2011ApJ...728L..12K

We report deep Green Bank Telescope spectroscopy in the redshifted NH₃ (1,1), CS 1-0, and H₂ CO 0₀₀-1₀₁lines from the z ∼ 0.685 absorber toward B0218+357. The inversion (NH₃) and rotational (CS, H₂CO) line frequencies have different dependences on the proton-electron mass ratio µ, implying that a comparison between the line redshifts is sensitive to changes in µ. A joint three-component fit to the NH₃, CS, and H₂ CO lines yields [Δµ/µ] = (-3.5±1.2)x10^–7, from z ∼ 0.685 to today, where the error includes systematic effects from comparing lines from different species and possible frequency-dependent source morphology. Two additional sources of systematic error remain, due to time variability in the source morphology and velocity offsets between nitrogen-bearing and carbon-bearing species. We find no statistically significant ( ≥ 3σ) evidence for changes in µ and obtain the stringent 3σ constraint, [Δµ/µ] < 3.6x10^–7, over 6.2 Gyr; this is the best present limit on temporal changes in µ from any technique, and for any look-back time, by a factor ≳ 5.