Improved Dark Photon Sensitivity from the Dark SRF Experiment

TL;DR

This work reanalyzes the Dark SRF pathfinder data by incorporating a refined jittering model for high-$Q$ resonators, based on the Lorentzian jitter-spectrum framework of Ref. $\text{cui2025}$. By replacing a severe power-suppression assumption with a perturbative correction $\alpha$ that remains small ($\alpha\approx0.15$) and restricting the data window to mitigate slow drift, the authors achieve an approximate order-of-magnitude improvement in the dark-photon exclusion bound for masses below $6\,\mu\mathrm{eV}$, with an associated $\sim 4$-order-of-magnitude enhancement in the effective SNR. This refined Dark SRF bound translates into a world-leading laboratory limit on the SM photon mass, $m_\gamma<1.6\times10^{-15}\ \mathrm{eV}=2.9\times10^{-48}\ \mathrm{g}$, via the longitudinal-mode mapping of a kinetically mixed dark photon. The paper also outlines upgrade paths for a next-generation Dark SRF in a dilution refrigerator, including higher frequency operation, reduced cavity volume, active frequency stabilization, and potential Josephson-parametric amplification to further boost sensitivity.

Abstract

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 $6\,\rm μeV$ and translates to the best laboratory-based limit on the photon mass $m_γ<2.9\times 10^{-48}\,\rm g$.

Figures (1)

• Figure 1: Refined dark-photon exclusion bound, based on the Dark SRF pathfinder run, utilizing our improved jittering treatment. Our new result (solid line) improves the published result (dashed line) Romanenko2023 by about an order of magnitude. See the text for details.