Photon trapped in lab brings physicists closer to identifying hypothetical dark counterpart

Scientists believe they have found a mass range for the particles called dark photons.

While photons (light particles) have no mass, their as yet unseen counterpart, dark photons, do. Now experimental physicists may be able to measure that mass.

As part of the experiment, an ordinary, massless photon was confined in a superconducting radio frequency cavity (SRF) by physicists at the US Department of Energy’s Fermi National Accelerator Laboratory (Fermilab).

The physicists working as part of the Dark SRF project at Fermilab demonstrated never before seen sensitivity. Their experiment provides the most well-defined mass range for the dark photon to date. The results are published in Physical Review Letters.

Photons are the particles of light that allow us to see all the ordinary matter around us. The are also the particles which transmit electromagnetic force.

But observations of distant galaxies suggests that there is more to the universe than “normal” visible matter. Gravitational anomalies in galaxies has led physicists to hypothesise the existence of “dark matter” which is five times more abundant than ordinary matter.

While the simplest models of this elusive substance suggest that one undiscovered type of particle accounts for all the universe’s dark matter, many scientists suspect that there is a whole array of dark particles and forces that are yet to be discovered – some of which may have hidden interactions which known particles and forces.

One such dark particle is the theoretical dark photon.

“The dark photon is a copy similar to the photon we know and love, but with a few variations,” says co-author Roni Harnik, a researcher at the Superconducting Quantum Materials and Systems (SQMS) Center hosted by Fermilab.

It’s theorised that unlike an ordinary photon, the dark photon would have a mass. Once produced, photons and dark photons could transform into each other at specific intervals which are determined by the properties of the dark photon.

Researchers employed a light-shining-through-wall type of experiment to look for dark photons.

This method uses two hollow, metallic cavities which store and harness electromagnetic energy with high efficiency to detect the transformation of an ordinary photon into its dark counterpart.

Trapping a photon in one cavity, the researchers look for the emergence of photons in the other.

SRF cavities are primarily used in particle accelerators, but researchers at the SQMS Center have begun using them in other experiments on quantum computing as well as dark matter searches. This experiment marks the first time an SRF cavity has been used in a light-shining-through-wall experiment.

Using hollow chunks of niobium cooled to -271°C (only 2°C above absolute zero), the researchers were able to efficiently store photons in the cavities.

Such SRF cavities can now be produced to look for dark photons of different frequencies to cover various parts of the potential mass range for dark photons.

“SRF cavities open many new search possibilities,” says co-author Zhen Liu, an SQMS Center researcher from the University of Minnesota. “The fact we covered new parameter regions for the dark photon’s mass shows their successfulness, competitiveness and great promise for the future.”

“The Dark SRF experiment has paved the way for a new class of experiments under exploration at the SQMS Center,” says Anna Grassellino, director of the SQMS Center and co-pricipal investigator of the experiment. “From dark matter to gravitational waves searches, to fundamental tests of quantum mechanics, these world’s-highest-efficiency cavities will help us uncover hints of new physics.”