SPUD: SPectroscopy for Ultralight Dark matter

One of the most important scientific goals of the next decade is to uncover the nature of Dark Matter (DM). Several DM models characterized by a low mass (well below 1 eV) scalar field are thought to be detectable by table-top atomic, molecular, and optical experiments [1]. These candidate DM particles carry a mass small enough such that they behave as a classical coherent field whose interactions with Standard Model particles manifest as fluctuations in the fundamental constants of nature [2]. Spectroscopy experiments are attractive platforms for observing such fluctuations, with the structure of atoms determined largely by the values of the fine structure constant and mass of the electron, with molecular systems benefiting further from additional dependence on nuclear masses, enabling characterization of these DM candidates across multiple channels of interactions via measurement of a single observable. We have designed the experiment SPectroscopy for Ultralight Dark matter (SPUD) that aims to impove the limits on variations of the fundamental constants due to DM as revealed by the variations in the spectra of molecules, allowing for the search of ultralight bosonic DM in the 10^-7 to 10^-4 eV range. We conduct precision measurement of logarithmic variations in the absorption spectrum to compute bounds on the strength of DM coupling to the Standard Model. This work expands upon previous similar experiments conducted with an Iodine (I₂) system by searching at higher mass ranges [3].