Global 3-group symmetry and t Hooft anomalies in axion electrodynamics

We investigate a higher-group structure of massless axion electrodynamics in $(3+1)$ dimensions. By using the background gauging method, we show that the higher-form symmetries necessarily have a global semistrict 3-group (2-crossed module) structure, and exhibit t Hooft anomalies of the 3-group. In particular, we find a cubic mixed t Hooft anomaly between 0-form and 1-form symmetries, which is specific to the higher-group structure.