High-accuracy calculation of the blackbody radiation shift in the 133Cs primary frequency standard.

The blackbody radiation (BBR) shift is an important systematic correction for the atomic frequency standards realizing the SI unit of time. Presently, there is controversy over the value of the BBR shift for the primary 133Cs standard. At room temperatures, the values from various groups differ at the 3x10(-15) level, while modern clocks are aiming at 10(-16) accuracies. We carry out high-precision relativistic many-body calculations of the BBR shift. For the BBR coefficient beta at T=300 K, we obtain beta=-(1.710+/-0.006)x10(-14), implying 6x10(-17) fractional uncertainty. While in accord with the most accurate measurement, our 0.35% accurate value is in a substantial (10%) disagreement with recent semiempirical calculations. We identify an oversight in those calculations.