Threshold and resonance phenomena in ultracold ground-state collisions

We study the magnetic-field dependence of the cross sections for elastic and inelastic collisions of pairs of ultracold cesium atoms in a magnetic trap, calculated with the coupled-channels method. We pay special attention to atoms in the f=3, ${\mathit{m}}_{\mathit{f}}$=-3 weak-field seeking state of the lower hyperfine manifold. The cross sections show a pronounced resonance structure. We discuss its origin, starting from the pure bound singlet and triplet rovibrational ${\mathrm{Cs}}_2$ states and introducing perturbations due to the hyperfine and Zeeman interactions. We also discuss the role of the centrifugal barrier in the final collision channel in reducing the loss of atoms from the trap due to transitions induced by the magnetic dipole-dipole interaction.