Measurement of the zero crossing in a Feshbach resonance of fermionic [Formula Presented]

We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of [Formula Presented] To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a [Formula Presented] laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for [Formula Presented] consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at [Formula Presented] in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT. 5555 2002 The American Physical Society.