We analyze OGLE-2007-BLG-050, a high magnification microlensing event (A
~ 432) whose peak occurred on 2 May, 2007, with pronounced finite-source
and parallax effects. We compute planet detection efficiencies for this
event in order to determine its sensitivity to the presence of planets
around the lens star. Both finite-source and parallax effects permit a
measurement of the angular Einstein radius \theta_E = 0.48 +/- 0.01 mas
and the parallax \pi_E = 0.12 +/- 0.03, leading to an estimate of the
lens mass M = 0.50 +/- 0.14 M_Sun and its distance to the observer D_L =
5.5 +/- 0.4 kpc. This is only the second determination of a reasonably
precise (<30%) mass estimate for an isolated unseen object, using any
method. This allows us to calculate the planetary detection efficiency
in physical units (r_\perp, m_p), where r_\perp is the projected
planet-star separation and m_p is the planet mass. When computing planet
detection efficiency, we did not find any planetary signature and our
detection efficiency results reveal significant sensitivity to
Neptune-mass planets, and to a lesser extent Earth-mass planets in some
configurations. Indeed, Jupiter and Neptune-mass planets are excluded
with a high confidence for a large projected separation range between
the planet and the lens star, respectively [0.6 - 10] and [1.4 - 4] AU,
and Earth-mass planets are excluded with a 10% confidence in the lensing
zone, i.e. [1.8 - 3.1] AU.
