While pure exchange gives a simple
distribution for the (and
) direction in
interference results in a
forward-backward asymmetry. If one introduces
The angular orientation of a 3- or 4-jet event may be described in terms of three angles , and ; for 2-jet events only and are necessary. From a standard orientation, with the along the axis and the in the plane with , an arbitrary orientation may be reached by the rotations in azimuthal angle, in polar angle, and in azimuthal angle, in that order. Differential cross sections, including QFD effects and arbitrary beam polarizations have been given for 2- and 3-jet events in refs. [Ols80,Sch80]. We use the formalism of ref. [Ols80], with translation from their terminology according to and . The resulting formulae are tedious, but straightforward to apply, once the internal jet configuration has been chosen. 4-jet events are approximated by 3-jet ones, by joining the two gluons of a event and the and of a event into one effective jet. This means that some angular asymmetries are neglected [Ali80a], but that weak effects are automatically included. It is assumed that the second-order 3-jet events have the same angular orientation as the first-order ones, some studies on this issue may be found in [Kör85]. Further, the formulae normally refer to the massless case; only for the QED 2- and 3-jet cases are mass corrections available.
The main effect of the angular distribution of multijet events is to smear the lowest-order result, i.e. to reduce any anisotropies present in 2-jet systems. In the parton-shower option of the program, only the initial axis is determined. The subsequent shower evolution then de facto leads to a smearing of the jet axis, although not necessarily in full agreement with the expectations from multijet matrix-element treatments.