Many different possibilities are open for the decay of heavy onia resonances. Of special interest are the decays into three gluons or two gluons plus a photon, since these offer unique possibilities to study a `pure sample' of gluon jets. A routine for this purpose is included in the program. It was written at a time where the expectations were to find toponium at PETRA energies. Given the large value of the top mass, weak decays dominate, to the extent that the top quark decays weakly even before a bound toponium state is formed, and thus the routine will be of no use for top. The charm system, on the other hand, is far too low in mass for a jet language to be of any use. The only application is therefore likely to be for , which unfortunately also is on the low side in mass.
The matrix element for
is (in lowest order)
Another process is
, obtained by replacing
a gluon in
by a photon. This process has the
same normalized cross section as the one above, if e.g. is taken
to refer to the photon. The relative rate is [Kol78]
In the present implementation the angular orientation of the and events is given for the onium case [Kol78] (optionally with beam polarization effects included), i.e. weak effects have not been included, since they are negligible at around 10 GeV.
It is possible to start a perturbative shower evolution from either of the two states above. However, for the phase space for additional evolution is so constrained that not much is to be gained from that. We therefore do not recommend this possibility. The shower generation machinery, when starting up from a configuration, is constructed such that the photon energy is not changed. This means that there is currently no possibility to use showers to bring the theoretical photon spectrum in better agreement with the experimental one.
In string fragmentation language, a state corresponds to a closed string triangle with the three gluons at the corners. As the partons move apart from a common origin, the string triangle expands. Since the photon does not take part in the fragmentation, the state corresponds to a double string running between the two gluons.