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 σ_y=1.2·10^-11 τ^-1/2 and a repeatability of 3·10^-13; its accuracy has been evaluated to be at the level of 1·10^-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_Im _e/m_p, with g_I the gyromagnetic ratio of Cs, obtaining the limit 2.6·^-13 yr.^-1 for the relative rate of change. From this value one can deduce a limit for the relative rate of change of g_pm_e/m_p and of α 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