The hunt for dark matter is one of the central tasks of modern physics. Among the candidates are dark photons, which, unlike ordinary photons, carry a tiny mass and interact with matter extremely weakly. The idea is that dark photons can convert into ordinary photons at the boundary between media with different dielectric properties. To catch such a faint signal, researchers build special detectors called haloscopes. Sitting alongside this work in the archive are studies of pulsars, axions and dark photons using mirror telescopes, laser detectors and resonant cavities. They are all after the same class of particles, but with different methods. This work stands out because it uses an open Fabry-Perot resonator instead of a closed cavity, which makes it possible to keep a large volume at high frequencies.
The DALI prototype (PoP) is housed at the Instituto de Astrofísica de Canarias. It consists of a stack of 20 yttria-stabilized zirconium dioxide (ZrO₂) plates measuring 100×100 mm and 1 mm thick, placed inside a copper mirror. The plates form a Fabry-Perot resonator tuned to a frequency near 6.907 GHz. The signal is collected by a horn antenna, amplified by a cryogenic low-noise amplifier and digitized. The whole setup sits inside a Faraday cage to suppress external interference. Data were recorded over 36 hours of effective time between 17 February and 5 March 2026. Each spectrum was averaged from 9540 fast Fourier transforms, giving a frequency resolution of about 954 Hz.
Over 36 hours of observations in the 6.886909-6.917922 GHz band (corresponding to dark photon masses of 28.4820-28.6102 μeV), no statistically significant excess signal was found. Sensitivity reached χ ≲ 6.7967×10⁻¹⁴ at a frequency of 6.902112 GHz. This is the strongest laboratory limit in this frequency range. Of the 10841 bins in the grand spectrum, 9555 (88.1%) remained unmasked, and in all of them dark photons are excluded at a confidence level of 90%. A few narrowband candidates were flagged for rechecking in future runs, but their local significance does not exceed 77.9%, so they are not treated as a detection.
The work shows that an open Fabry-Perot resonator can serve effectively as a haloscope for dark photon searches at frequencies where traditional closed cavities lose volume. That opens a path to scaling up the setup without sacrificing sensitivity. Unlike neighboring studies that use closed resonators, mirror telescopes or pulsars, this one applies an interferometric approach with ZrO₂ plates for the first time. The result shows that even a scaled-down prototype can compete with the best laboratory limits.
The full-scale DALI will have 100 times the aperture area and a phased antenna array for coherent signal summing. That should improve sensitivity by several orders of magnitude. There are also plans to tune the resonator frequency to cover a wider range of dark photon masses. Better matching between the antenna and the resonator (a higher coupling coefficient β) also promises a gain in sensitivity.
The result matters for dark matter physics and particle physics because it closes off part of the parameter space for dark photons. It is also useful for developing future gravitational-wave and axion detectors, since it relies on similar technology.
Rescanning the candidate frequencies with improved hardware and longer integration time. Extending the frequency range and raising the coupling coefficient to boost sensitivity.
The work speaks directly to the question of what dark matter actually is: is it made of particles, and if so, which ones? The dark photon is one of the most motivated candidates, and searching for it in the lab complements astrophysical observations. The absence of a signal in this mass range narrows the territory where dark photons might hide, which helps build more accurate models.
A sensitivity of χ ≲ 6.8×10⁻¹⁴ means that if a dark photon and an ordinary photon mixed with that strength, there would be roughly one dark photon for every trillion ordinary ones. It is like looking for one special grain of sand on a beach hundreds of kilometers long.
The idea of hidden photons that permeate our Universe and only rarely reveal themselves echoes the "dark sector" concept in science fiction, for instance in Liu Cixin's novel "The Dark Forest," where invisible particles play a key role in the plot.