PRL New research at Fermilab lights the way to 'dark photons'

The dark photon is one of the mediating particles constructed by theorists to bridge the world of visible matter and the world of dark matter. It is a vector canonical bosonic particle, and it is called a dark photon because it interacts essentially like a photon, interacting only with particles with an electric charge.

It could be a bridge to the world of dark matter, or dark matter itself.

Now, scientists working on the superconducting radiofrequency (SRF) experiment at the U.S. Department of Energy's Fermi National Accelerator Laboratory have demonstrated unprecedented sensitivity in an experimental setup used to search for dark photon particles.

-- This experiment provides the world's best constraints on the existence of dark matter photons in a specific mass range.

"Search for Dark Photons with Superconducting Radio Frequency Cavities," the results of the experiment were recently published in Physical Review Letters.

The light we see as ordinary matter in the world is made up of particles called photons. But ordinary matter makes up only a small fraction of all matter. Our universe is filled with an unknown substance called dark matter, which makes up 85 percent of all matter.

Just as electrons have copies that are different in some ways (including muons, etc.), dark photons can be different from ordinary photons and have mass. Theoretically, once created, photons and dark photons can transform into each other at a specific rate set by the properties of dark photons.

Left: The SRF experimental setup, consisting of two 1.3 GHz chambers; Right: Sketch of the SRF electronic system.

A new 95% C.L. exclusion limit for the dark photon parameter space. The experimental result is shown as a blue curve with the upper region excluded.

To search for dark photons, the researchers performed an experiment called "light-shining-through-wall (LSW)". This method uses two hollow metal cavities to detect the conversion of ordinary photons to dark matter photons. Scientists store ordinary photons in one cavity while the other is empty. They then look for photons in the cavities.

The SRF cavities used by the collaborators are hollow niobium blocks, and when cooled to ultra-low temperatures, these cavities work well for storing photons or packets of electromagnetic energy. In the "dark SRF" experiment, the scientists cooled the SRF cavities in a liquid helium bath to about 2 K, close to absolute zero. At this temperature, electromagnetic energy flows effortlessly through the niobium, which allows these cavities to efficiently store photons.

Researchers at Fermilab's SQMS Center have many years of expertise in the use of SRF cavities, which are primarily used in particle gas pedals. Because of the SRF cavities' ability to efficiently store and utilize electromagnetic energy, researchers at the SQMS Center are now using SRF cavities for other purposes, such as quantum computing and dark matter searches.

The research team can now use SRF cavities with different resonant frequencies to cover various parts of the potential mass range of dark photons. This is because the peak sensitivity of the dark photon mass is directly related to the frequency of ordinary photons stored in one of the SRF cavities.

Compared to previous searches, this new experimental setup improves the sensitivity and covers a new dark photon parameter space. Utilizing a cavity with an operating frequency of 1.3 GHz, the team provides the world's best constraints for proposing a new exclusion limit for dynamical mixing: ε = 1.6 × 10-9 and 2.1 × 10-7-5.7 × 10-6 eV for dark photons . In addition, these data set a competitive experimental limit for the Standard Model photon mass by searching for longitudinal photon polarization.

The team has also followed up and cross-checked the experiments several times.The SRF cavities open up many new search possibilities and cover new parameter regions of the dark photon mass - demonstrating their success, competitiveness and great promise for the future.

From left to right, Anna Grassellino, SQMS Center Director; Roni Harnik, SQMS Science Advancement Leader; and Alexander Romanenko, SQMS Technology Advancement Leader.

The Dark SRF experiment paves the way for a new class of experiments being explored by the SQMS Center in which these cavities can be used as extremely sensitive detectors, said Anna Grassellino, SQMS Center Director and co-PI of the experiment, "From dark matter to gravitational wave searches to fundamental tests of quantum mechanics, these 'cavities' will help us discover hints of new physics."

Reference link:

[1]https://phys.org/news/2023-07-ultra-sensitivity-dark-photon.html

[2]https://scitechdaily.com/probing-the-abyss-fermilabs-dark-srf-experiment-illuminates-the-search-for-dark-photons/

[3]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.261801

[4]https://interestingengineering.com/innovation/dark-photon-search-ultra-sensitive