Monopoles, Scattering, and Generalized Symmetries

Understand

We reconsider the problem of electrically charged, massless fermions scattering off magnetic monopoles.

• The interpretation of the outgoing states has long been a puzzle as, in certain circumstances, they necessarily carry fractional quantum numbers.
• We argue that consistency requires such outgoing particles to be attached to a topological co-dimension 1 surface, which ends on the monopole.
• This surface cannot participate in a 2-group with the magnetic 1-form symmetry and is often non-invertible.