MSEL = 21, 22, 24
The of subprocess 141 contains the full interference structure for couplings to fermion pairs. With MSTP(44) it is possible to pick only a subset, e.g. only the pure piece. The couplings of the to quarks and leptons in the first generation can be set via PARU(121) - PARU(128), in the second via PARJ(180) - PARJ(187) and in the third via PARJ(188) - PARJ(195). The eight numbers correspond to the vector and axial couplings of down-type quarks, up-type quarks, leptons and neutrinos, respectively. The default corresponds to the same couplings as that of the Standard Model , with axial couplings and vector couplings . This implies a resonance width that increases linearly with the mass. By a suitable choice of the parameters, it is possible to simulate just about any imaginable scenario, with full interference effects in cross sections and decay angular distributions and generation-dependent couplings. The conversion from the coupling conventions in a set of different models in the literature to those used in PYTHIA can be found in [Cio05].
The coupling to the decay channel is regulated by PARU(129) - PARU(130). The former gives the strength of the coupling, which determines the rate. The default, PARU(129) = 1., corresponds to the `extended gauge model' of [Alt89], wherein the coupling is used, scaled down by a factor , to give a partial width into this channel that again increases linearly. If this factor is cancelled, by having PARU(129) proportional to , one obtains a partial width that goes like the fifth power of the mass, the `reference model' of [Alt89]. In the decay angular distribution one could imagine a much richer structure than is given by the one parameter PARU(130).
Other decay modes include , predicted in left-right symmetric models (see PARU(145) and ref. [Coc91]), and a number of other Higgs decay channels, see sections and .
The of subprocess 142 so far does not contain interference with the Standard Model -- in practice this should not be a major limitation. The couplings of the to quarks and leptons are set via PARU(131) - PARU(134). Again one may set vector and axial couplings freely, separately for the and the decay channels. The defaults correspond to the structure of the Standard Model , but can be changed to simulate a wide selection of models. One possible limitation is that the same Cabibbo-Kobayashi-Maskawa quark mixing matrix is assumed as for the standard .
The coupling can be set via PARU(135) - PARU(136). Further comments on this channel as for ; in particular, default couplings again agree with the `extended gauge model' of [Alt89]. A channel is also included, in analogy with the one, see PARU(146).
The boson (particle code 41) of subprocess 144 represents one possible scenario [Ben85a] for a horizontal gauge boson, i.e. a gauge boson that couples between the generations, inducing processes like . Experimental limits on flavour-changing neutral currents forces such a boson to be fairly heavy.
A further example of new gauge groups follows right after this.