Enhanced sensitivity to ultralight bosonic dark matter in the spectra of the linear radical SrOH

Coupling between Standard Model particles and theoretically well-motivated ultralight dark matter (UDM) candidates can lead to time variation of fundamental constants including the proton-to-electron mass ratio $\mu\equiv m_{p}/m_{e}\approx1836$. The presence nearly-degenerate vibrational energy levels different character in polyatomic molecules result significantly enhanced relative shifts molecular spectra originating from $\partial_{t}\mu$, relaxing experimental complexity required for high-sensitivity measurements. We analyze amplification UDM effects spectrum laser-cooled strontium monohydroxide (SrOH). SrOH is first and, so far, only molecule be directly laser cooled sub-millikelvin temperatures, opening possibility long coherence times. Because high enhancement factors ($\left|Q_{\mu}\right|\approx10^{3}$), measurements $\tilde{X}\left(200\right)\leftrightarrow\tilde{X}\left(03^{1}0\right)$ rovibrational transitions microwave regime $\sim10^{-17}$ fractional uncertainty $\delta\mu/\mu$ with one day integration. Furthermore, ultracold provides a promising platform suppressing systematic errors. More complex SrOR radicals additional modes arising larger ligands R could even greater factors.