Searching for DArk Matter with a Non-Equal Delay inteferometer: The DAMNED experiment.

We report an experiment that compares the frequency of a clock (an ultra-stable optical cavity in this case) at time $t$ to its own frequency some time $t-T$ earlier, by "storing" the output signal (photons) in a fibre delay line. In ultra-light oscillating dark matter (DM) models such an experiment is sensitive to coupling of DM to the standard model fields, through oscillations of the cavity and fibre lengths and of the fibre refractive index. Additionally, the sensitivity is significantly enhanced around the mechanical resonances of the cavity. We report no evidence of DM for masses in the [$4.1\times 10^{-11}$, $8.3\times 10^{-10}$]~eV region. In addition, we improve constraints on the involved coupling constants by one order of magnitude in a standard galactic DM model, at the mass corresponding to the resonant frequency of our cavity. In a recently proposed model of a DM relaxation halo around the Earth we improve on existing constraints over the whole DM mass range, by up to 6 orders of magnitude.