  In the hierarchical structure formation model of the universe, galaxy
clusters are assembled through a series of mergers. Accordingly, it is expected
that galaxy clusters in the early universe are actively forming and dynamically
young. Located at a high redshift of z=1.71, SpARCS1049+56 offers a unique look
into the galaxy cluster formation process. This cluster has been shown to be
rich in cluster galaxies and to have intense star formation. Its high redshift
pushes a weak-lensing analysis beyond the regime of the optical spectrum into
that of the infrared. Equipped with deep Hubble Space Telescope Wide Field
Camera 3 UVIS and IR observations, we present a weak-lensing characterization
of SpARCS1049+56. As few IR weak-lensing studies have been performed, we
discuss the details of PSF modeling and galaxy shape measurement for an IR
weak-lensing procedure and the systematics that come with the territory. It
will be critical to understand these systematics in future weak-lensing studies
in the IR with the next generation space telescopes such as JWST, Euclid, and
WFIRST. Through a careful analysis, the mass distribution of this young galaxy
cluster is mapped and the convergence peak is detected at a 3.3 sigma level.
The weak-lensing mass of the cluster is estimated to be
$3.5\pm1.2\times10^{14}\ \text{M}_\odot$ and is consistent with the mass
derived from a mass-richness scaling relation. This mass is extreme for a
cluster at such a high redshift and suggests that SpARCS1049+56 is rare in the
standard $\Lambda$CDM universe.
