Torsion groups of subexponential growth cannot act on finite-dimensional CAT(0)-spaces without a fixed point

We show that finitely generated torsion groups of subexponential growth must have a global fixed point whenever they act by isometry on a finite dimensional complete CAT(0)-space. The same statement for simple groups of subexponential growth is also shown. The method of proof uses ultralimits, equivariant harmonic maps, subharmonic functions, horofunctions and random walks.