A null test of General Relativity: Long term comparison of H-masers and Cs-fountains

We compare the long-term fractional frequency variation of four hydrogen masers that are part of an ensemble of clocks comprising the National Institute of Standards and Technology,(NIST), Boulder, timescale with the fractional frequencies of primary frequency standards operated by leading metrology laboratories in the United States, France, Germany, Italy and the United Kingdom for a period extending more than 14 years. The measure of the assumed variation of non-gravitational interaction,(LPI parameter, $\beta$)---within the atoms of H and Cs---over time as the earth orbits the sun, has been constrained to $\beta=(2.2 \pm 2.5)\times 10^{-7}$, a factor of two improvement over previous estimates. Using our results together with the previous best estimates of $\beta$ based on Rb vs. Cs, and Rb vs. H comparisons, we impose the most stringent limits to date on the dimensionless coupling constants that relate the variation of fundamental constants such as the fine-structure constant and the scaled quark mass with strong(QCD) interaction to the variation in the local gravitational potential. For any metric theory of gravity $\beta=0$.