Angular momentum content of a virtual graviton
  We show that a virtual graviton has a J=0 component, which serves to cancel the J=2, J_z=0 component when the graviton is on shell. In contrast, a massive graviton has no J=0 component either on or off shell. This difference is responsible for the van Dam-Veltman-Zakharov discontinuity. 
Duane Dicus (U. of Texas at Austin) and Scott Willenbrock (U. of   Illinois at Urbana-Champaign)