RWTH Aachen Particle Physics Theory

Schlagwort: ‘physics’

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. 

TTK Outreach: A Universe of Possibilities Probabilities

January 30th, 2018 | by

The universe may not be full of possibilities –most of it is dark and fatal– but what it does have in abundance are probabilities. Most of us know about Newton’s three laws of motions. Especially the third which, taken out of context, apparently makes for a good argument justifying revenge. For centuries, Newton’s laws made perfect sense: an object’s position and velocity specified at a certain time gives us complete knowledge of its future position and velocity aka its trajectory. Everything was neat and simple and well-defined. So imagine our surprise when we found out that Newton’s laws, valid as they are on large scales, completely break down, on smaller ones. We cannot predict with 100% certainty the motion of an atom in the same way that we can predict the motion of a car or a rocket or a planet. And the heart of this disagreement is quantum mechanics. So today let’s talk about two of the main principles of quantum mechanics: duality and uncertainty.


new doc 2018-01-28 16.38.08_1We begin with light. For a long time, no one seemed to be quite sure what light is. More specifically, we didn’t know if Light was a bunch of particles or a wave. Experiments verified both notions. We could see light interfering and diffracting much like two water waves would. At the same time, we had phenomena such as the photoelectric effect which could only be explained if Light was assumed to be made of particles. It is important to dwell on this dichotomy for a bit. Waves and particles lie on the opposite ends of a spectrum. At any given instant of time, a wave is spread out. It has a momentum, proportional to the speed with which it is traveling, but it makes no sense to talk of a definite, single position of a wave by its very definition. A particle, on the other hand, is localized. So the statement, ‘Light behaves as a wave and a particle’, is inherently non-trivial. It is equivalent to saying, ‘I love and hate pineapple on my pizza’, or ‘I love science fiction and hate Doctor Who.’

But nature is weird. And Light is both a particle and a wave, no matter how counter-intuitive this idea is to our tiny human brains. This is duality. And it doesn’t stop just at Light. In 1924, de Broglie proposed that everything exhibits a wave-like behavior. Only, as things grow bigger and bigger, their wavelengths get smaller and smaller and hence, unobservable. For instance, the wavelength of a cricket ball traveling at a speed of 50km/h is approximately 10-34 m.

And it is duality which leads us directly to the second principle of quantum mechanics.


The idea of uncertainty, or Heisenberg’s Uncertainty principle, is simple: you can’t know the exact position and momentum of an object simultaneously. In popular science, this is often confused with something called the observer’s effect: the idea that you can’t make a measurement without disturbing the system in some unknowable way. But uncertainty is not a product of measurement, neither a limitation imposed by experimental inadequacy. It is a property of nature, derived directly from duality.

From our very small discussion about waves and particles above, we know that a wave has a definite momentum and a particle has a definite position. Let’s try to create a ‘particle’ out of a wave, or in other words, let’s try to localize a wave. It’s not that difficult actually. We take two waves of differing wavelengths (and hence differing momenta) and superimpose them. At certain places, the amplitudes of the waves would add up, and in others, they would cancel out. If we keep on adding more and more waves with slightly differing momenta, we would end up with a ‘wave-packet’, which is the closest we can get to a localized particle.

Screen Shot 2018-01-28 at 4.45.06 PM

                                                                                      Image taken from these lecture notes.


Even now, there is a small, non-zero ‘spread’ in the amplitude of the wave-packet. We can say that the ‘particle’ exists somewhere in this ‘spread’, but we can’t say exactly where. Secondly, we’ve already lost information on the exact momenta of the wave and so there is an uncertainty there as well. If we want to minimize the position uncertainty, we’d have to add more waves, implying a larger momentum uncertainty. If we want a smaller momentum uncertainty, we would need a larger wave-packet and hence automatically increase the position uncertainty. This is what Heisenberg quantified in his famous equation:

Δx Δp ≥ h/4π

And so we come to probabilities. At micro-scales statements such as, ‘the particle is in the box’, are meaningless. What we can say is, ‘the particle has a 99% probability of being in the box’. From Newton’s deterministic universe (which is still valid at large scales) we transition to quantum mechanics’ probabilistic one where impossible sounding ideas become reality.

The Doctor once said, “The universe is big, it’s vast and complicated, and ridiculous. And sometimes, very rarely, impossible things just happen and we call them miracles.” Or you know, at small enough scales, a manifestation of quantum mechanics. And that is fantastic.