  We report the analysis of OGLE-2019-BLG-0960, which contains the smallest
mass-ratio microlensing planet found to date (q = 1.2--1.6 x 10^{-5} at
1-sigma). Although there is substantial uncertainty in the satellite parallax
measured by Spitzer, the measurement of the annual parallax effect combined
with the finite source effect allows us to determine the mass of the host star
(M_L = 0.3--0.6 M_Sun), the mass of its planet (m_p = 1.4--3.1 M_Earth), the
projected separation between the host and planet (a_perp = 1.2--2.3 au), and
the distance to the lens system (D_L = 0.6--1.2 kpc). The lens is plausibly the
blend, which could be checked with adaptive optics observations. As the
smallest planet clearly below the break in the mass-ratio function (Suzuki et
al. 2016; Jung et al. 2019), it demonstrates that current experiments are
powerful enough to robustly measure the slope of the mass-ratio function below
that break. We find that the cross-section for detecting small planets is
maximized for planets with separations just outside of the boundary for
resonant caustics and that sensitivity to such planets can be maximized by
intensively monitoring events whenever they are magnified by a factor A > 5.
Finally, an empirical investigation demonstrates that most planets showing a
degeneracy between (s > 1) and (s < 1) solutions are not in the regime (|log s|
>> 0) for which the "close"/"wide" degeneracy was derived. This investigation
suggests a link between the "close"/"wide" and "inner/outer" degeneracies and
also that the symmetry in the lens equation goes much deeper than symmetries
uncovered for the limiting cases.

Earth and Planetary Astrophysics;Astrophysics of Galaxies;Solar and Stellar Astrophysics