Feshbach resonances in cesium at ultralow static magnetic fields

We have observed Feshbach resonances for Cs-133 atoms in two different hyperfine states at static magnetic fields of a few milligauss. These resonances are unusual for two main reasons. First, they are the lowest static-field resonances investigated up to now, and we explain their multipeak structure in these ultralow fields. Second, they are robust with respect to temperature effects. We have measured them using an atomic fountain clock and reproduced them using coupled-channels calculations, which are in excellent agreement with our measurements. We show that these are s-wave resonances due to a very weakly bound state of the triplet molecular Hamiltonian. We also describe a model explaining their asymmetric shape in the regime where the kinetic energy dominates over the coupling strength.