Homoclinic RG flows, or when relevant operators become irrelevant

Abstract: We study an $\mathcal{N}=1$ supersymmetric quantum field theory with $O(M)\times O(N)$ symmetry. Working in $3-\epsilon$ dimensions, we calculate the beta functions up to second loop order and analyze in detail the Renormalization Group (RG) flow and its fixed points. We allow $N$ and $M$ to assume general real values, which results in them functioning as bifurcation parameters. In studying the behaviour of the model in the space of $M$ and $N$, we demarcate the region where the RG flow is non-monotonic and determine curves along which Hopf bifurcations take place. At a number of points in the space of $M$ and $N$ we find that the model exhibits an interesting phenomenon: at these points the RG flow possesses a fixed point located at real values of the coupling constants $g_i$ but with a stability matrix $\left(\frac{\partial \beta_i}{\partial g_j}\right)$ that is not diagonalizable and has a Jordan block of size two with zero eigenvalue. Such points correspond to logarithmic CFTs and represent Bogdanov-Takens bifurcations, a type of bifurcation known to give rise to a nearby homoclinic orbit - an RG flow that originates and terminates at the same fixed point. In the present example, we are able to employ analytic and numeric evidence to display the existence of the homoclinic RG flow.