Abstract :
It is shown that the space of cohomology classes of the SU(1,1) /U(1) coset at negative level k contains states of relevant conformal dimensions. These states correspond to the energy density operator of the associated nonlinear sigma model. We exhibit that there exists a subclass of relevant operators forming a closed fusion algebra. We make use of these operators to perform renormalizable perturbations of the SU(1,1)/U(1) coset. In the infra-red limit, the perturbed theory flows to another conformal model. We identify one of the perturbative conformal points with the SU(2)/U(1) coset at positive level. From the point of view of the string target space geometry, the given renormalization group flow maps the noncompact geometry described by the SU(1,1)/U(1) coset into the sphere described by the SU(2)/U(1) coset. This exhibits a new mechanism of topology change in the space of string compactifications.
