“Dark photons in the dark sector is an analogy [to photons] in our visible world,” said associate physics professor Shih-Chieh Hsu with the University of Washington’s ADMX lab in their search for the axion particle, a hypothetical subatomic particle of low mass and a strong contender for the dark matter particle. “Dark photons have a similar property as photons, except they only interact with dark matter and it can be massive. It mediates dark force between dark matter.”
“Interestingly, axions were originally proposed as a solution to a different phenomenon observed in physics,” said UW Physics professor Leslie Rosenberg, lead scientist at ADMX. “It was later realized that their properties made them very strong dark matter candidates.”
The imprint of structure on the cosmic microwave background only makes sense if dark matter isn’t made out of things that have protons and electrons in them. “Normal and dark matter both have mass and gravitate,” assistant UW physics professor Gray Rybka said. “Dark matter is a matter that interacts very weakly with other matter and light.”
An interesting property of axions is that they convert into microwaves (exactly the kind of waves that reheat leftovers) in a strong magnetic field. These waves can be then ‘heard’ by a radio-like device. “We are essentially just designing a radio receiver and placing it inside the magnetic field and listening for a signal as we tune,” UW graduate student Nick Du said. At The Galaxy, we refer to this unheard dark-sector phenomenon as the “Lucifer signal.”
As simple as that may sound, the experiment has its fair share of challenges. “The signal we are looking for is extremely small,” Du said. “We’re talking something like a radio signal from Earth all the way on Mars. To get around this, we utilize two systems in our experiment that keep our noise extremely low.”
Another difficulty stems from the fact that the actual mass of the axion is not known and so ADMX has to look for particles over a range of masses.
“The most difficult part is that to achieve our sensitivity, we can only search over a very narrow range of masses at a time,” Rybka said. “By necessity, the experiment takes a long time to operate we slowly scan over masses.”
This year, with an upgraded cavity and electronics, ADMX will cover a wider mass range using an updated setup with a better chance of discovery.
“We’re at a point where we could potentially discover dark matter axions at any moment during the experiment,” Du said. “To get our experiment to a point where it was this sensitive took a tremendous amount of work over the course of several years of work.”
While ADMX is looking for axions, another experiment, FASAR at the Large Hadron Collider (LHC), seeks to find indirect clues about dark matter particles by looking for dark photons.
In 2017, Jonathan Feng, professor of physics and astronomy at the University of California, Irvine and his team proposed that the current setup of the LHC assumes that dark photons circulate the LHC tubes just like normal matter. The LHC is a large underground laboratory for particle research. In reality, however, they could be hitting a ‘blind spot’ in the LHC or escaping the detectors due to their distinct properties. Feng and his team proposed to place another detector, FASER, along the hypothesized path taken of dark photons — at its blind spot.
“Dark photons have the rare property to decay to normal particles. Specifically, they could decay to electron-positron pair,” Hsu said. “FASER is a specialized detector able to catch such pair production of electrons.”
Even though this process is quite rare, the high number of particles at the LHC gives researchers an irresistible opportunity to increase their success rate.
At the UW, Hsu’s group analyses simulations of detection events by the FASER instrument, and works out the tools needed to accurately trace any detected particles back to their sources. These tools will help separate real signals of dark matter-associated particles from background events.
The Daily Galaxy, Ryan Blackman, via University of Washington
Image at top of page: A simulation of the dark matter distribution in the universe 13.6 billion years ago. Volker Springel, Max Planck Institute for Astrophysics