  We perform a density-matrix renormalization group (DMRG) study of the S=1/2
Heisenberg antiferromagnet on the kagome lattice to identify the conjectured
spin liquid ground state. Exploiting SU(2) spin symmetry, which allows us to
keep up to 16,000 DMRG states, we consider cylinders with circumferences up to
17 lattice spacings and find a spin liquid ground state with an estimated per
site energy of -0.4386(5), a spin gap of 0.13(1), very short-range decay in
spin, dimer and chiral correlation functions and finite topological
entanglement gamma consistent with gamma=ld 2, ruling out gapless, chiral or
non-topological spin liquids. At the same time, DMRG results provide strong
evidence for a gapped topological Z2 spin liquid.
