Coplanarity In Twistor Space Of N=4 Next-To-MHV One-Loop Amplitude   Coefficients
  Next-to-MHV one-loop amplitudes in N=4 gauge theory can be written as a linear combination of known multivalued functions, called scalar box functions, with coefficients that are rational functions. We consider the localization of these coefficients in twistor space and prove that all of them are localized on a plane. The proof is done by studying the action of differential operators that test coplanarity on the unitarity cuts of the amplitudes. 
Ruth Britto, Freddy Cachazo, Bo Feng