**Existence and Stability of Semilocal Cosmic Strings**

The Semilocal Strings are Metastable Vortex Configurations which appear in theories
where the vacuum manifold may not have non-trivial topological properties. Their
metastabity is realized for a **finite sector** of parameter space. It
is due to the effects of gauge fields whose symmetry group is selected to affect only a **subspace**
of the vacuum manifold. This subspace (if isolated) would have non-trivial
topological properties.

The simplest model that incorporates semilocal strings was first studied by **Vachaspati
and Achucarro** (**Phys.Rev.D44:3067-3071,1991**).
The stability of the semilocal vortices was first studied analyticaly by **Hindmarsh**
(**Phys.Rev.Lett.68:1263-1266,1992**)
and using numerical simulations by **Achucarro, Kuijken, Perivolaropoulos and
Vachaspati** (**Nucl.Phys.B388:435-456,1992**).
The most interesting physical application of Semilocal Strings is their Embedding
in the **Standard Electroweak Model** which was first done by **Vachaspati**
(**Nucl.Phys.B397:648-671,1993**).
The **Stability** of these **Electroweak Strings** was first
studied by **James, Perivolaropoulos and Vachaspati** (**Phys.Rev.D46:5232-5235,1992**)
who found that there is a finite sector in the parameter space of the electroweak model
where these strings are **metastable**. This sector however does not include
the experimentally measured values of the Standard Model Parameters. This was the first
time that metastable soliton solutions were found in the Standard Electroweak Model.
A conceptual generalization of semilocal strings was recently done by **Axenides,
Perivolaropoulos and Tomaras** (**Phys.Rev.D58:103512,1998**).

The simplest model that incorporates semilocal strings has **a single free
parameter** called beta (â). For **beta > 1** the semilocal
strings were shown to be **unstable** towards decay to the trivial vacuum
configuration while for **beta < 1** they are **metastable**.

The following links lead to plots corresponding to frames of the simulation of the
evolution of an isolated semilocal string at T=0, T=60 and T=100 (wth timestep size
dt=0.2) for beta=1.1 and beta=0.9. More details can be found in **Nucl.Phys.B388:435-456,1992.**

**Parameter beta = 1.1 (â=1.1):** The frames
indicate the distribution of the energy density in 2d space at evolution times: T=0, T=60,
T=100 which correspond to timesteps N=0, N=300, N=500. The instability of the initial
configuration is demonstrated.

**Parameter beta = 0.9 (â=0.9):** The frames
indicate the distribution of the energy density in 2d space at evolution times: T=0, T=60,
T=100 which correspond to timesteps N=0, N=300, N=500. The stability of the initial
configuration is demonstrated.

Click here to dowload the full Origin 5.0 file with graphs and data. You may also download a single zipped ps-file showing the evolution of a stable or unstable semilocal string including the three frames of evolution.