HERWIG for and p Physics

Overview HERWIG is a general purpose event generator for simulating Hadron Emission Reactions With Interfering Gluons, which includes any combination of lepton, hadron or photon scattering in a single packagee1]. The emphasis is rmly on the perturbative description of jet evolution, and in particular the treatment of coherence eeects in that evolution. The philosophy of HERWIG is to provide as complete as possible an implementation of perturbative QCDD2,3], coupled with as simple as possible a model of non-perturbative eeectss4,5], uniformly for as many processes as possible. It gives a good description of high-energy data with relatively few adjustable parameters , most of which are well constrained by those data. This means that when moving to new types of collision, such as and p, at correspondingly lower energy, it is highly predictive, with only a couple of parameters that can be freely tuned. This, of course, is a great advantage for understanding the physical processes, the extent to which they can be described by perturbative QCD, and how similar they are in diierent reactions. But on the other hand, if the aim is simply to nd a good parametrisation of the experimental nal state, then the lack of free parameters can certainly be a nuisance. To illustrate the process by which HERWIG generates events, we describe the reaction with the most steps, doubly-resolved photon scattering in e e ? events. The various other combinations can generally be obtained from this picture by replacing one or more steps by simpler equivalents. When HERWIG is presented with an ee collision and asked to produce a hadronic event, the rst step is to emit photons from the electrons. This is done using the Equivalent Photon Approximation, retaining the full dependence on the photon transverse momentum and virtuality, within user-deened limits. This is important both as a tool for calculating experimental corrections and in reducing the theoretical ambiguities inherent in the EPAA6]. Since the photons in general have a non-negligible transverse momentum, the entire event is then boosted to their centre-of-mass frame, where they collide head-on.