Comparing Observed Stellar Kinematics and Surface Densities in a Low
  Latitude Bulge Field to Galactic Population Synthesis Models
  We present an analysis of Galactic bulge stars from Hubble Space Telescope
(HST) Wide Field Camera 3 (WFC3) observations of the Stanek window
(l,b=[0.25,-2.15]) from two epochs approximately two years apart. This dataset
is adjacent to the provisional Wide-field Infrared Survey Telescope (WFIRST)
microlensing field. Proper motions are measured for approximately 115,000 stars
down to 28th mag in V band and 25th mag in I band, with accuracies of 0.5 mas
yr$^{-1}$ (20 km s$^{-1}$) at I $\approx$ 21. A cut on the longitudinal proper
motion $\mu_l$ allows us to separate disk and bulge populations and produce
bulge-only star counts that are corrected for photometric completeness and
efficiency of the proper-motion cut. The kinematic dispersions and surface
density in the field are compared to the nearby SWEEPS sight-line, finding a
marginally larger than expected gradient in stellar density. The observed bulge
star counts and kinematics are further compared to the Besan\c{c}on, Galaxia,
and GalMod Galactic population synthesis models. We find that most of the
models underpredict low-mass bulge stars by $\sim$33% below the main-sequence
turnoff, and upwards of $\sim$70% at redder J and H wavebands. While
considering inaccuracies in the Galactic models, we give implications for the
exoplanet yield from the WFIRST microlensing mission.
