Semilocal Defects
  I analyze the interplay of gauge and global symmetries in the theory of topological defects. In a two-dimensional model in which both gauge symmetries and {\it exact} global symmetries are spontaneously broken, stable vortices may fail to exist even though magnetic flux is topologically conserved. Following Vachaspati and Ach\'ucarro, I formulate the condition that must be satisfied by the pattern of symmetry breakdown for finite-energy configurations to exist in which the conserved magnetic flux is spread out instead of confined to a localized vortex. If this condition is met, vortices are always unstable at sufficiently weak gauge coupling. I also describe the properties of defects in models with an ``accidental'' symmetry that is partially broken by gauge boson exchange. In some cases, the spontaneously broken accidental symmetry is not restored inside the core of the defect. Then the structure of the defect can be analyzed using an effective field theory; the details of the physics responsible for the spontaneous symmetry breakdown need not be considered. Examples include ``semilocal'' domain walls and vortices that are classically unstable, but are stabilized by loop corrections, and ``semilocal'' magnetic monopoles that have an unusual core structure. Finally, I examine the general theory of the ``electroweak strings'' that were recently discussed by Vachaspati. These arise only in models with gauge boson ``mixing,'' and can always end on magnetic monopoles. Cosmological implications are briefly discussed. 
John Preskill