PYTHIA 6.4 Physics and Manual

conservation, and even more so if one does not. The main supersymmetric machineryand process selection is inherited from SPythia [Mre97], however with manyimprovements in the event generation chain. Many different SUSY scenarios havebeen proposed, and the program is flexible enough to allow input from several ofthese, in addition to the ones provided internally.• The possibility of extra dimensions at low energies has been a topic of much study inrecent years, but has still not settled down to some standard scenarios. Its inclusioninto Pythia is also only in a very first stage.This is by no means a survey of all interesting physics. Also, within the scenarios studied,not all contributing graphs have always been included, but only the more importantand/or more interesting ones. In many cases, various approximations are involved in thematrix elements coded.2.1.2 Resonance DecaysAs we noted above, the bulk of the processes above are of the 2 → 2 kind, with veryfew leading to the production of more than two final-state particles. This may be seenas a major limitation, and indeed is so at times. However, often one can come quite farwith only one or two particles in the final state, since showers will add the required extraactivity. The classification may also be misleading at times, since an s-channel resonanceis considered as a single particle, even if it is assumed always to decay into two final-stateparticles. Thus the process e + e − → W + W − → q 1 q ′ 1 q 2 q ′ 2 is classified as 2 → 2, althoughthe decay treatment of the W pair includes the full 2 → 4 matrix elements (in the doublyresonant approximation, i.e. excluding interference with non-WW four-fermion graphs).Particles which admit this close connection between the hard process and the subsequentevolution are collectively called resonances in this manual. It includes all particlesin mass above the b quark system, such as t, Z 0 , W ± , h 0 , supersymmetric particles, andmany more. Typically their decays are given by electroweak physics, or physics beyondthe Standard Model. What characterizes a (Pythia) resonance is that partial widthsand branching ratios can be calculated dynamically, as a function of the actual massof a particle. Therefore not only do branching ratios change between an h 0 of nominalmass 100 GeV and one of 200 GeV, but also for a Higgs of nominal mass 200 GeV, thebranching ratios would change between an actual mass of 190 GeV and 210 GeV, say.This is particularly relevant for reasonably broad resonances, and in threshold regions.For an approach like this to work, it is clearly necessary to have perturbative expressionsavailable for all partial widths.Decay chains can become quite lengthy, e.g. for supersymmetric processes, but followa straight perturbative pattern. If the simulation is restricted to only some set of decays,the corresponding cross section reduction can easily be calculated. (Except in some rarecases where a nontrivial threshold behaviour could complicate matters.) It is thereforestandard in Pythia to quote cross sections with such reductions already included. Notethat the branching ratios of a particle is affected also by restrictions made in the secondaryor subsequent decays. For instance, the branching ratio of h 0 → W + W − , relative toh 0 → Z 0 Z 0 and other channels, is changed if the allowed W decays are restricted.The decay products of resonances are typically quarks, leptons, or other resonances,e.g. W → qq ′ or h 0 → W + W − . Ordinary hadrons are not produced in these decays,but only in subsequent hadronization steps. In decays to quarks, parton showers areautomatically added to give a more realistic multijet structure, and one may also allowphoton emission off leptons. If the decay products in turn are resonances, further decaysare necessary. Often spin information is available in resonance decay matrix elements.This means that the angular orientations in the two decays of a W + W − pair are properlycorrelated. In other cases, the information is not available, and then resonances decayisotropically.12