Precision determination of the mass function of dark matter halos

The predicted mass function of dark matter halos is essential in connecting observed galaxy-cluster counts and models of galaxy clustering to the properties of the primordial density field. We determine the mass function in the concordance Lambda CDM cosmology, as well as its uncertainty, using sixteen 1024(3) particle nested-volume dark matter simulations spanning a mass range of over 5 orders of magnitude. Using the nested volumes and single-halo tests, we find and correct for a systematic error in the friends-of-friends halo-finding algorithm. We find a fitting form and full error covariance for the mass function that successfully describes the simulations' mass function and is well behaved outside the simulations' resolutions. Estimated forecasts of uncertainty in cosmological parameters from future cluster-count surveys receive a negligible contribution from remaining statistical uncertainties in the central cosmology multiplicity function. There exists a potentially nonnegligible cosmological dependence (nonuniversality) of the halo multiplicity function.