Introduction

In this chapter we consider the way in which gluons are introduced into the Lund string fragmentation model. They are treated as internal excitations on the massless relativistic string (the MRS) similar to a sudden ‘hammer hit’ on an ordinary classical string. Thus they will be initially well localised in space-time. But we will find that they quickly disperse their energy-momentum to the surrounding string. This property means that the gluon excitation disappears and reappears periodically as a localised energy-momentum-carrying entity during the string cycle.

We will start as usual with a classical mechanics scenario and study some simple modes of motion of the MRS in order to get acquainted with the notion of an internal excitation. We start with the mode which has acquired the poetical name of ‘the dance of the butterfly’. It certainly does exhibit the grace and the beauty that goes with this name. After a brief snapshot description of the appearance of this mode in space coordinates we proceed to a description in space-time. This will lead us to the general equations of motion for the MRS and to an understanding of the way the string is built up in terms of moving wave fronts.

After that we consider more complex modes, although there is no reason to go into too many details.