  We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb.
The planet was detected in a high-magnification microlensing event (with peak
magnification A_max = 628) in which the primary lens transited the source,
resulting in a dramatic smoothing of the peak of the event. The angular extent
of the region of perturbation due to the planet is significantly smaller than
the angular size of the source, and as a result the planetary signature is also
smoothed out by the finite source size. Thus the deviation from a single-lens
fit is broad and relatively weak (~ few percent). Nevertheless, we demonstrate
that the planetary nature of the deviation can be unambiguously ascertained
from the gross features of the residuals, and detailed analysis yields a fairly
precise planet/star mass ratio of q = 0.0026+/-0.0004, in accord with the large
significance (\Delta\chi^2=1070) of the detection. The planet/star projected
separation is subject to a strong close/wide degeneracy, leading to two
indistinguishable solutions that differ in separation by a factor of ~8.5.
Upper limits on flux from the lens constrain its mass to be M < 0.75 M_Sun
(assuming it is a main-sequence star). A Bayesian analysis that includes all
available observational constraints indicates a primary in the Galactic bulge
with a mass of ~0.2-0.5 M_Sun and thus a planet mass of ~ 0.5-1.3 M_Jupiter.
The separation and equilibrium temperature are ~0.6-1.1AU (~5.3-9.7AU) and
~103K (~34K) for the close (wide) solution. If the primary is a main-sequence
star, follow-up observations would enable the detection of its light and so a
measurement of its mass and distance.
