Collisional frequency shifts in 133Cs fountain clocks.

We present a theoretical analysis of the density dependent frequency shift in Cs fountain clocks using the highly constrained binary collision model described by Leo et al. [Phys. Rev. Lett. 85, 2721 (2000)]. We predict a reversal in the clock shift at temperatures near 0.08 microK. Our results show that s waves dominate the collision process. However, as a consequence of the large scattering lengths in Cs the clock shift is strongly temperature dependent and does not reach a constant Wigner-law value until temperatures are less than 0.1 nK.