Global event measures, like sphericity or thrust, can only be
used to determine the jet axes for back-to-back 2-jet events. To
determine the individual jet axes in events with three or more
jets, or with two (main) jets which are not back-to-back,
cluster algorithms are customarily used. In these, nearby
particles are grouped together into a variable number of
clusters. Each cluster has a well-defined direction, given by a
suitably weighted average of the constituent particle
directions. The cluster algorithms traditionally used in and in
physics differ in several respects. The former tend to be
spherically symmetric, i.e. have no preferred axis in space, and
normally all particles have to be assigned to some jet. The
latter pick the beam axis as preferred direction, and make use
of variables related to this choice, such as rapidity and
transverse momentum; additionally only a fraction of all
particles are assigned to jets. This reflects a difference in
the underlying physics: in collisions, the beam remnants