We present the analysis of the microlensing event OGLE-2015-BLG-1670 , detected in a high-extinction field , very close to the Galactic plane .
Due to the dust extinction along the line of sight , this event was too faint to be detected before it reached the peak of magnification .
The microlensing light-curve models indicate a high-magnification event with a maximum of A _ { \mathrm { max } } \gtrsim 200 , very sensitive to planetary deviations .
An anomaly in the light curve has been densely observed by the microlensing surveys MOA , KMTNet , and OGLE .
From the light-curve modeling , we find a planetary anomaly characterized by a planet-to-host mass ratio , q = \left ( 1.00 ^ { +0.18 } _ { -0.16 } \right ) \times 10 ^ { -4 } , at the peak recently identified in the mass-ratio function of microlensing planets .
Thus , this event is interesting to include in future statistical studies about planet demography .
We have explored the possible degeneracies and find two competing planetary models resulting from the s \leftrightarrow 1 / s degeneracy .
However , because the projected separation is very close to s = 1 , the physical implications for the planet for the two solutions are quite similar , except for the value of s .
By combining the light-curve parameters with a Galactic model , we have estimated the planet mass M _ { 2 } = 17.9 ^ { +9.6 } _ { -8.8 } M _ { \oplus } and the lens distance D _ { \mathrm { L } } = 6.7 ^ { +1.0 } _ { -1.3 } \mathrm { kpc } , corresponding to a Neptune-mass planet close to the Galactic bulge .
Such events with a low absolute latitude ( |b| \approx 1.1 \mathrm { deg } ) are subject to both high extinction and more uncertain source distances , two factors that may affect the mass measurements in the provisional Wide Field Infrared Survey Telescope fields .
More events are needed to investigate the potential trade-off between the higher lensing rate and the difficulty in measuring masses in these low-latitude fields .