Thermal Stabilisation of Superconducting Sigma Strings and their Drum Vortons
We discuss various issues related to stabilized embedded strings in a thermal background. In particular, we demonstrate that such strings will generically become superconducting at moderately low temperatures, thus enhancing their stability. We then present a new class of defects - drum vortons - which arise when a small symmetry breaking term is added to the potential. We display these points within the context of the O(4) sigma model, relevant for hadrodynamics below the QCD scale. This model admits `embedded defects' (topological defect configurations of a simpler - in this case O(2) symmetric - model obtained by imposing an embedding constraint) that are unstable in the full model at zero temperature, but that can be stabilised (by electromagnetic coupling to photons) in a thermal gas at moderately high termperatures. It is shown here that below the embedded defect stabilisation threshold, there will still be stabilized cosmic string defects. However, they will not be of the symmetric embedded vortex type, but of an `asymmetric' vortex type, and are automatically superconducting. In the presence of weak symmetry breaking terms, such as arise naturally when using the O(4) model for hadrodynamics, the strings become the boundary of a new kind of cosmic sigma membrane, with tension given by the pion mass. The string current would then make it possible for a loop to attain a (classically) stable equilibrium state that differs from an ``ordinary'' vorton state by the presence of a sigma membrane stretched across it in a drum like configuration. Such defects will however be entirely destabilised if the symmetry breaking is too strong, as is found to be the case -- due to the rather large value of the pion mass -- in the hadronic application of the O(4) sigma model.