Meet the dark photon

Dark matter, the invisible substance that makes up over a quarter of the universe, remains one of physics’ most stubborn enigmas. In the hunt for dark matter, physicists are pushing past the Standard Model of particle physics, the best current model for describing existence, to the ‘dark side’ beyond. That may sound like something from Star Wars, but dark particles, like the theoretical dark photon, hold promising implications for both cosmology and particle physics research.

What is a dark photon?

We owe the photon – one of the fundamental particles in the Standard Model – a lot. Radio waves, X-rays, and even visible light are all due to these particles, which are the force carriers for electromagnetism. But if photons are critical to electromagnetism, what could their possible dark counterpart help us understand?

“A dark photon is a hypothetical particle,” explains Zach Weiner, a postdoctoral researcher at Perimeter Institute. He describes the particle as a “cousin to the photon.”  

“The dark photon doesn’t interact with ordinary matter like electrons, protons and neutrons, or, if it does, the interaction is so weak that we haven’t detected it yet,” says Weiner. “In that sense, it’s dark to us.”

Another Perimeter researcher, Junwu Huang, is also interested in exploring what’s beyond the Standard Model. In particular, he investigates dark matter candidates: how they could be produced in the early universe, what they mean for cosmology and astrophysics, and how to design experiments to look for them.  

Huang says that dark matter candidates fall under two regimes. In the first, dark matter behaves like ping pong balls and can hit one another and scatter. In the second, the particles are lightweight and therefore behave like waves in most of the systems researchers are interested in.

Dark photons fall into this second, wavy category in a group called light bosons. “The difference between light bosons and other dark matter candidates is that they are very light,” explains Huang. “For the same energy density, there’s a much larger number density.” In other words, for the same energy density as other dark matter candidates, there must be a whole lot of them.

“A key concept is the occupation number – essentially density times the proper power of wavelength of the particle. For light boson dark matter, the occupation number is much bigger than one.”

Huang is interested in how this large occupation number of dark photons changes the dynamics in astronomy and cosmology. “We’re trying to understand how this occupation number leads to new dynamics,” says Huang, “And how the particles behave in ways that are different from what people are used to.”

Dark photons in the early universe

Dark photons are also curious because, unlike their Standard Model cousins, they have mass. Particles related to the photon in the Standard Model, like the electroweak W and Z boson, have mass through the Higgs mechanism. So, it’s possible that a dark photon gets mass in a similar but simplified way – a dark Higgs mechanism, so to speak.

But this is where a problem pops up.

In 2023, Huang and fellow Perimeter researcher Will East published a paper showing that when dark photons are given mass through a dark Higgs mechanism, they can collapse into a network called cosmic strings. Since dark matter must have been created in the early universe, at some point dark photons would have to be produced and could trigger this string formation. In these string configurations, the dark photons are not free particles and can't be a candidate for dark matter. “If you want to imagine a model of dark photons with mass as your dark matter, you have to avoid this collapse into a cosmic string,” says Weiner. 

“In order to make dark photons viable as a dark matter candidate, you have to make the coupling to the dark Higgs very, very small,” says Weiner. “At the same time, that would make any experimental signature of the dark photon, very, very small.”

Essentially, if the simplest mechanisms for producing dark photons in the early universe are correct, prospects for detecting dark matter go to zero.  

While completing his postdoc at the University of Washington, Weiner published a pair of papers with co-author David Cyncynates seeking models that avoid cosmic string formation. They used East and Huang’s research to set thresholds that would avoid cosmic string formation. This allowed them to set concrete experimental targets for future dark photon experiments.

The search for dark photons

There aren’t a lot of dedicated experiments searching specifically for dark photons and dark photons alone,” says Weiner. “It turns out that laboratory signatures of dark photons often resemble the signatures of another dark matter candidate –- axions – well enough that axion-specific experiments can revise their analysis to say something about dark photons.”

“This is one of the reasons David and I were motivated to look into this – because we will have these dark photon searches for free,” says Weiner. “Future experiments are going to explore this parameter space, and so we’d like to understand what we can say about the findings.”

The challenging part of searching for dark matter is the tension between cosmological models and the sensitivity of lab experiments. “The cosmology dictates how weakly coupled this thing has to be,” says Huang. “The question is whether we are sensitive enough on Earth to design (an experiment) to look for it. It’s always a competition.”  

So what does all this mean for the photon’s dark, mysterious cousin? There’s a chance that dark photons exist but aren’t the main cause of dark matter: they collapse into cosmic strings instead and influence phenomenon like gravitational waves. But it’s also possible that more research could make them a leading candidate. Either way, further studies of this dark relative of the photon may illuminate our understanding of the origins and evolution of our universe.