Lepton-hadron collisions

The main option for photoproduction and Deeply Inelastic Scattering (DIS) physics is provided by the 'gamma/lepton' option as beam or target in a PYINIT call, see section . The $Q^2$ range to be covered, and other kinematics constraints, can be set by CKIN values. By default, when the whole $Q^2$ range is populated, obviously photoproduction dominates.

The older DIS process 10, $\ell \mathrm{q}\to \ell' \mathrm{q}'$, includes $\gamma^0/\mathrm{Z}^0/\mathrm{W}^{\pm}$ exchange, with full interference, as described in section . The $\mathrm{Z}^0/\mathrm{W}^{\pm}$ contributions are not implemented in the 'gamma/lepton' machinery. Therefore process 10 is still the main option for physics at very high $Q^2$, but has been superseded for lower $Q^2$. Radiation off the incoming lepton leg is included by MSTP(11)=1 and off the outgoing one by MSTJ(41)=2 (both are default). Note that both QED and QCD radiation (off the $\mathrm{e}$ and the $\mathrm{q}$ legs, respectively) are allowed to modify the $x$ and $Q^2$ values of the process, while the conventional approach in the literature is to allow only the former. Therefore an option (on by default) has been added to preserve these values by a post-facto rescaling, MSTP(23)=1. Further comments on HERA applications are found in [Sjö92b].