Remnants . 14 2.4 Fragmentation . 15 2.5 Decays . 18 3 Program Overview 20 3.1 Update History . 21 3.2 Program Installation . 31 3.3 Program Philosophy . 32 3.4 Manual Conventions . 34 3.5 Getting Started with JETSET . 35 3.6 Getting Started with PYTHIA . 38 4 Monte Carlo Techniques 43 4.1 Selection From a Distribution . 43 4.2 The Veto Algorithm . 45 4.3 The Random Number Generator . 47 5 The Event Record 51 5.1 Particle Codes . 51 5.2 The Event Record . 56 5.3 How The Event Record Works . 60 5.4 The HEPEVT Standard . 63 6 Hard Processes in JETSET 67 6.1 Annihilation Events in the Continuum . 67 6.2 Decays of Onia Resonances . 77 6.3 Routines and Common Block Variables . 78 6.4 Examples . 84 7 Process Generation in PYTHIA 86 7.1 Parton Distributions . 86 7.2 Kinematics and Cross section for a 2 ! 2 Process . 90 7.3 Resonance Production . 92 7.4 Cross-section Calculations . 95 7.5 2 ! 3 and 2 ! 4 Processes . 101 7.6 Resonance Decays . 103 7.7 Nonperturbative Processes . 105 8 Physics Processes in PYTHIA 111 8.1 The Process Classification Scheme . 111 8.2 QCD Processes . 116 8.3 Electroweak Gauge Bosons . 120 8.4 Higgs Production . 127 8.5 Non-Standard Physics . 132 8.6 Main Processes by Machine . 135 9 The PYTHIA Program Elements 138 9.1 The Main Subroutines . 138 9.2 Switches for Event Type and Kinematics Selection . 142 9.3 The General Switches and Parameters . 147 9.4 General Event Information . 163 9.5 How to include external processes in PYTHIA . 167 9.6 How to run PYTHIA with varying energies . 172 9.7 Other Routines and Common Blocks . 174 9.8 Examples . 187 10 Initial- and Final-State Radiation 188 10.1 Shower Evolution . 188 10.2 Final-State Showers . 191 10.3 Initial-State Showers . 197 10.4 Routines and Common Block Variables . 204 11 Beam Remnants and Underlying Events 210 11.1 Beam Remnants . 210 11.2 Multiple Interactions . 213 11.3 Pile-up Events . 220 11.4 Common Block Variables . 221 12 Fragmentation 226 12.1 Flavour Selection . 226 12.2 String Fragmentation . 230 12.3 Independent Fragmentation . 238 12.4 Other Fragmentation Aspects . 240 13 Particles and Their Decays 242 13.1 The Particle Content . 242 13.2 Masses, Widths and Lifetimes . 243 13.3 Decays . 245 14 The JETSET Program Elements 251 14.1 Definition of Initial Configuration or Variables . 251 14.2 The JETSET Physics Routines . 253 14.3 Event Study and Data Listing Routines . 256 14.4 The General Switches and Parameters . 261 14.5 Couplings . 271 14.6 Further Parameters and Particle Data . 275 14.7 Miscellaneous Comments . 281 14.8 Examples . 284 15 Event Analysis Routines 288 15.1 Event Shapes . 288 15.2 Cluster Finding . 292 15.3 Event Statistics . 296 15.4 Routines and Common Block Variables . 298 16 Summary and Outlook 309 References 310 Index of Subprograms and Common Block Variables 320 1 Introduction Multiparticle production is the most characteristic feature of current high-energy physics. Today, observed particle multiplicities are typically between ten and a hundred, and with future machines this range will be extended upwards. The bulk of the multiplicity is found in jets, i.e. in bunches of hadrons (or decay products of hadrons) produced by the hadronization of quarks and gluons. The Complexity of High-Energy Processes To first approximation, all processes have a simple structure at the level of interactions between the fundamental objects of nature, i.e. quarks, leptons and gauge bosons. For instance, a lot can be understood about the structure of hadronic events at LEP just from the `skeleton' process e+e− ! Z0 ! qq. Corrections to this picture can be subdivided, arbitrarily but conveniently, into three main classes. Firstly, there are bremsstrahlung-type modifications, i.e. the emission of additional final-state particles by branchings such as e ! eγ or q ! qg. Because of the largeness of the strong coupling constant αs, and because of the presence of the triple gluon ver- tex, QCD emission off quarks and gluons is especially prolific. We therefore speak about `parton showers', wherein a single initial parton may give rise to a whole bunch of par- tons in the final state. Also photon emission may give sizeable effects in e+e− and ep processes. The bulk of the bremsstrahlung corrections are universal, i.e. do not depend on the details of the process studied, but only on one or a few key numbers, such as the momentum transfer scale of the process. Such universal corrections may be included to arbitrarily high orders, using a probabilistic language. Alternatively, exact calculations of bremsstrahlung corrections may be carried out order by order in perturbation the- ory, but rapidly the calculations then become prohibitively complicated and the answers correspondingly lengthy.