Torbjörn Sjöstrand

A Brief Introduction to PYTHIA 8.1

Abstract in Undetermined The PYTHIA program is a standard tool for the generation of high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multihadronic final state. It contains a library of hard processes and models for initial-and final-state parton showers, multiple parton-parton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and interfaces to external programs. While previous versions were written in Fortran, PYTHIA 8 represents a complete rewrite in C++. The current release is the first main one after this transition, and does not yet in every respect replace the old code. It does contain some new physics aspects, on the other hand, that should make it an attractive option especially for LHC physics studies. (Less)

High-energy physics event generation with PYTHIA 5.7 and JETSET 7.4

Pythia and JETSET are the two main components of the “Lund Monte Carlo” program suite. They can be used to generate high-energy-physics “events”, i.e. sets of outgoing particles produced in the interactions between two incoming particles. Ideally these events should have the same average behaviour and the same fluctuations as real data. The underlying physics is not understood well enough to give an exact description; the programs therefore contain a combination of analytical results and various models. Several event classes are considered, within and beyond the standard model. We give a summary of the main physics component of the current versions, Pythia 5.7 and Jetset 7.4: hard scattering matrix elements, parton distributions, initial-and final-state radiation, multiple interactions, beam remnants, fragmentation and decays. A brief outline is also given of some programming aspects. A detailed physics description and manual is available as a separate publication.

High-Energy-Physics Event Generation with PYTHIA 6.1

Pythia version 6 represents a merger of the Pythia 5, Jetset 7 and SPythia programs, with many improvements. It can be used to generate high-energy-physics ‘events’, i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions. The underlying physics is not understood well enough to give an exact description; the programs therefore contain a combination of analytical results and various models. The emphasis in this article is on new aspects, but a few words of general introduction are included. Further documentation is available on the web.

Parton fragmentation and string dynamics

While muchhasbeenlearnedrecentlyaboutquark andgluon interactionsin theframeworkof perturbativeQuantumChromodynamics,the relationbetweencalculatedpartonpropertiesandobservedhadrondensitiesinvolves modelswhere dynamicsandjet empincal ruleshaveto be combined.Thepurposeof this article is to describea presentlysuccessful approachwhich is basedon a cascadejet model usingStringdynamics.It can readily lead to Monte Carlo jet programmesof greatuse when analyzingdata. Production processesin an iterative cascadeapproach,with tunneling in a constantforce field, arereviewed. Expecteddifferencesbetweenquark and gluon jets are discussed.Low transversemomentum phenomenaare alsoreviewed with emphasison hyperon polarization. In so far as this approachusesa fragmentationschemebasedon String dynamics,it wasdeemedappropriateto alsoincludeunderthesamecovera specialreport on theClassicaltheoryof relativisticStrings,seenasthe classicallimit of theDual Resonancemodel. TheEquationsof motion and interactionamongstringsarepresented. Single ordersfor this issue PHYSICSREPOR1’S(Review Sectionof PhysicsLetters)97, Nos.2 & 3 (1983)31—171. Copies of this issue may be obtainedat the price givenbelow. All ordersshouldbe sentdirectly to the Publisher.Ordersmust be accompaniedby check. Single issuepriceDfl. 79.00, postageincluded. 34 B. Andersson et a!., Patton fragmentation and siring dynamics testsof the theory,in particularof the perturbativeQCD structure,containse.g.nonscalingdeviations from the partonmodel. There are at this point already some difficulties becauseit is well-known that any finite energy hadronicdistributionwill containnonscalingcontributions,sometimesto an evenlargerdegreethanthe inherent scale breaking effects of the theory. Further the pencilsharpenergyand momentumdistributions from the single partonsareessentiallydistorted, widenedin transversedirectionsandeven the basicquantumnumberslike chargeandstrangeness etc.seemto havebeentransportedsometimes ratherfar away in longitudinal rapidity space.It has thereforebecomeincreasinglyobvious over the yearsthat in order to compareexperimentto basictheory,it is necessaryto havereliabledescriptionsof the transferfrom the partonicstageto the hadronicone,i.e. to haveconsistentmodelsfor the process of partonfragmentation. Suchmodelsmayon the onehandbe lookedupon solely as phenomenological parametrizationsand rulesof thumb in order to obtain a translationfrom onelanguageto another.As such theyare useful for analysis of experiment as well as for the planning. On the other hand one may as always in connectionwith phenomenologytry to obtain a dynamicalframeworkthat servesas a motivation anda generalizingprinciple for theconstructions. It shouldbekept in mind, however,that thereareno easilyavailablemeasuresof the successof such a venture.As Bacontold us a long time ago,it is actuallyonly possibleto learnthat oneis wrong by a comparisonbetweenmodel calculationsand experimentalfindings. If the predictionagreesthereis no reassurancethat one is evenworking in the right basicdirection (althoughthereis evidentlya possible reasonto feelsomeconfidence!). A modestmeasureof successwould be a demandthat the numberof phenomenological parameters andthe variation in size of their valuesare nonincreasingfunctionsof time as well as the numberof independent experimentalfindings. It is alsoof evidentinterestthat thesamebasicschemeis applicable in different contextssuch as different partonic processesand different parts of phase-space. Several schemeswith a more or less profound theoreticalfoundationhavebeensuggestedbut we will in this review be concernedonly with iterative cascadejet modelsbasedupon string dynamics.The present experiencefrom thesemodelsshowsthat at leastthe above-mentionedcriteriafor successarefulfilled. We will in this reviewmostlydiscussa possibledynamicalframeworkbehindthe modelsandwe will only usecomparisonsto datain order to demonstratemattersof principle.One of the nice featuresof the models is their stochasticstructurewhich readily lends itself to an implementationin terms of computergeneration.Severalsuch MonteCarlojet programsareavailable[21for the interestedreader to makemuchmore detailedcomparisons. Our approachwill primarily be of a semi-classicalnature,i.e. we will at most placesmakeuseof a classicaldynamicalframeworkfor our considerations. We will, however,at all necessaryplacespoint to the basicquantummechanicalconstraints. It is well-known that by meansof a careful choice of dynamicalvariablesit is often possibleto circumvent such constraints.As an example we note that it is in general not possible to give independentvaluesto canonicallyconjugatevariablessuch as momentum(p) andposition(x) dueto Heisenberg’suncertaintyprinciple:

The Lund Monte Carlo for jet fragmentation and e+ e- physics - jetset version 6.3 - an update

Abstract Title of program: JETSET 6.3 Catalogue number: AATJ Program obtainable from: CPC Program Library, Queens University of Belfast, N. Ireland (see application form in this issue) Computer for which the program is designed and others on which it is operable: ND, VAX, IBM, CDC and others with a FORTRAN 77 compiler Computer: ND-570; Installation: University of Lund, Lund, Sweden Operating system: SINTRAN III - VSX/500 Programming language used: FORTRAN 77 High speed storage required: 50 Kwords No. of bits in word: 32 Peripherals used: terminal for input, terminal or line printer for output No. of lines in combined program and test deck: 6173

An introduction to PYTHIA 8.2

The Pythia program is a standard tool for the generation of events in high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multiparticle final state. It contains a library of hard processes, models for initial- and final-state parton showers, matching and merging methods between hard processes and parton showers, multiparton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and several interfaces to external programs. Pythia 8.2 is the second main release after the complete rewrite from Fortran to C++, and now has reached such a maturity that it offers a complete replacement for most applications, notably for LHC physics studies. The many new features should allow an improved description of data.

General-purpose event generators for LHC physics

We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hardscattering matrix elements for processes of interest, at both leading and nextto-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the Ariadne, Herwig++, Pythia 8 and Sherpa generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.

Transverse-momentum-ordered showers and interleaved multiple interactions

Abstract.We propose a sophisticated framework for high-energy hadronic collisions, wherein different QCD physics processes are interleaved in a common sequence of falling transverse-momentum values. Thereby phase-space competition is introduced between multiple parton-parton interactions and initial-state radiation. As a first step we develop new transverse-momentum-ordered showers for initial- and final-state radiation, which should be of use also beyond the scope of the current article. These showers are then applied in the context of multiple interactions, and a few tests of the new model are presented. The article concludes with an outlook on further aspects, such as the possibility of a shower branching giving partons participating in two different interactions.

Stable massive particles at colliders

We review the theoretical motivations and experimental status of searches for stable massive particles (SMPs) which could be sufficiently long-lived as to be directly detected at collider experiments. The discovery of such particles would address a number of important questions in modern physics including the origin and composition of dark matter in the universe and the unification of the fundamental forces. This review describes the techniques used in SMP-searches at collider experiments and the limits so far obtained on the production of SMPs which possess various colour, electric and magnetic charge quantum numbers. We also describe theoretical scenarios which predict SMPs along with the phenomenology needed to model their production at colliders and interactions with matter. In addition, the interplay between collider searches and open questions in cosmology such as dark matter composition is addressed.

Interleaved Parton Showers and Tuning Prospects

General-purpose Monte Carlo event generators have become important tools in particle physics, allowing the simulation of exclusive hadronic final states. In this article we examine the Pythia 8 generator, in particular focusing on its parton-shower algorithms. Some relevant new additions to the code are introduced, that should allow for a better description of data. We also implement and compare with 2 → 3 real-emission QCD matrix elements, to check how well the shower algorithm fills the phase space away from the soft and collinear regions. A tuning of the generator to Tevatron data is performed for two PDF sets and the impact of first new LHC data is examined.