Topological Origin of Chiral Symmetry Breaking in QCD and in Gravity

We show that the assumption of non-zero topological susceptibility of the vacuum in a fermion-free version of a theory, such as gravity or QCD, suffices to conclude the following: Once N massless fermion flavors are added to the theory, they break the chiral flavor symmetry dynamically, down to a subgroup that would be anomaly-free under gauging; In both theories, the pseudo-Goldstone corresponding to axial U(1)-symmetry becomes massive; In QCD as well as in gravity the massless fermions are eliminated from the low energy spectrum of the theory. All the above conclusions are reached without making an assumption about confinement. Some key methods of our approach are: Reformulation of topological susceptibility in the language of a three-form gauge theory; Utilization of gravity in the role of a spectator interaction for the chiral anomaly-matching in QCD; Gauging chiral symmetries and matching their anomalies using the spectator Green-Schwarz type axions. Our observations suggest that breaking of chiral symmetries in QCD and in gravity can be described in unified topological language, and seemingly-disconnected phenomena, such as, the generation of eta'-meson mass in QCD and breaking of global chiral symmetry by gravity may share a secret analogy. The described phenomenon may shed a new light - via contribution of micro black holes into the gravitational topological susceptibility of the vacuum - on incompatibility between black holes and global symmetries. It appears that explicit breaking is not the sole possibility, and like QCD, gravity may break global symmetries dynamically. As an useful byproduct, matching of gravitational anomalies provides a selection tool for compositeness, eliminating possibility of massless composite fermions where standard gauge anomaly matching would allow for their existence.