Hadron-hadron collisions

The default is to include QCD jet production by $2 \to 2$ processes, see section . Since the differential cross section is divergent for $p_{\perp}\to 0$, a lower cut-off has to be introduced. Normally that cut-off is given by the user-set $p_{\perp\mathrm{min}}$ value in CKIN(3). If CKIN(3) is chosen smaller than a given value of the order of 2 GeV (see PARP(81) and PARP(82)), then low-$p_{\perp}$ events are also switched on. The jet cross section is regularized at low $p_{\perp}$, so as to obtain a smooth joining between the high-$p_{\perp}$ and the low-$p_{\perp}$ descriptions, see further section . As CKIN(3) is varied, the jump from one scenario to another is abrupt, in terms of cross section: in a high-energy hadron collider, the cross section for jets down to a $p_{\perp\mathrm{min}}$ scale of a few GeV can well reach values much larger than the total inelastic, non-diffractive cross section. Clearly this is nonsense; therefore either $p_{\perp\mathrm{min}}$ should be picked so large that the jet cross section be only a fraction of the total one, or else one should select $p_{\perp\mathrm{min}}= 0$ and make use of the full description.

If one switches to MSEL = 2, also elastic and diffractive processes are switched on, see section . However, the simulation of these processes is fairly primitive, and should not be used for dedicated studies, but only to estimate how much they may contaminate the class of non-diffractive minimum-bias events.

Most processes can be simulated in hadron colliders, since the bulk of PYTHIA processes can be initiated by quarks or gluons. However, there are limits. Currently we include no photon or lepton parton distributions, which means that a process like $\gamma \mathrm{q}\to \gamma \mathrm{q}$ is not accessible. Further, the possibility of having $\mathrm{Z}^0$ and $\mathrm{W}^{\pm}$ interacting in processes such as 71-77 has been hardwired process by process, and does not mean that there is a generic treatment of $\mathrm{Z}^0$ and $\mathrm{W}^{\pm}$ distributions.

The emphasis in the hadron-hadron process description is on high energy hadron colliders. The program can be used also at fixed-target energies, but the multiple interaction model for underlying events then may break down and has to be used with caution. The limit of `safe' applicability is somewhere at around 100 GeV. Only with the simpler model obtained for MSTP(82) = 1 can one go arbitrarily low.