Once the flavours and have been selected, a choice is made between the possible multiplets. The relative composition of different multiplets is not given from first principles, but must depend on the details of the fragmentation process. To some approximation one would expect a negligible fraction of states with radial excitations or non-vanishing orbital angular momentum. Spin counting arguments would then suggest a 3:1 mixture between the lowest lying vector and pseudoscalar multiplets. Wave function overlap arguments lead to a relative enhancement of the lighter pseudoscalar states, which is more pronounced the larger the mass splitting is [And82a].
In the program, six meson multiplets are included. If the nonrelativistic classification scheme is used, i.e. mesons are assigned a valence quark spin and an internal orbital angular momentum , with the physical spin denoted , , then the multiplets are:
In the program, the spin is first chosen to be either 0 or 1. This is done according to parameterized relative probabilities, where the probability for spin 1 by default is taken to be 0.5 for a meson consisting only of and quark, 0.6 for one which contains as well, and for quarks with or heavier quark, in accordance with the deliberations above.
By default, it is assumed that , such that only pseudoscalar and vector mesons are produced. For inclusion of production, four parameters can be used, one to give the probability that a state also has , the other three for the probability that a state has and either 0, 1, or 2. Experimentally a non-negligible rate of production is observed. This is visible in the appropriate invariant-mass plots, but does not have a significant impact on event shapes in general, i.e. comparably good descriptions can be obtained with and without mesons. The reason is likely that the mass spectrum of intermediate string pieces, as reconstructed from the primary hadrons, is not too dissimilar from a smeared-out setup with higher mesonic states. That is, the string model already implicitly contains higher-excited mesons.
flavour-diagonal meson states
it is also necessary to include mixing into the physical mesons.
This is done according to a parameterization, based on the mixing angles
given in the Review of Particle Properties [PDG88]. In particular,
the default choices correspond to