PYTHIA 6.4 Physics and Manual

Monte Carlo world, the rôle of the machine, namely to produce events, is taken by theevent generators described in this report. The behaviour of the detectors — how particlesproduced by the event generator traverse the detector, spiral in magnetic fields, shower incalorimeters, or sneak out through cracks, etc. — is simulated in programs such as Geant[Bru89]. Be warned that this latter activity is sometimes called event simulation, which issomewhat unfortunate since the same words could equally well be applied to what, here,we call event generation. A more appropriate term is detector simulation. Ideally, theoutput of this simulation has exactly the same format as the real data recorded by thedetector, and can therefore be put through the same event reconstruction and physicsanalysis chain, except that here we know what the ‘right answer’ should be, and so cansee how well we are doing.Since the full chain of detector simulation and event reconstruction is very timeconsuming,one often does ‘quick and dirty’ studies in which these steps are skippedentirely, or at least replaced by very simplified procedures which only take into accountthe geometric acceptance of the detector and other trivial effects. One may then use theoutput of the event generator directly in the physics studies.There are still many holes in our understanding of the full event structure, despitean impressive amount of work and detailed calculations. To put together a generatortherefore involves making a choice on what to include, and how to include it. At best,the spread between generators can be used to give some impression of the uncertaintiesinvolved. A multitude of approximations will be discussed in the main part of this report,but already here is should be noted that many major approximations are related to thealmost complete neglect of non-‘trivial’ higher-order effects, as already mentioned. Itcan therefore only be hoped that the ‘trivial’ higher order parts give the bulk of theexperimental behaviour. By and large, this seems to be the case; for e + e − annihilation iteven turns out to be a very good approximation.The necessity to make compromises has one major implication: to write a good eventgenerator is an art, not an exact science. It is therefore essential not to blindly trustthe results of any single event generator, but always to make several cross-checks. Inaddition, with computer programs of tens of thousands of lines, the question is not whetherbugs exist, but how many there are, and how critical their positions. Further, an eventgenerator cannot be thought of as all-powerful, or able to give intelligent answers to illposedquestions; sound judgement and some understanding of a generator are necessaryprerequisites for successful use. In spite of these limitations, the event-generator approachis the most powerful tool at our disposal if we wish to gain a detailed and realisticunderstanding of physics at current or future high-energy colliders.1.3 The Origins of the Current ProgramOver the years, many event generators have appeared. A recent comprehensive overviewis the Les Houches guidebook to Monte Carlo event generators [Dob04]. Surveys ofgenerators for e + e − physics in general and LEP in particular may be found in [Kle89,Sjö89, Kno96, Lön96, Bam00], for high-energy hadron–hadron (pp) physics in [Ans90,Sjö92, Kno93, LHC00], and for ep physics in [HER92, HER99]. We refer the readerto those for additional details and references. In this particular report, the two closelyconnected programs Jetset and Pythia, now merged under the Pythia label, will bedescribed.Jetset has its roots in the efforts of the Lund group to understand the hadronizationprocess, starting in the late seventies [And83]. The so-called string fragmentationmodel was developed as an explicit and detailed framework, within which the long-rangeconfinement forces are allowed to distribute the energies and flavours of a parton configurationamong a collection of primary hadrons, which subsequently may decay further.This model, known as the Lund string model, or ‘Lund’ for short, contained a number3