Measurement of the blackbody radiation shift of the 133Cs hyperfine transition in an atomic fountain

We used a Cs fountain to measure the Stark shift of the ground-state hyperfine transition frequency in cesium (9.2 GHz) due to the electric field of the blackbody radiation. The relative shift at 300 K deduced from our measurements, including the leading and the second-order term in temperature, is (−1.45±0.09)×10−14 and agrees with our recent theoretical evaluation (−1.51±0.07)×10−14 [Micalizio Phys. Rev. A 69, 053401 (2004)]. These values differ from that currently used (−1.735±0.003)×10−14, with significant implications on frequency standards accuracy, on clocks comparison and on a variety of high-precision physics tests, such as the time stability of fundamental constants.