We resolve the inner region of a massive cluster forming in a cosmological \Lambda CDM simulation with a mass resolution of 2 \times 10 ^ { 6 } { M _ { \odot } } and before z=4.4 even 3 \times 10 ^ { 5 } { M _ { \odot } } .
This is a billion times less than the clusters final virial mass and a substantial increase over current \Lambda CDM simulations .
We achieve this resolution using a new multi-mass refinement procedure and are now able to probe a dark matter halo density profile down to 0.1 percent of the virial radius .
The inner density profile of this cluster halo is well fitted by a power-law \rho \propto r ^ { - \gamma } down to the smallest resolved scale .
An inner region with roughly constant logarithmic slope is now resolved , which suggests that cuspy profiles describe the inner profile better than recently proposed profiles with a core .
The cluster studied here is one out of a sample of six high resolution cluster simulations of Diemand et al .
( 12 ) and its inner slope of about \gamma = 1.2 lies close to the sample average .