Ultra-precise mid-infrared molecular spectroscopy for testing the stability of µ, the electron-to-proton mass ratio – Ultimos

Molecules are sensitive probes of the stability of the electron-to-proton mass ratio µ, in particular when line centres in spectra of cosmic objects are compared to corresponding laboratory values. This approach, which relies on the observations of transitions in the micro- and millimetre wave (MW/MMW) regions, provides today the most stringent constraints on the time and spatial variations of µ, but is limited by the accuracy of available laboratory data.

The Ultiµos project will provide a solution to this using an innovative approach for an order of magnitude improvement of the laboratory spectroscopic data. We will use suitable combinations of molecular transitions in the mid-infrared (MIR) that allow to determine the required transition in the MW/MMW region. The control of the frequency of MIR quantum cascade lasers reaches today sub-Hz levels both in terms of accuracy and stability. These performances are obtained at 10 µm in our laboratories by comparison with a frequency comb referenced to the International System of Units (SI) thanks to the REFIMEVE network (distribution of an optical frequency referenced to the SI by LNE-SYRTE, the national institute of time-frequency metrology). We aim at significantly extending the range of accessible wavelengths around 6 µm, 9 µm and 13 µm. This provides a unique platform of complementary and SI-traceable spectrometers, capable of probing MIR absorbing molecules with a record resolution, allowing us to improve on MW/MMW transitions currently used for testing the stability of µ. We will in particular exploit the very high sensitivity of MIR transition frequency combinations in methanol and ammonia to variations of µ predicted by models seeking new physics beyond the standard model. Ultiµos will thus refine constraints on a drifting-µ and the proposed spectroscopic investigations will help identifying new molecules and transitions for future Earth/space comparison campaigns in search for varying fundamental constants.