Stability of the Proton-to-Electron Mass Ratio

We report a limit on the fractional temporal variation of the proton-to-electron mass ratio as ${1}{({m}P/{m}ₑ)}{{∂}}{{∂}t}({m}P/{m}ₑ)=({-}3.8±{}5.6)×{}{10}^{{-}14} {yr}^{{-}1}$, obtained by comparing the frequency of a rovibrational transition in ${SF}₆$ with the fundamental hyperfine transition in Cs. The ${SF}₆$ transition was accessed using a ${CO}₂$ laser to interrogate spatial 2-photon Ramsey fringes. The atomic transition was accessed using a primary standard controlled with a Cs fountain. This result is direct and model-free.