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In the past few years it has become clear that topological defects, and in particular strings, will be endowed with a considerably richer structure than previously envisaged. In generic grand unified models the Higgs field, responsible for the existence of cosmic strings, will have interactions with other fundamental fields. This should not surprise us, for well understood low energy particle theories include field interactions in order to account for the well measured masses of light fermions, like the familiar electron, and for the masses of gauge bosons W and Z discovered at CERN in the eighties. Thus, when one of these fundamental (electromagnetically charged) fields present in the model condenses in the interior space of the string, there will appear electric currents flowing along the string core.

Even though these strings are the most attractive ones, the fact of them having electromagnetic properties is not actually fundamental for understanding the dynamics of circular string loops. In fact, while in the uncharged and non current-carrying case symmetry arguments do not allow us to distinguish the existence of rigid rotations around the loop axis, the very existence of a small current breaks this symmetry, marking a definite direction, which allows the whole loop configuration to rotate. This can also be viewed as the existence of spinning particle-like solutions trapped inside the core. The stationary loop solutions where the string tension gets balanced by the angular momentum of the charges is what Davis and Shellard [1988] dubbed vortons.

Vorton configurations do not radiate classically. Because they have loop shapes, implying periodic boundary conditions on the charged fields, it is not surprising that these configurations are quantized. At large distances these vortons look like point masses with quantized electric charge (actually they can have more than a hundred times the electron charge) and angular momentum. They are very much like particles, hence their name. They are however very peculiar, for their characteristic size is of order of their charge number (around a hundred) times their thickness, which is essentially some fourteen orders of magnitude smaller than the classical electron radius. Also, their mass is often of the order of the energies of grand unification, and hence vortons would be some twenty orders of magnitude heavier than the electron.

But why should strings become conducting in the first place? The physics inside the core of the string differs somewhat from outside of it. In particular the existence of interactions among the Higgs field forming the string and other fundamental fields, like that of charged fermions, would make the latter loose their masses inside the core. Then, only small energies would be required to produce pairs of trapped fermions and, being effectively massless inside the string core, they would propagate at the speed of light. These zero energy fermionic states, also called zero modes, endow the string with currents and in the case of closed loops they provide the mechanical angular momentum support necessary for stabilizing the contracting loop against collapse.

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