s-wave elastic collisions between cold ground-state ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ atoms

We have measured the elastic-scattering cross section of ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ atoms in the ‖F=1,${\mathit{m}}_{\mathit{F}}$=-1〉 ground state at 25 μK. The cross section is almost purely s wave at these temperatures and has a value of (5.4±1.3)×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^2$. We have searched for the predicted Feshbach-type resonances in the elastic cross section [Tiesinga et al., Phys. Rev. A 46, 1167 (1992)] as a function of magnetic field. There are no resonances with a magnetic-field width ≥2 G over a magnetic-field range of 15–540 G.