Progress toward detection of ultralight dark matter with cryogenic optical cavities*

Virialized ultralight fields (VULFs), ϕ, form a category of dark matter (DM) candidates where their coupling (d_e and d_me) to baryonic matter is expected to result in the oscillation of fundamental constants, such as the fine-structure constant (δα/α ∝ d_eϕ) and electron mass (δm_e/m_e ∝ d_meϕ), at the DM Compton frequency (f_ϕ = mc²/(2πħ)). This implies that the Bohr radius (a₀ = ħ/(m_ecα)), and hence the size of atoms, would also oscillate at frequency f_ϕ. Here c & ħ are the speed of light & the reduced Planck's constant respectively.

In this talk, I will present progress toward experimental implementation of a VULF detection scheme, with f_ϕ ⊆ [100 Hz, 10 kHz], involving measurement of differential length change of 2 cryogenic optical cavities using a common laser. One of the cavities has its mirrors attached to a rigid spacer; the latter is expected to oscillate at f_ϕ. The second (non-rigid) cavity has its mirrors separated by vacuum. Moreover, the non-rigid cavity will be isolated, using a pendulum system, such that the effect on the cavity length due to oscillation of the suspension points (at f_ϕ) is suppressed by almost 10 orders of magnitude. This apparatus is projected to extend the exploration of d_e and d_me by 2 orders of magnitude compared to bounds set by other experiments.