Fermionic zero modes on a toroidal cosmic string

Abstract

We consider a toroidal configuration of cosmic string in 3+1 dimensions in an abelian Higgs model, a compactification of the Nielsen-Olesen string. This object is classically unstable. We explicitly compute the number of permitted zero modes for majorana fermions coupled to such a string. As in the case of indefinitely long strings, there are |n| zero modes for winding number sector n, and correspondingly, induced fermionic charge n/2 which canbe fractional. According to a previously proved result, this implies quantum mechanical stability for objects with odd winding number. The result is of significance to cosmology in classes of unified theories permitting such cosmic strings.