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RWTH Aachen Particle Physics Theory

Looking for Dark Matter: Trembles beneath the surface

April 2nd, 2019 | by

The ever-elusive nature of dark matter is of interest not only from a theoretical point of view –we are, after all, missing 85% of the Universe– but also from an experimental one. How exactly does one study something invisible? We can point our telescopes at the sky and study how light bends around clusters of apparent emptiness, hinting that there must be something there. We can do this with ancient light (commonly known as the CMB) and figure out exactly how much of this invisible substance is present in our Universe. We can study the structure of galaxies and galaxy clusters, the motion of stars bound to them. Everywhere we look, it seems, there is evidence for something massive lurking in the shadows. But none of these things tell us anything about what a dark matter particle* actually looks like. We cannot “trap” a dark matter particle for it can, quite literally, slip through walls. We don’t have access to it in the way we have access to, say electrons. But as humans, we are tenacious. And as scientists, we are creative. The minor problem of dark matter’s invisibility won’t stop us in our quest for answers.

In this series of (hopefully) short posts, we’ll cover the three conventional avenues of dark matter detection. The elevator pitch for these techniques? Shake It, Make It or Break It. ‘It’ being dark matter.

Shaking dark matter is as fun as it sounds. In the simplest of terms, it involves waiting for a dark matter particle to strike a target atom placed inside a detector. As a result of the collision, the state of the target atom changes. It could either get ionized, resulting in the production of free electrons which we can ‘see’ in the detector. It could absorb the energy of the dark particle and then release it in the form of a photon. Or it could release this energy as heat. In all of these cases, the collision results in a visible/detectable signature.

In a direct detection experiment, we study the effects of a dark particle colliding with a target atom like Xenon.

There are two important questions to ask now:
1. Where does the initial dark matter particle come from?
2. How can we be sure that the electrons/photons/heat we detect is actually caused by this collision? 


The answer to the first questions is fairly straight-forward. From various other measurements, we know that there is a constant (ish) dark matter flux through Earth. At any given instant, we are being bombarded with dark matter particles. To get an idea of how strong this bombardment is, consider these numbers. The dark matter flux (total dark matter mass passing through one cubic centimeter every second) is approximately 0.3\ \mathrm{GeV}/\mathrm{cm}^3. (GeV is the standard unit of mass in physics. 1 GeV is approximately 10^{-27} kg.) So if we assume a dark matter mass of 1 GeV, this amounts to roughly 0.3 dark matter particles per centimeter cubed. The average volume of a human body is 66400 cubic centimeters. Which means that in one second, about 20,000 dark matter particles zip through every person on Earth. Given that dark matter is weakly interacting, it is no surprise that we don’t notice this constant flux. But given that the flux is substantial, we can hope to detect these particles by building detectors that are large enough. A larger detector volume means more dark particles passing through and hence a higher probability of collision.

Which automatically brings us to question two. How can we be sure that the cause of a signal in our detector is a dark matter-target atom collision. The short answer to this is we’re extremely meticulous. For every experiment, there is a ‘background’ that we need to be aware of. A background is basically “noise” which makes it hard to see the actual signal. In this case, any other particle colliding with the target would give rise to a background signal. A major part of these experiments is to account for all possible backgrounds and reduce this noise as much as possible. The first step in doing this is to build the detector underground. The Earth’s crust provides a natural shielding against most of the stray particles hanging around in our corner of the universe. Beneath that are layers of concrete to further stop any particles getting inside the detector. Note that since the dark matter particles are incredibly weakly interacting, they have no problem shuttling through the Earth’s crust or any of our other protective layers. Another important factor is the choice of the target atom itself. Since we want to avoid spurious signals, an atom that decays radioactively or is otherwise reactive would be a poor choice for the target. Some of the best target atoms are inert gases such as Xenon or Argon.

The Xenon 1T detector located in Italy. The tank (left) is filled with liquid Xenon. For scale, a three-level office complex on the right. (Source)

This is (very briefly) how a conventional direct detection experiment works**. We hide out in the depths of Earth, waiting. The obvious follow-up question is what happens if we don’t see a signal. Does it mean the dark matter paradigm is dead in the water (or liquid Xenon)? The answer is Not Quite. The lack of a signal is valuable information as well. For example, it can be used to infer that the dark matter-target interaction is weaker than we thought (meaning the probability of a collision is even smaller resulting in the absence of a signal). In this way, we can use the “null results” of an experiment to set limits on the strength of the dark matter — standard model interaction. So even though we don’t “detect” dark matter, we end up with more knowledge than we started with. And that, in the end, is the true spirit of science.

Up next: Can we make dark matter at colliders?

*The assumption that dark matter is composed of particles is well-motivated but it might very well be that dark matter is something more exotic such as primordial black holes. 
**Detectors operate on the same principle (some kind of dark matter — particle collision) but can be experimentally realised in different ways. 

2 responses to “Looking for Dark Matter: Trembles beneath the surface”

  1. Paul Hess says:

    Great article, thanks Saniya!

    Quick question: When we say we can know or measure the amount of dark matter flux, is it a statistical average of all dark matter in our (galaxy?), or is it a number specific to what we think or can actually measure that is happening here on earth?

    • Saniya Heeba says:

      Excellent question! We “know” the local dark matter flux by studying the motion of stars in our Galaxy and building detailed models which tell us what the dark matter distribution should look like. There is a small subtlety as to what happens here on Earth. Although the dark matter flux is constant, the relative velocity of incoming dark particles changes with the motion of Earth. Larger velocities mean more energetic collisions and so stronger signals. We account for this variation when we calculate the number of collisions we expect to see.

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