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We present Keck/NIRC2 adaptive optics imaging of planetary microlensing eventMOA-2007-BLG-400 that resolves the lens star system from the source. We findthat the MOA-2007-BLG-400L planetary system consists of a $1.71\pm 0.27 M_{\rmJup}$ planet orbiting a $0.69\pm 0.04M_{\odot}$ K-dwarf host star at a distanceof $6.89\pm 0.77\,$kpc from the Sun. So, this planetary system probably residesin the Galactic bulge. The planet-host star projected separation is only weaklyconstrained due to the close-wide light curve degeneracy; the 2$\sigma$projected separation range is 0.6--$7.2\,$AU. This host mass is at the top endof the range of masses predicted by a standard Bayesian analysis that assumesthat all stars have an equal chance of hosting a star of the observed massratio. This and the similar result for event MOA-2013-BLG-220 suggests thatmore massive stars may be more likely to host planets with a mass ratio in the$0.002 < q < 0.004$ range that orbit beyond the snow line. These results alsoindicate the importance of host star mass measurements for exoplanets found bymicrolensing. The microlensing survey imaging data from NASA's Nancy GraceRoman Space Telescope (formerly WFIRST) mission will be doing mass measurementslike this for a huge number of planetary events. This host lens is the highestcontrast lens-source detected in microlensing mass measurement analysis (thelens being 10$\times$ fainter than the source). We present an improved methodof calculating photometry and astrometry uncertainties based on the Jackknifemethod, which produces more accurate errors that are $\sim$$2.5 \times$ largerthan previous estimates.

MOA-2007-BLG-400 A Super-Jupiter Mass Planet Orbiting a Galactic BulgeK-dwarf Revealed by Keck Adaptive Optics Imaging