Why do you think there is a fifth force?
The four fundamental forces of physics – gravitational, electromagnetic, strong nuclear and weak nuclear – are pretty well understood. But there’s always the chance that there’s another one we just haven’t noticed, perhaps because it’s incredibly weak. People have been looking for a new one for a long time. But the forces we’re looking for now are different in that they act primarily on dark matter. Just as regular matter consists of a range of particles and forces, I’m motivated by the idea that dark matter is just the lightest, most stable component of an undiscovered “dark sector” of particles and forces.
Any reason to think such a dark sector exists?
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It makes increasing sense to consider it. We know dark matter exists, that it interacts gravitationally – in other words, it has mass – and that the vast majority of it is probably embodied in a particular type of particle. Scientists latched on to the idea that dark matter is mostly composed of particles called weakly interacting massive particles (see “Shadow worlds: Have we seen our first glimpse of dark forces?“). But searches for those WIMPs – with underground detectors and the Large Hadron Collider, for example – haven’t turned up anything, and we’re running out of room where we might find them. So if dark matter isn’t just WIMPs, one solution is that there are different sorts of dark particles that interact with each other via their own set of forces.
Explore the shadow worlds: Have we seen our first glimpse of dark forces?
So dark matter could be pretty diverse stuff?
Sure. The standard model of particle physics has lots of particles, with a set, including the photon, that carry the forces. This ordinary matter only accounts for about one-sixth of the universe’s matter. The rest is dark matter, so why wouldn’t it be as diverse? If you open that conceptual door, you’re suddenly looking at a lot of new possibilities. But to help us get started, we’re just considering the simplest option at the moment, which is a dark force analogous to electromagnetism, so we came up with the term “dark photons”.
It all sounds a little out there.
It was a challenge early on because not everyone took these ideas seriously as they were outside the mainstream. That’s changing but nevertheless the students and scientists attracted to this search tend to be clever and crazy in equal parts.
How do you go about hunting for dark photons?
The theory is that dark photons mix with regular photons by a process called kinetic mixing. That means a dark photon can turn into a regular photon, and vice versa – though most likely at some very, very low rate. So, in principle, if you have an experiment where you produce lots of high-energy photons, you’ll also produce dark photons at some much lower rate.
And how do you detect dark photons?
Dark photons can’t be massless like regular photons. If they were, it would contradict our understanding of how dark matter behaves. In fact, they could have an incredibly wide range of masses. That means that although we can’t see the dark photons directly, we can hunt for them the same way we hunt for any other particle that has mass.
Are you trying this already?
Yes, our at the Accelerator Facility (JLab) uses a beam of high-energy electrons that we fire into a tungsten foil target. When you do that – and the electrons suddenly hit that obstacle – you get deceleration radiation. That radiation is essentially a beam of photons, and, if dark photons exist, the collisions will radiate those too, at a lower rate. What happens next depends on whether or not dark photons are the lightest particle of the dark sector. Our experiment assumes that they are, which means they must decay via kinetic mixing to regular matter such as electron-positron pairs, which we can detect.
Have you found anything so far?
We had a practice run in early 2015 where the principal purpose was to get the experiment working, but we did take a few days’ worth of high quality data. And we ran the experiment for a while earlier this year. In the coming months, when we’ve analysed these data sets, we have a chance of seeing something new.
What if dark photons are heavier than you think?
We presume that the lightest dark matter particle makes up the bulk of dark matter. If the dark photon is not the lightest particle in the dark sector, then instead of decaying back to regular matter, it will pretty much always decay to dark matter. That means we can’t see it with our current experiment, but it actually leads to some interesting possibilities. If I have an experiment with a really thick tungsten target and create a bunch of dark photons that are all moving really fast, they are all going to decay to dark matter – so I have now essentially created a dark matter beam. We will have lost the ability to detect dark photons, but gained the ability to detect dark matter itself. It’s a win-win situation.
A dark matter beam sounds awesome…
The cool thing about this is that it would produce dark matter at high energy. The direct-detection experiments we have for dark matter, such as and are trying to detect dark matter orbiting our galaxy at relatively low velocities. When it bumps into the detector it only deposits a very small amount of energy that is really hard for us to spot. That’s why we have to bury these detectors in mines – the surrounding earth screens out many interfering signals. But if I had a high-energy dark-matter beam I could just point it at a standard particle detector.
“People attracted to this search tend to be clever and crazy in equal parts”
Is anyone attempting to do this?
There is a plan to create one of these “beam dump experiments” at JLab in Virginia, but here at the SLAC National Accelerator Laboratory we’re proposing something we call a missing momentum experiment. The idea is to try to create the dark photons in the same way, but rather than worry about detecting them, we measure the incoming and outgoing momentum for each individual electron in the beam, looking to see if any momentum went missing. That would mean something had been produced in the target that was invisible to our detector – dark matter. This would be a very sensitive way of finding hints that dark photons exist, though it wouldn’t help us study them in detail. In March, approved a first-generation experiment of this sort, called NA64.
What would definitive detections of dark matter mean for humanity?
It would be like the Copernican revolution – another confirmation that we are not at the centre of the universe, and that what we thought was the entire universe is just a tiny slice. It’s one thing to be intellectually convinced of that, as we are, but another to confront it face to face.
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Tim Nelson is a physicist at the SLAC National Accelerator Laboratory in Menlo Park, California. He is working on the Heavy Photon Search experiment
This article appeared in print under the headline “I’m shooting for a beam of dark light”
