Anomalies of non-Abelian finite groups via cobordism

Abstract

We use cobordism theory to analyse anomalies of finite non-abelian symmetries in 4 spacetime dimensions. By applying the method of `anomaly interplay', which uses functoriality of cobordism and naturality of the $\eta$-invariant to relate anomalies in a group of interest to anomalies in other (finite or compact Lie) groups, we derive the anomaly for every representation in many examples motivated by flavour physics, including $S_3$, $A_4$, $Q_8$, and $\mathrm{SL}(2,\mathbb{F}_3)$. In the case of finite abelian groups, it is well known that anomalies can be `truncated' in a way that has no effect on low-energy physics, by means of a group extension. We extend this idea to non-abelian symmetries. We show, for example, that a system with $A_4$ symmetry can be rendered anomaly-free, with only one-third as many fermions as na\"ively required, by passing to a larger symmetry. As another example, we find that a well-known model of quark and lepton masses utilising the $\mathrm{SL}(2,\mathbb{F}_3)$ symmetry is anomalous, but that the anomaly can be cancelled by enlarging the symmetry to a $\mathbb{Z}/3$ extension of $\mathrm{SL}(2,\mathbb{F}_3)$.