Categorification of Donaldson-Thomas invariants by Perverse Sheaves

We show that there is a natural perverse sheaf on the moduli space of semistable sheaves on a smooth projective Calabi-Yau 3-fold which is locally the perverse sheaf of vanishing cycles for a local Chern-Simons functional. This gives us a cohomology theory whose Euler numbers are Donaldson-Thomas invariants.