Vortex Solutions in Two-Higgs Systems and $\tan\beta$

Abstract

Classical vortex solutions in various two-Higgs systems are studied. The systems we consider include the standard model with two Higgs doublets, in which case the vortex appears as part of a string-like object. The Higgs potentials contain several different couplings in general and the spontaneous symmetry breaking involves with two different vacuum expectation values. In particular it is shown that the existence of such a solution in general requires a specific ratio of the two Higgs vacuum expectation values, i.e. $\tan\beta$, and some inequalities between different Higgs couplings. This ratio can be determined in terms of the couplings in the Higgs potential. The Higgs masses are also computed in this case. (1+2)-d solutions are topological so that they are topologically stable and the Bogomol'nyi bound is saturated for some couplings. Some comments on the stabilization of (1+3)-d solutions are also given. Thus, as long as such a defect can be formed in the early universe, stable or not, $\tan\beta$ is no longer an independent free parameter in the theory.