Results of the first search for dark photons using a MADMAX prototype

While many research groups worldwide have been searching for dark matter over the past decades, detecting it has so far proved very challenging, thus very little is known about its possible composition and physical properties. Two promising dark matter candidates (i.e., hypothetical particles that dark matter could be made of) are axions and dark photons.

The MAgnetized Disk and Mirror Axion eXperiment (MADMAX) is a large research effort aimed at detecting axions or dark photons using a sophisticated instrument comprised of a stack of sapphire disks and a reflective mirror. In a recent paper published in Physical Review Letters, the MADMAX collaboration published the results of the first search for dark photons performed using a prototype of their detector.

“The primary goal of MADMAX is to detect dark matter in the form of axions or dark photons,” Jacob Mathias Egge, first author of the paper, told Phys.org. “These two hypothetical particles are popular candidates for what dark matter might consist of. In our recent paper, we describe the results of a search for dark photons using a small-scale prototype.”

Compared to most previous and ongoing dark matter searches, MADMAX relies on a different type of detector. A key objective of the recent work by the researchers involved in the experiment, therefore, was to confirm that the detector they are utilizing actually works and can be used to search for axions and dark photons.

“Dark photons are essentially heavier cousins of the massless photons (i.e., particles of light),” explained Egge. “As a consequence, dark photons from the dark matter halo that surrounds us can spontaneously (but very rarely) convert to ordinary photons with a frequency that depends on the unknown dark photon mass. In our case, we tried to detect these excess photons with a frequency around 20 GHz.”

The experimental setup used by the researchers is designed to first boost the conversion between dark photons and ordinary photons using a resonator. This resonator has a unique design characterized by a stack of parallel dielectric disks.

An advantage of the team’s design is that it enables the creation of resonators that are far larger than the wavelength of converted dark photons. This is a notable difference compared to previously developed instruments, which can often only probe significantly lower frequencies.

“A microwave receiver system then measures the power coming out of this dielectric stack,” said Egge. “A dark photon signal would show up as a constant oscillation with a well-defined frequency amidst random noise from thermal blackbody radiation. In Fourier space, this is a narrow peak above a noisy baseline, which is what we ultimately searched for. Unfortunately, we did not find any such signals other than artificial ones likely coming from the nearby airport.”

While the first search for dark photons performed using the MADMAX prototype did not pick up any signals that could be linked to these elusive particles, it demonstrated the potential of the experiment itself. In fact, even with a small-scale prototype, the researchers were able to pick up signals with a sensitivity that was almost three orders of magnitude greater than that attained in previous experiments, while also covering a large unexplored parameter space in a single experimental run.

“Since the core detector concept has now been proven to work, we can now easily expand our reach in the next upgraded iterations, increasing our chances of a detection,” added Egge. “The next big upgrades for MADMAX are already underway. We are preparing to cool down the entire setup to 4 K which will significantly reduce thermal noise.

“Next, we will incrementally increase the size of the resonator, which boosts a potential dark matter signal, by using more disks and also increase their size. By precisely moving the disks we can tune the resonance frequency and expand the mass range in which we are sensitive.”

In their future experiments, the researchers plan to operate the MADMAX detector in a strong magnetic field, as this would allow them to simultaneously search for both dark photons and axions. Ultimately, their experiments could help to set new constraints on the masses of both these dark matter candidates, potentially contributing to their detection.

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