Improved Dark Photon Sensitivity from the Dark SRF Experiment

We report the refined dark-photon exclusion bound from Dark SRF’s pathfinder run. Our new result is driven by improved theoretical modeling of frequency instability in high-quality resonant experiments. Our analysis leads to a constraint that is an order of magnitude stronger than previously reported (corresponding to a signal-to-noise ratio that is four orders of magnitude larger). This result represents the world-leading constraint on non-dark-matter dark photons over a wide range of masses below $6\mu \mathrm{e}\mathrm{V}$ and translates to the best laboratory-based limit on the photon mass $m_{\gamma }<2.9\times {10}^{-48}\mathrm{g}$.