  As a young massive cluster in the Central Molecular Zone, the Arches cluster
is a valuable probe of the stellar Initial Mass Function (IMF) in the extreme
Galactic Center environment. We use multi-epoch Hubble Space Telescope
observations to obtain high-precision proper motion and photometric
measurements of the cluster, calculating cluster membership probabilities for
stars down to 1.8 M$_{\odot}$ between cluster radii of 0.25 pc -- 3.0 pc. We
achieve a cluster sample with just ~8% field contamination, a significant
improvement over photometrically-selected samples which are severely
compromised by the differential extinction across the field. Combining this
sample with K-band spectroscopy of 5 cluster members, we forward model the
Arches cluster to simultaneously constrain its IMF and other properties (such
as age and total mass) while accounting for observational uncertainties,
completeness, mass segregation, and stellar multiplicity. We find that the
Arches IMF is best described by a 1-segment power law that is significantly
top-heavy: $\alpha$ = 1.80 $\pm$ 0.05 (stat) $\pm$ 0.06 (sys), where dN/dm
$\propto$ m$^{-\alpha}$, though we cannot discount a 2-segment power law model
with a high-mass slope only slightly shallower than local star forming regions
($\alpha$ = 2.04$^{+0.14}_{-0.19}$ $\pm$ 0.04) but with a break at
5.8$^{+3.2}_{-1.2}$ $\pm$ 0.02 M$_{\odot}$. In either case, the Arches IMF is
significantly different than the standard IMF. Comparing the Arches to other
young massive clusters in the Milky Way, we find tentative evidence for a
systematically top-heavy IMF at the Galactic Center.
