Magnetic Scattering: Pairwise Little Group and Pairwise Helicity

This lecture delves into the properties of magnetic scattering, focusing on the pairwise little group and pairwise helicity. It covers the construction of the magnetic S-matrix for 2→2 electric-magnetic scattering, helicity flip, and monopole harmonics. The instructor discusses the multi-particle representations of the Poincare group, emphasizing the importance of pairwise spinor-helicity variables. The lecture explores the implications of the pairwise little group on angular momentum and the S-matrix transformation. It concludes with a detailed analysis of the general 3-point S-matrix and the partial wave expansion for magnetic cases, including the massless limit and higher partial waves.