First Microlens Mass Measurement: PLANET Photometry of EROS BLG-2000-5

We analyze PLANET photometric observations of the caustic-crossing binary lens microlensing event, EROS BLG-2000-5, and find that modeling the observed light curve requires incorporation of the microlens parallax and the binary orbital motion. The projected Einstein radius ((r) over tilde (E) = 3.61+/-0.11 AU) is derived from the measurement of the microlens parallax, and we are also able to infer the angular Einstein radius (theta(E) = 1.38+/-0.12 mas) from the finite source effect on the light curve, combined with an estimate of the angular size of the source given by the source position in a color-magnitude diagram. The lens mass, M = 0.612+/-0.57 M-circle dot is found by combining these two quantities. This is the fit time that parallax effects are detected for a caustic-crossing event and also the first time that the lens mass degeneracy has been completely broken through photometric monitoring alone. The combination of (r) over tilde (E) and theta(E) also allows us to conclude that the lens lies in the near side of the disk, within 2.6 kpc of the Sun, while the radial velocity measurement indicates that the source is a Galactic bulge giant.