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.