Other decays

For onia spin 1 resonances, decay channels into a pair of leptons are explicitly given. Hadronic decays of the $\mathrm{J}/\psi$ are simulated using the flavour generation model introduced for charm. For $\Upsilon$ a fraction of the hadronic decays is into $\mathrm{q}\overline{\mathrm{q}}$ pairs, while the rest is into $\mathrm{g}\mathrm{g}\mathrm{g}$ or $\mathrm{g}\mathrm{g}\gamma$, using the matrix elements of eq. (). The $\eta_c$ and $\eta_b$ are both allowed to decay into a $\mathrm{g}\mathrm{g}$ pair, which then subsequently fragments. In $\Upsilon$ and $\eta_b$ decays the partons are allowed to shower before fragmentation, but energies are too low for showering to have any impact.

Default branching ratios are given for resonances like the $\mathrm{Z}^0$, the $\mathrm{W}^{\pm}$, the $\t$ or the $\mathrm{h}^0$. When PYTHIA is initialized, these numbers are replaced by branching ratios evaluated from the given masses. For $\mathrm{Z}^0$ and $\mathrm{W}^{\pm}$ the branching ratios depend only marginally on the masses assumed, while effects are large e.g. for the $\mathrm{h}^0$. In fact, branching ratios may vary over the Breit-Wigner resonance shape, something which is also taken into account in the PYRESD description. Therefore the simpler resonance treatment of PYDECY is normally not so useful, and should be avoided. When it is used, a channel is selected according to the given fixed branching ratios. If the decay is into a $\mathrm{q}\overline{\mathrm{q}}$ pair, the quarks are allowed to shower and subsequently the parton system is fragmented.