In search of the enigmatic dark photons

The light that allows us to see the ordinary matter of our world is made up of particles called photons. But ordinary matter only represents a small fraction of all the matter in the cosmos. Our universe is filled with an unknown substance called dark matter, which comprises 85% of all matter. The Standard Model that describes known particles and forces is incomplete.

In theorists’ simplest version, an undiscovered type of particle could explain all the dark matter in the universe. But many scientists suspect that the dark sector of the universe has many different particles and forces; some of them could have hidden interactions with the particles and forces of ordinary matter.

Just as the electron has copies that differ in some respects, like the muon and the tau, the dark photon would be different from the ordinary photon. In theory, once produced, photons and dark photons could transform into each other at a specific rate set by the properties of the dark photon.

To search for dark photons, the researchers perform a type of experiment that could be briefly described as “light shining through the wall.” This method uses two hollow metal cavities to detect the transformation of an ordinary photon into a dark matter photon. Scientists store ordinary photons in one cavity and leave the other empty. They then look for the appearance of photons in the empty cavity.

Researchers at the Center for Superconducting Quantum Materials and Systems (SQMS), attached to the Fermi National Accelerator Laboratory, in the United States, have years of experience working with radiofrequency superconducting cavities, which are mainly used in particle accelerators.

Alexander Romanenko’s team, from the Center for Superconducting Quantum Materials and Systems, realized the good potential of radiofrequency superconducting cavities to achieve higher sensitivity than the cavities used in previous experiments.

Scientists working on the Dark SRF experiment at the US Department of Energy’s Fermi National Accelerator Laboratory have shown that their experimental system based on radio frequency superconducting cavities has unprecedented sensitivity for the kind of work that requires searching for dark photons.

Standing next to a part of the Dark SRF experiment are (from left to right) Anna Grassellino, Roni Harnik and Alexander Romanenko, respectively the director of the Center for Superconducting Quantum Materials and Systems (SQMS) and two section heads. (Photo: Reidar Hahn, Fermilab)

The researchers trapped massless ordinary photons in radiofrequency superconducting cavities to search for the transition of those photons to their hypothetical dark sector counterparts.

The experiment, although it has not led to any definitive revelation, has delimited in the most restrictive way achieved so far the range of masses in which dark photons must be.

This experiment represents the first demonstration of the use of radiofrequency superconducting cavities to carry out an experiment of the type described.

The radiofrequency superconducting cavities used by Romanenko and his collaborators are hollow pieces of niobium. When cooled to ultra-low temperatures, these cavities store photons, or packets of electromagnetic energy, very well. For the Dark SRF experiment, the scientists cooled the radio-frequency superconducting cavities in a bath of liquid helium to about 271 degrees Celsius below zero (2 degrees above absolute zero, 273 degrees Celsius below zero, the lowest temperature allowed by the laws of physics).

At this temperature, electromagnetic energy flows effortlessly through the niobium, making these cavities effective for storing photons.

Researchers can now use RF superconducting cavities with different resonance frequencies to cover various parts of the potential mass range of dark photons. This is because the sensitivity peak in the dark photon mass is directly related to the frequency of the regular photons stored in one of the RF superconducting cavities. (Fountain: NCYT by Amazings)