Anomaly Enforced Gaplessness and Symmetry Fractionalization for $Spin_G$ Symmetries

TL;DR

This paper explores how certain gravitational anomalies in quantum field theories impose strict constraints on RG flows, requiring symmetry fractionalization for gapped phases on non-spin manifolds, especially in 4d gauge theories with fermions.

Contribution

It derives new constraints on RG flows from $ ext{Z}_2$-valued gravitational anomalies, linking them to symmetry fractionalization in gapped phases on non-spin manifolds.

Findings

$ ext{Z}_2$ gravitational anomalies cannot be matched by symmetry-preserving gapped phases without fractionalization.

Examples in 4d gauge theories with fermions illustrate the necessity of symmetry fractionalization.

Anomalies activate only on non-spin manifolds, constraining possible IR phases.

Abstract

Symmetries and their anomalies give strong constraints on renormalization group (RG) flows of quantum field theories. Recently, the identification of a theory's global symmetries with its topological sector has provided additional constraints on RG flows to symmetry preserving gapped phases due to mathematical results in category and topological quantum field theory. In this paper, we derive constraints on RG flows from $\mathbb{Z}_2$-valued pure- and mixed-gravitational anomalies that can only be activated on non-spin manifolds. We show that such anomalies cannot be matched by a unitary, symmetry preserving gapped phase without symmetry fractionalization. In particular, we discuss examples that commonly arise in $4d$ gauge theories with fermions.