Semilocal cosmic strings

We show that the presence of global symmetries in a Lagrangian can lead to U(1) flux-tube solutions even when the vacuum manifold is simply connected. We provide a model in which these flux tubes, called "semilocal" strings, occur and explicitly prove the existence of stable string solutions by Bogomol'nyi's method. The formation of semilocal strings in the early Universe and the evolution of the network is discussed. We also show that semilocal magnetic monopoles cannot exist.