Semilocal string formation in two dimensions

Abstract

We present a toy model for the investigation of the formation of semilocal strings, where a planar symmetry is employed to reduce the system to two dimensions. We approximate the symmetry breaking using an extension of the Vachaspati-Vilenkin algorithm, where we throw down random phases for the scalar fields and then find the gauge field configuration which minimizes gradient energy in this fixed scalar background. We show this procedure reproduces the standard estimate for the formation rate of cosmic strings. For semilocal strings the configurations generated by this method are ambiguous, and we numerically evolve the configurations forward in time to identify which regions form strings. We find a significant rate of formation, depending on the ratio of couplings $β$. For low $β$ the formation rate is about one quarter that of cosmic strings; this falls as $β$ is increased and above $β= 1$, where the string solution is dynamically unstable, no semilocal strings form. We show the results are robust by examining different initial conditions where, as expected in a thermal environment, the initial scalar field need not be on the vacuum. We discuss implications for the three-dimensional case.