Multimonopoles and closed vortices in SU(2) Yang-Mills-Higgs theory

We review classical monopole solutions of the SU(2) Yang-Mills-Higgs theory. The first part is a pedagogical introduction into to the basic features of the celebrated 't Hooft - Polyakov monopole. In the second part we describe new classes of static axially symmetric solutions which generalise 't Hooft - Polyakov monopole. These configurations are either deformations of the topologically trivial sector or the sectors with different topological charges. In both situations we construct the solutions representing the chains of monopoles and antimonopoles in static equilibrium. The solutions of another type are closed vortices which are centred around the symmetry axis and form different bound systems. Configurations of the third type are monopoles bounded with vortices. We suggest classification of these solutions which is related with 2d Poincare index.Comment: 34 pages, 8 figures. Invited contribution prepared for Review Volume "Etudes on Theoretical Physics" dedicated to 65th anniversary of the Department of Theoretical Physics, Belarus State University, Minsk. Based on a work in collaboration with Jutta Kunz and Burkhard Kleihaus. Any comments and suggestions, especially with respect to references, are welcom

Fractional Hopfions in the Faddeev-Skyrme model with a symmetry breaking potential

We construct new solutions of the Faddeev-Skyrme model with a symmetry breaking potential admitting $S^1$ vacuum. It includes, as a limiting case, the usual $SO(3)$ symmetry breaking mass term, another limit corresponds to the potential $m^2 \phi_1^2$, which gives a mass to the corresponding component of the scalar field. However we find that the spacial distribution of the energy density of these solutions has more complicated structure, than in the case of the usual Hopfions, typically it represents two separate linked tubes with different thicknesses and positions. In order to classify these configurations we define a counterpart of the usual position curve, which represents a collection of loops $\mathcal{C}_1, \mathcal{C}_{-1}$ corresponding to the preimages of the points $\vec \phi = (\pm 1 \mp \mu, 0,0)$, respectively. Then the Hopf invariant can be defined as $Q= {\rm link} (\mathcal{C}_1,\mathcal{C}_{-1})$. In this model, in the sectors of degrees $Q=5,6,7$ we found solutions of new type, for which one or both of these tubes represent trefoil knots. Further, some of these solutions possess different types of curves $\mathcal{C}_1$ and $\mathcal{C}_{-1}$.Comment: 22 pages, 129 figure