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, obtained by comparing the frequency of a rovibrational transition in SF6 with the fundamental hyperfine transition in Cs. The SF6 transition was accessed using a CO2 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. 
Alexander Shelkovnikov (LPL), Robert James Butcher (LPL), Christian   Chardonnet (LPL), Anne Amy-Klein (LPL)