MOA-2010-BLG-311: A Planetary Candidate below the Threshold of Reliable Detection

We analyze MOA-2010-BLG-311, a high magnification (A _max > 600) microlensing event with complete data coverage over the peak, making it very sensitive to planetary signals. We fit this event with both a point lens and a two-body lens model and find that the two-body lens model is a better fit but with only Δχ² ~ 80. The preferred mass ratio between the lens star and its companion is q = 10^{-3.7 ± 0.1}, placing the candidate companion in the planetary regime. Despite the formal significance of the planet, we show that because of systematics in the data the evidence for a planetary companion to the lens is too tenuous to claim a secure detection. When combined with analyses of other high-magnification events, this event helps empirically define the threshold for reliable planet detection in high-magnification events, which remains an open question.