The merging of jets

In perturbative QCD matrix elements, the invariant mass of parton pairs is normally peaked at low values, due to the singularities of the q->qg and g->gg subprocesses. Whereas pairs at large invariant masses give rise to separately visible jets, one may expect a continuous merging back into one single jet, q or g, when the qg or gg invariant mass becomes small. Within the framework of string fragmentation, we outline an explicit model in which this takes place, and study some of the resulting properties. (orig.)