Towards laser-cooling and trapping of SrOH to probe ultralight dark matter

If dark matter is an ultralight scalar particle, it can coherently oscillate at its Compton frequency and induce a corresponding oscillation in the value of the proton-to-electron mass ratio, $\mu$. Since molecular rotational and vibrational energies depend on $\mu$, precise measurements of rovibrational spectra over time would reveal this signature of dark matter as a time-dependent resonance frequency. We anticipate that precision microwave spectroscopy of a trapped sample of SrOH molecules will provide ~$10^{-17}$ fractional sensitivity to changes in $\mu$ due to an accidental near-degeneracy between vibrational states. We report on high-resolution vibrational branching ratio measurements enabling the design of a laser-cooling scheme with over 10,000 photon scatters before loss to an unaddressed vibrational state–more than enough to form a MOT and load an optical trap. Only 8 lasers are required for this scheme, fewer than for any other known polyatomic molecule cooling cycle. We describe progress towards laser cooling and trapping of SrOH, the first step in a high-sensitivity measurement of $\mu$ variation using ultracold SrOH molecules.