The Particle Content

In order to describe both current and potential future physics, a number of different particles are needed. A list of some particles, along with their codes, is given in section . Here we therefore emphasize the generality rather than the details.

Four full generations of quarks and leptons are included in the program, although indications from LEP strongly suggest that only three exist in Nature. The PDG naming convention for the fourth generation is to repeat the third one but with a prime: $\b '$, $\t '$ $\tau'$ and $\nu'_{\tau}$. Quarks may appear either singly or in pairs; the latter are called diquarks and are characterized by their flavour content and their spin. A diquark is always assumed to be in a colour antitriplet state.

The colour neutral hadrons may be build up from the five lighter coloured quarks (and diquarks). Six full meson multiplets are included and two baryon ones, see section . In addition, $\mathrm{K}_{\mathrm{S}}^0$ and $\mathrm{K}_{\mathrm{L}}^0$ are considered as separate particles coming from the `decay' of $\mathrm{K}^0$ and $\overline{\mathrm{K}}^0$ (or, occasionally, produced directly).

Other particles from the Standard Model include the gluon $\mathrm{g}$, the photon $\gamma$, the intermediate gauge bosons $\mathrm{Z}^0$ and $\mathrm{W}^{\pm}$, and the standard Higgs $\mathrm{h}^0$. Non-standard particles include additional gauge bosons, $\mathrm{Z}'^0$ and $\mathrm{W}'^{\pm}$, additional Higgs bosons $\H ^0$, $\mathrm{A}^0$ and $\H ^{\pm}$, a leptoquark $\L _{\mathrm{Q}}$ a horizontal gauge boson $\mathrm{R}^0$, technicolor and supersymmetric particles, and more.

From the point of view of usage inside the programs, particles may be subdivided into three classes, partly overlapping.

95.

A parton is generically any object which may be found in the wave function of the incoming beams, and may participate in initial- or final-state showers. This includes what is normally meant by partons, i.e. quarks and gluons, but here also leptons and photons. In a few cases other particles may be classified as partons in this sense.

96.

A resonance is an unstable particle produced as part of the hard process, and where the decay treatment normally is also part of the hard process. Resonance partial widths are perturbatively calculable, and therefore it is possible to dynamically recalculate branching ratios as a function of the mass assigned to a resonance. Resonances includes particles like the $\mathrm{Z}^0$ and other massive gauge bosons and Higgs particles, in fact everything with a mass above the $\b$ quark and additionally also a lighter $\gamma^*$.

97.

Hadrons and their decay products, i.e. mesons and baryons produced either in the fragmentation process, in secondary decays or as part of the beam remnant treatment, but not directly as part of the hard process (except in a few special cases). Hadrons may be stable or unstable. Branching ratios are not assumed perturbatively calculable, and can therefore be set freely. Also leptons and photons produced in decays belong to this class. In practice, this includes everything up to and including $\b$ quarks in mass (except a light $\gamma^*$, see above).

Usually the subdivision above is easy to understand and gives you the control you would expect. Thus the restriction on the allowed decay modes of a resonance will directly affect the cross section of a process, while this is not the case for an ordinary hadron, since in the latter case there is no precise theory knowledge on the set of decay modes and branching ratios.