Quantifying the Universality of the Stellar Initial Mass Function in Old
  Star Clusters
  We present a new technique to quantify cluster-to-cluster variations in the
observed present-day stellar mass functions of a large sample of star clusters.
Our method quantifies these differences as a function of both the stellar mass
and the total cluster mass, and offers the advantage that it is insensitive to
the precise functional form of the mass function. We applied our technique to
data taken from the ACS Survey for Globular Clusters, from which we obtained
completeness-corrected stellar mass functions in the mass range 0.25-0.75
M$_{\odot}$ for a sample of 27 clusters. The results of our observational
analysis were then compared to Monte Carlo simulations for globular cluster
evolution spanning a range of initial mass functions, total numbers of stars,
concentrations, and virial radii.
  We show that the present-day mass functions of the clusters in our sample can
be reproduced by assuming an universal initial mass function for all clusters,
and that the cluster-to-cluster differences are consistent with what is
expected from two-body relaxation. A more complete exploration of the initial
cluster conditions will be needed in future studies to better constrain the
precise functional form of the initial mass function. This study is a first
step toward using our technique to constrain the dynamical histories of a large
sample of old Galactic star clusters and, by extension, star formation in the
early Universe.
