Magnetic scattering: pairwise little group and pairwise helicity

<!--HTML--><p>I discuss how to construct a Lorentz-invariant S-matrix for the&nbsp;scattering&nbsp;of electrically and magnetically charged particles. A key ingredient is a revision of our fundamental understanding of multi-particle representations of the Poincaré&nbsp;group. Surprisingly, the asymptotic states for electric-magnetic&nbsp;scattering&nbsp;transform with an additional&nbsp;little&nbsp;group&nbsp;phase, associated with pairs of electrically and magnetically charged particles. I will discuss the general construction of such states. The resulting "pairwise&nbsp;helicity" is identified with the quantized "cross product" of charges&nbsp;e1 g2- e2 g1 for&nbsp; every charge-monopole&nbsp;pair, and represents the extra angular momentum stored in the asymptotic electromagnetic field. We define a new kind ofpairwise&nbsp;spinor-helicity&nbsp;variable, which serves as an additional building block for electric-magnetic&nbsp;scattering&nbsp;amplitudes. We then construct the most general 3-point S-matrix elements, as well as the full partial wave decomposition for the 2 -&gt; 2 fermion-monopole&nbsp;S-matrix. In particular, we derive the famous&nbsp;helicity&nbsp;flip in the lowest partial wave as a simple consequence of a generalized spin-helicity&nbsp;selection rule, as well as the full angular dependence for the higher partial waves. We will also discuss a possible direction for resolving Callan's ``semiton" problem: the&nbsp;scattering&nbsp;amplitude of a positron on a GUT&nbsp;monopole&nbsp;which apparently does not have an allowed final state. We will show that using entagled pairwise helicity spinors a simple possible s-wave final state does exist.</p> TH colloquia: https://cern.zoom.us/j/67346292748?pwd=ZnRkQWh0ZXVFQTc5K256QW5NcEcrdz09