Anomalies and symmetric mass generation for Kaehler-Dirac fermions

TL;DR

This paper reveals a unique gravitational anomaly for massless Kaehler-Dirac fermions, constraining their interactions and symmetries, with implications for topological insulators and lattice models.

Contribution

It uncovers a mixed gravitational anomaly for KD fermions, linking symmetry breaking to topological and lattice model constraints, and explores anomaly implications in various dimensions.

Findings

Anomaly depends on Euler characteristic of background space.

Symmetry reduces to Z4 in even dimensions, forbidding bilinear terms.

Constraints imply a minimum of eight Dirac or sixteen Majorana fermions for consistency.

Abstract

We show that massless Kaehler-Dirac (KD) fermions exhibit a mixed gravitational anomaly involving an exact $U(1)$ symmetry which is unique to KD fields. Under this $U(1)$ symmetry the partition function transforms by a phase depending only on the Euler character of the background space. Compactifying flat space to a sphere we learn that the anomaly vanishes in odd dimensions but breaks the symmetry down to $Z_4$ in even dimensions. This $Z_4$ is sufficient to prohibit bilinear terms from arising in the fermionic effective action. Four fermion terms are allowed but require multiples of two flavors of KD field. In four dimensional flat space each KD field can be decomposed into four Dirac spinors and hence these anomaly constraints ensure that eight Dirac fermions or, for real representations, sixteen Majorana fermions are needed for a consistent interacting theory. These constraints on fermion number agree with known results for topological insulators and recent work on discrete anomalies rooted in the Dai-Freed theorem. Our work suggests that KD fermions may offer an independent path to understanding these constraints. Finally we point out that this anomaly survives intact under discretization and hence is relevant in understanding recent numerical results on lattice models possessing massive symmetric phases.