Anomalous TQFTs for Fermionic Systems via Symmetry Extension

This paper investigates nonperturbative global anomalies in 4D fermionic systems, particularly Weyl fermions, focusing on mixed gauge-gravitational anomalies. It proposes a symmetry-extension construction to cancel these anomalies using anomalous topological quantum field theories (TQFTs). The key idea is to replace an anomalous fermionic system with a discrete gauge TQFT, offering a new perspective on low-energy physics and potentially addressing issues like the Standard Model's anomalies.