Soft gluons at large angles in hadron collisions

A general discussion is presented of the single logarithmic soft factor thatappears in two scale QCD observables in processes involving four partons. Wetreat it as the ``fifth form factor'', accompanying the four collinear singularSudakov form factors attached to colliding and outgoing hard partons. The fifthform factor is expressed in terms of the Casimir operators (squared colourcharges) of irreducible representations in the crossing $t$- and $u$-channels.As an application we revisit the problem of large angle radiation in $gg\to gg$and give a relatively simple solution and interpretation of the results. Wefound an unexpected symmetry of the soft anomalous dimension under exchange ofinternal and external variables of the problem whose existence calls forexplanation.Comment: 29 pages, pictures generated with feynmf graphic packag