In the monopole background, multi-fermion operators exhibit condensation. The phases of the operators are "axions" in the sense that they are topologically coupled to the electromagnetic field. We propose that an ``excited'' closed axion string is the fermion itself. We use this picture to solve a long-standing puzzle in the scattering process of a monopole and a massless fermion. It has been...
Axion models are phenomenologically relevant QFTs that exhibit higher group global symmetries. Higher groups describe the case when different types of higher form symmetries combine together in a non-trivial structure. In this talk, we will discuss how the emergence of these higher groups can be used to infer non-trivial constraints on the RG flows of theories that flow to axion-Yang-Mills in the IR.