SPUD: SPectroscopy for Ultralight Dark matter

One of the most important scientific goals of the next century 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 to produce a classical coherent field whose interactions with Standard Model particles manifest as fluctuations in the fundamental constants (FCs) [2]. Spectroscopy experiments are attractive platforms for observing such fluctuations because the structure of atoms is determined largely by the values of the fine structure constant and the mass of the electron. Molecular systems benefit further from additional dependence on nuclear masses, enabling characterization of DM candidates across multiple channels of interactions via measurement of a single observable. We have designed the experiment SPectroscopy for Ultralight Dark matter (SPUD) to improve measurements correlated to fundamental constants and enable the search for ultralight bosonic DM in the 10^-7 to 10^-4 eV range. We conduct precision measurements of logarithmic variations in the absorption spectrum to compute bounds on the strength of DM coupling to the Standard Model. These precision measurements are enabled by the addition of a filter cavity used to reduce fast laser frequency noise. This work expands upon previous similar experiments conducted with an Iodine (I₂) system [3].