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Decays

A large fraction of the particles produced by fragmentation are unstable and subsequently decay into the observable stable (or almost stable) ones. It is therefore important to include all particles with their proper mass distributions and decay properties. Although involving little deep physics, this is less trivial than it may sound: while a lot of experimental information is available, there is also very much that is missing. For charm mesons, it is necessary to put together measured exclusive branching ratios with some inclusive multiplicity distributions to obtain a consistent and reasonably complete set of decay channels, a rather delicate task. For bottom even less is known, and for some $\mathrm{B}$ baryons only a rather simple phase-space type of generator has been used for hadronic decays.

Normally it is assumed that decay products are distributed according to phase space, i.e. that there is no dynamics involved in their relative distribution. However, in many cases additional assumptions are necessary, e.g. for semileptonic decays of charm and bottom hadrons one needs to include the proper weak matrix elements. Particles may also be produced polarized and impart a non-isotropic distribution to their decay products. Many of these effects are not at all treated in the program. In fact, spin information is not at all carried along, but has to be reconstructed explicitly when needed.

This normal decay treatment makes use of a set of tables where branching ratios and decay modes are stored. It encompasses all hadrons made out of $\d $, $\u $, $\mathrm{s}$, $\c $ and $\b $ quarks, and also the leptons. The decay products are hadrons, leptons and photons. Some $\b\overline{\mathrm{b}}$ states are sufficiently heavy that they are allowed to decay to partonic states, like $\Upsilon \to \mathrm{g}\mathrm{g}\mathrm{g}$, which subsequently fragment, but these are exceptions.

You may at will change the particle properties, decay channels or branching ratios of the above particles. There is no censorship of what is allowed or not allowed, beyond energy-momentum, spin, and (electrical and colour) charge conservation. There is also no impact e.g. on the cross section of processes, since there is no way of knowing e.g. if the restriction to one specific decay of a particle is because that decay is of particular interest to us, or because recent measurement have shown that this indeed is the only channel. Furthermore, the number of particles produced of each species in the hadronization process is not known beforehand, and so cannot be used to correctly bias the preceding steps of the generation chain. All of this contrasts with the class of `resonances' described above, in section [*].


next up previous contents
Next: Program Overview Up: Hadronization Previous: String Fragmentation   Contents
Stephen Mrenna 2007-10-30