Scientists set new limits on dark photon existence

Scientists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have achieved a groundbreaking milestone in their Dark SRF experiment. Their experimental setup, which aims to detect elusive particles known as dark photons, has demonstrated unparalleled sensitivity.

The researchers utilized superconducting radio frequency cavities to trap and study ordinary, massless photons in search of potential transitions into their hypothesized dark counterparts. Recently published in Physical Review Letters, this experiment has provided the most stringent constraint to date on the existence of dark photons within a specific mass range.

Roni Harnik, a researcher at the Superconducting Quantum Materials and Systems Center hosted at Fermilab, and co-author of the study, explained that dark photons are akin to copies of the familiar photons we are aware of, but with some distinct differences.

Ordinary matter, which comprises only a small fraction of all matter, is composed of photons that allow us to perceive light. The majority of matter in the universe, approximately 85%, is made up of an unknown substance called dark matter. This enigmatic realm of particles and forces remains beyond the scope of the current Standard Model, which describes known particles and interactions but is deemed incomplete.

The simplest theoretical model posits that a single undiscovered type of dark matter particle could account for all dark matter in the universe. However, many scientists believe that the dark sector is more complex, potentially housing various particles and forces, some of which might have hidden interactions with ordinary matter.

Drawing parallels to the familiar electron, which has similar but distinct counterparts like the muon and tau, the dark photon is envisioned to be different from the regular photon and to possess mass. According to theory, photons and dark photons could potentially transform into each other at a specific rate dictated by the properties of the dark photon.

Left: the experimental setup for the Dark SRF experiment consisting of two 1.3 GHz cavities. Right: a sketch of the Dark SRF electronic system. Credit: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.261801

Innovative use of SRF cavities

Researchers at the SQMS Center within Fermilab have achieved a significant breakthrough in their quest to detect dark photons using a unique experimental method called the light-shining-through-wall experiment. This approach involves utilizing two hollow metallic cavities to observe the conversion of an ordinary photon into a dark photon. In one cavity, ordinary photons are stored, while the other remains empty, and scientists analyze whether photons appear in the empty cavity.

The SQMS Center researchers at Fermilab have extensive experience working with superconducting radio frequency (SRF) cavities primarily used in particle accelerators. However, they have now harnessed these cavities for other purposes, such as quantum computing and dark matter searches, due to their exceptional ability to efficiently store and manipulate electromagnetic energy.

Alexander Romanenko, the quantum technology thrust leader at the SQMS Center, recognized the potential of using SRF cavities for light-shining-through-wall experiments when he came across similar experiments with copper cavities. The superior sensitivity offered by SRF cavities over traditional ones used in previous experiments was apparent to him.

In this groundbreaking experiment, SRF cavities made of niobium, which are hollow chunks, are cooled to ultralow temperatures using liquid helium, nearing absolute zero at approximately 2 K. At such temperatures, niobium becomes an excellent conductor of electromagnetic energy, making these cavities exceptionally efficient at storing photons.

To cover various potential mass ranges for dark photons, researchers can now utilize SRF cavities with different resonance frequencies. The sensitivity to the mass of the dark photon depends on the frequency of the regular photons stored in one of the SRF cavities.

Zhen Liu, a physics and sensing team member from the University of Minnesota and co-author of the study, expressed that the team has explored various schemes to handle the challenges and opportunities presented by these ultra-high-quality superconducting cavities for the light-shining-through-wall experiment.

Through rigorous follow-ups and data analysis, the researchers have successfully opened up new parameter regions for the mass of the dark photon, showcasing the effectiveness and promise of SRF cavities for future experiments. These high-Q cavities offer exceptional sensitivity and open up possibilities for a new class of experiments at the SQMS Center, including searches for dark matter, gravitational waves, and fundamental tests of quantum mechanics. The director of the SQMS Center, Anna Grassellino, and co-PI of the experiment, highlighted the potential of these world-class cavities in uncovering hints of new physics across various fields.

Source: Fermi National Accelerator Laboratory

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