Improved Limit on a Temporal Variation of mp/me from Comparisons of Yb+ and Cs Atomic Clocks

Accurate measurements of different transition frequencies between atomic levels of the electronic and hyperfine structure over time are used to investigate temporal variations of the fine structure constant a and the proton-to-electron mass ratio mu. We measure the frequency of the S-2(1/2) -> F-2(7/2) electric octupole (E3) transition in Yb-171(+) against two caesium fountain clocks as f(E3) = 642 121 496 772 645.36 Hz with an improved fractional uncertainty of 3.9 x 10(-16). This transition frequency shows a strong sensitivity to changes of alpha. Together with a number of previous and recent measurements of the S-2(1/2) -> D-2(3/2) electric quadrupole transition in Yb-171(+) and with data from other elements, a least-squares analysis yields (1/alpha)(d alpha/dt) = -0.20(20) x 10(-16)/yr and (1/mu)(d mu/dt) = -0.5(1.6) x 10(-16)/yr, confirming a previous limit on d alpha/dt and providing the most stringent limit on d mu/dt from laboratory experiments.