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Hard Processes and Parton Distributions

In the original JETSET code, only two hard processes were available. The first and main one is $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0\to \mathrm{q}\overline{\mathrm{q}}$. Here the `$*$' of $\gamma^*$ is used to denote that the photon must be off the mass shell. The distinction is of some importance, since a photon on the mass shell cannot decay. Of course also the $\mathrm{Z}^0$ can be off the mass shell, but here the distinction is less relevant (strictly speaking, a $\mathrm{Z}^0$ is always off the mass shell). In the following we may not always use `$*$' consistently, but the rule of thumb is to use a `$*$' only when a process is not kinematically possible for a particle of nominal mass. The quark $\mathrm{q}$ in the final state of $\mathrm{e}^+\mathrm{e}^-\to \gamma^* / \mathrm{Z}^0\to \mathrm{q}\overline{\mathrm{q}}$ may be $\u $, $\d $, $\mathrm{s}$, $\c $, $\b $ or $\t $; the flavour in each event is picked at random, according to the relative couplings, evaluated at the hadronic c.m. energy. Also the angular distribution of the final $\mathrm{q}\overline{\mathrm{q}}$ pair is included. No parton-distribution functions are needed.

The other original JETSET process is a routine to generate $\mathrm{g}\mathrm{g}\mathrm{g}$ and $\gamma \mathrm{g}\mathrm{g}$ final states, as expected in onium 1$^{--}$ decays such as $\Upsilon$. Given the large top mass, toponium decays weakly much too fast for these processes to be of any interest, so therefore no new applications are expected.



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Stephen Mrenna 2007-10-30