Low-dimensional manifolds with non-negative curvature and maximal   symmetry rank
  We classify closed, simply connected $n$-manifolds of non-negative sectional curvature admitting an isometric torus action of maximal symmetry rank in dimensions $2\leq n\leq 6$. In dimensions $3k$, $k=1,2$ there is only one such manifold and it is diffeomorphic to the product of $k$ copies of the 3-sphere. 
Fernando Galaz-Garcia and Catherine Searle
math.DG