  We present the analysis of the simultaneous high resolution images from the
{\it Hubble Space Telescope} and Keck Adaptive Optics system of the planetary
event OGLE-2012-BLG-0950 that determine that the system consists of a $0.58 \pm
0.04 \rm{M}_\odot$ host star orbited by a $39\pm 8 \rm{M}_\oplus$ planet of at
projected separation of $2.54 \pm 0.23\,$AU. The planetary system is located at
a distance of $2.19\pm 0.23$ kpc from Earth. This is the second microlens
planet beyond the snow line with a mass measured to be in the mass range
$20$--$80 \rm{M}_\oplus$. The runaway gas accretion process of the core
accretion model predicts few planets in this mass range, because giant planets
are thought to be growing rapidly at these masses and they rarely complete
growth at this mass. So, this result suggests that the core accretion theory
may need revision. This analysis also demonstrates the techniques that will be
used to measure the masses of planets and their host stars by the WFIRST
exoplanet microlensing survey: one-dimensional microlensing parallax combined
with the separation and brightness measurement of the unresolved source and
host stars to yield multiple redundant constraints on the masses and distance
of the planetary system.
