Frequency comparison between Cs and Mg standards and stability of fundamental constants

The frequency comparisons between Cs and Mg atomic standards performed during a period of 420 days are reported with an analysis of the effect of the temperature on the stability and the repeatability of the Mg output frequency. The Mg prototype shows a short-term stability /spl sigma//sub y/=1.2/spl middot/10/sup -11/ /spl tau//sup -1/2/ and a repeatability of 3/spl middot/10/sup -13/; its accuracy has been evaluated to be at the level of 1/spl middot/10/sup -12/. The results reported here for the frequency comparisons may well be interpreted from the point of view of the time stability of the quantity g/sub I/m/sub e//m/sub p/, with g/sub I/ the gyromagnetic ratio of Cs, obtaining the limit 2.6/spl middot//sup -13/ yr./sup -1/ for the relative rate of change. From this value one can deduce a limit for the relative rate of change of g/sub p/m/sub e//m/sub p/ and of /spl alpha/ too, The results here reported may also be interpreted as a "null" gravitational red-shift experiment allowing a new test of the Einstein equivalence principle.