Witten-Veneziano from Green-Schwarz
  We consider the U(1) problem within the AdS/CFT framework. We explain how the Witten-Veneziano formula for the eta' mass is related to a generalized Green-Schwarz mechanism. The closed string mode, that cancels the anomaly of the gauged U(1) axial symmetry, is identified with the eta' meson. In a particular set-up of D3-branes on a C3/(Z3xZ3) orbifold singularity, the eta' meson is a twisted-sector R-R field. 
Adi Armoni (CERN)