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SUMMARY:Magnetic scattering: pairwise little group and pairwise helicity -
  Csaba Csaki\, Cornell
DTSTART:20211028T150000Z
DTEND:20211028T160000Z
UID:TALK163246@talks.cam.ac.uk
CONTACT:85816
DESCRIPTION: I discuss how to construct a Lorentz-invariant S-matrix for t
 he scattering of electrically and magnetically charged particles. A key in
 gredient is a revision of our fundamental understanding of multi-particle 
 representations of the Poincaré group. Surprisingly\, the asymptotic stat
 es for electric-magnetic scattering transform with an additional little gr
 oup phase\, associated with pairs of electrically and magnetically charged
  particles. I will discuss the general construction of such states. The re
 sulting "pairwise helicity" is identified with the quantized "cross produc
 t" of charges e1 g2- e2 g1 for  every charge-monopole pair\, and represent
 s the extra angular momentum stored in the asymptotic electromagnetic fiel
 d. We define a new kind of pairwise spinor-helicity variable\, which serve
 s as an additional building block for electric-magnetic scattering amplitu
 des. We then construct the most general 3-point S-matrix elements\, as wel
 l as the full partial wave decomposition for the 2 -> 2 fermion-monopole S
 -matrix. In particular\, we derive the famous helicity flip in the lowest 
 partial wave as a simple consequence of a generalized spin-helicity select
 ion rule\, as well as the full angular dependence for the higher partial w
 aves. We will also discuss a possible direction for resolving Callan's ``s
 emiton" problem: the scattering amplitude of a positron on a GUT monopole 
 which apparently does not have an allowed final state. We will show that u
 sing entagled pairwise helicity spinors a simple possible s-wave final sta
 te does exist.
LOCATION:Online (Zoom)
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