Conformal geometry of null hexagons for Wilson loops and scattering   amplitudes
  The cross-ratios do not uniquely fix the class of conformally equivalent configurations of null polygons. In view of applications to Wilson loops and scattering amplitudes we characterise all conformal classes of null hexagon configurations belonging to given points in cross-ratio space. At first this is done for the ordered set of vertices. Including the edges, we then investigate the equivalence classes under conformal transformations for null hexagons. This is done both for the set of null hexagons closed in finite domains of Minkowski space as well as for the set including those closed via infinity. 
Harald Dorn, Hagen M\"unkler and Christian Spielvogel
hep-th hep-ph