IR considerations around causality

In nature, some UV features of the dynamics are reflected in IR quantities. In fully relativistic theories, this connection can be probed through the analyticity properties of the S-matrix, allowing to understand which IR theories respect the UV assumptions of quantum field theory.
In this talk, I explore the IR dynamical meaning of these analyticity bounds.
Such bounds are usually violated when a theory displays faster-than-light propagation for low-energy excitations. While superluminality by itself is not logically inconsistent, these theories also predict the formation of closed light-like curves at the classical level, making the cause-and-effect relation between different events ambiguous.
In this regard, I will show how quantum mechanical effects alter the prediction of this IR causal pathology. Low energy quanta strongly backreact on configurations with closed light-like curves, exciting unknown short-distance degrees of freedom and invalidating the classical predictions.
Thus, there is no obvious IR pathology associated with violating the S-matrix bounds, and these appear to be a purely UV statement phrased in terms of IR quantities.