Speaker
Description
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 known that when a charged fermion scatters off a monopole, the fermion in the s-wave component must flip its chirality, i.e., fermion number violation must happen. This fact has led to a puzzle; if there are two or more flavors of massless fermions, any superposition of the fermion states cannot be the final state of the s-wave scattering as it is forbidden by conservation of the electric and flavor charges. The unitary evolution of the state vector, on the other hand, requires some interpretation of the final states. We show that the final state should be an axion string that is interpreted as a new particle excitation of the theory.
