The Three Approaches for Detecting Dark Matter

By Don Lincoln, Fermilab

Scientists have tried many approaches like direct and indirect detection techniques, as well as large particle accelerators, trying to detect dark matter. Indirect detection assumes that antimatter dark matter didn’t disappear in the early moments of the big bang like ordinary matter did. None of the approaches have been successful, but they all have informed us of what dark matter isn’t. Let’s spend some time looking into them.

The Indirect Detection

Let’s first look at indirect detection. Indirect searches look for dark matter that exists “out there”, so to speak. Of course, in order to detect such signals, scientists have to consider two things. 

First, in order to see a lot of dark matter annihilations, you have to look where there is a lot of dark matter. The most obvious place is the center of galaxies. While dark matter exists in a cloud surrounding galaxies, it isn’t distributed with equal density. 

The second consideration is the distance from the Earth. If the annihilation products can spread out in every direction, they travel in a sphere that gets bigger and bigger. That means that a detector of fixed size sees a smaller and smaller fraction of the signal. 

Combining these two considerations, astronomers realized that the center of the Milky Way galaxy is the optimum place to look for dark matter annihilations. So, how do they look? There are many ways, but two are especially interesting. 

Looking for Neutrinos 

The first way to look for dark matter annihilation is searching for neutrinos. You need a huge detector. There are many detectors looking for this signal but let me tell you about two.

The first is a very big detector called the Super Kamiokande detector. It is an enormous tank of water buried deep underground in Japan. But that detector is dwarfed by another detector in Antarctica called IceCube. IceCube is literally a cubic kilometer of ice. That’s a cube a little more than half a mile on each side. And to see neutrinos, scientists have embedded 4,800 detectors throughout the cube to register light generated when a neutrino interacts with the ice.

In both cases, the detectors look for neutrinos coming from the general location of the galactic center that then interacts with their detector. Their story is a fascinating one, but the bottom line is that they see no unexplained excess of neutrinos from the center of the Milky Way.

This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.

Searching for Gamma Rays 

Another method of detecting the signature of cosmic dark matter annihilation is to look for gamma rays. There are also many facilities looking for extraterrestrial gamma rays. Some of them are ground-based telescopes, while others are satellites. Probably the most powerful of the detectors is the Large Area Telescope in the Fermi satellite. 

These detectors have certainly found gamma ray sources from the region surrounding the center of the Milky Way, but it’s essentially impossible to identify these gamma rays as having a dark matter origin.

After all, objects called pulsars, which are rapidly rotating neutron stars, can make gamma rays. And they’re not alone. The bottom line is that indirect searches are really hard. They’re useful, but probably not how we’re going to find a definitive signal.

Using a Large Particle Accelerator

Then there’s my favorite, which is to search for the creation of dark matter in large particle accelerators, like the Large Hadron Collider, which is operating in Europe. You smash together high-energy particles and hope that the energy makes dark matter and dark antimatter. Now, remember that dark matter doesn’t interact via electromagnetism or the strong force, which means that if you make dark matter, it just escapes the detector, well, undetected. Thus, you see it by not seeing it.

We know the energy and momentum of the colliding particles and we believe in energy and momentum conservation. If energy and momentum escape the detector undetected, then it results in an imbalance in what you see. An imbalance means an invisible thing got away and that thing could be dark matter.

Bottom line? We’ve found nothing. So, that’s a bit of a bummer to be sure. The finding is better than ruling out, but we have at least ruled out possible masses for dark matter particles, assuming, of course, that it is even possible to make dark matter at particle accelerators.

The Direct Detection 

And that leaves us with the third ability to find dark matter, which is just seeing it here on Earth. After all, if dark matter exists in all of space, then it stands to reason that there is dark matter here in the solar system. The prediction is that if you hold up your hand and make a fist, there is one dark matter particle in every fist-sized volume.

That’s already a tantalizing prospect but finding dark matter here on Earth means we need to have dark matter interact with ordinary matter in some detectable way. There are many options of how this might be accomplished, all of which involve somehow crashing dark matter into an atom of ordinary matter and seeing the ordinary matter recoil. This is called direct detection of dark matter. 

Luckily, there is an approach we can exploit if dark matter is real. If it is then dark matter in the galaxy doesn’t move at the same speed as the Earth. Thus, if this is true, then there is effectively a dark matter wind that blows through the Earth, caused by the relative motion of the dark matter and the Earth. It’s this dark matter wind that can smash together dark matter particles and ordinary matter particles in our detectors.

Common Questions about the Three Approaches for Detecting Dark Matter

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How Antimatter Was Discovered
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Cosmic Rays and the Discovery of the Unseen Particles