The dark photon search just got ultra-sensitive, thanks to new study

What are dark photons?

"The dark photon is a copy similar to the photon we know and love, but with a few variations," explained co-author Roni Harnik, a researcher at the Fermilab-hosted Superconducting Quantum Materials and Systems Center, in a press release. 

The researchers utilized a unique setup called the Dark SRF experiment, where they captured ordinary photons — the particles of light we encounter daily — using superconducting radio frequency (SRF) cavities. 

The goal was to observe whether these ordinary photons could transition into their hypothesized counterparts from the dark sector— a realm of the universe where dark matter resides.

Unlike ordinary matter, which makes up only a fraction of the universe, dark matter constitutes a staggering 85 percent of all matter, yet its nature remains elusive. 

The theoretical concept of dark photons suggests the existence of particles in the dark sector with mass, akin to the differences between familiar particles like electrons, muons, and taus. 

If proven true, these dark photons could transition into regular photons and vice versa at specific rates determined by their properties.

To shed light on this puzzle, scientists performed a 'light-shining-through-wall' experiment employing two hollow metallic cavities to conduct their investigation. 

Significantly, it was also the first time SRF cavities —in this case, made from hollow chunks of niobium— were used to carry out such an experiment.

They stored ordinary photons in one cavity while leaving the other empty, observing any emergence of photons in the latter, which could indicate the possible presence of dark photons.

Detecting dark photons

"It was immediately clear to me that we could demonstrate greater sensitivity with SRF cavities than cavities used in previous experiments," said Alexander Romanenko, SQMS Center quantum technology thrust leader. 

Scientists efficiently stored photons by cooling the cavities to extremely low temperatures, around 2 K (close to absolute zero), and submerging them in a bath of liquid helium. 

Thanks to this experiment, researchers can now utilize SRF cavities with different resonance frequencies to explore potential mass ranges for dark photons. 

This is because the sensitivity to the mass of these mysterious particles is directly linked to the frequency of regular photons stored in one of the SRF cavities.

Liu, who worked on the data analysis and the verification design, highlighted, " The team has done many follow-ups and cross-checks of the experiment." 

"SRF cavities open many new search possibilities. The fact we covered new parameter regions for the dark photon's mass shows their successfulness, competitiveness, and great promise for the future," she added. 

As the SQMS Center director, Anna Grassellino, explained, these high-efficiency cavities promise to be vital tools in our ongoing pursuit of uncovering hints of new physics.

The complete study was published in Physical Review Letters and can be found here.

 Study abstract:

We conduct the first “light-shining-through-wall” (LSW) search for dark photons using two state-of-the-art high-quality-factor superconducting radio frequency (SRF) cavities —Dark SRF—and report the results of its pathfinder run. Our new experimental setup enables improvements in sensitivity over previous searches and covers new dark photon parameter space. We design delicate calibration and measurement protocols to utilize the high-Q setup at Dark SRF. Using cavities operating at 1.3 GHz, we establish a new exclusion limit for kinetic mixing as small as ε=1.6×10−9 and provide the world’s best constraints on dark photons in the 2.1×10−7–5.7×10−6  eV mass range. Our result is the first proof of concept for the enabling role of SRF cavities in LSW setups, with ample opportunities for further improvements. In addition, our data set a competitive lab-based limit on the standard model photon mass by searching for longitudinal photon polarization.