We show that massless Kähler-Dirac fermions exhibit a mixed gravitational anomaly involving an exact U(1) symmetry which is unique to Kähler-Dirac 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 Z4 in even dimensions. This Z4 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 Kähler-Dirac field. In four dimensional flat space each Kähler-Dirac 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 Kähler-Dirac 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.
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)