Resident Expert: The Dark Matter Mystery

A purple haze shows dark matter flanking the 'Bullet Cluster'. NASA

Dark Matter is one of the biggest scientific mysteries of our time. Most of the universe is made up of it, but we still don't know its nature. What is the strange, unknown, invisible matter that holds the universe together? Why is it so hard to detect?

Francesco Arneodo, Visiting Professor of Physics and head of NYU Abu Dhabi's Physics Department, has been studying Dark Matter for the better part of two decades, and is a collaborating researcher on a historic Dark Matter experiment taking place in the world's largest underground laboratory in Italy. We sat down with this NYUAD Resident Expert to talk about how scientific understanding of Dark Matter is progressing and why the search for Dark Matter continues to challenge our planet's top researchers.

What is dark matter?

Essentially, we don’t know yet. All we know is that there is in the Universe much more mass than what we observe with standard astronomical techniques. There are several evidences for that, the most important and easy to explain is that spiral galaxies (like ours, the Milky Way) rotate as they had about 70% more mass than what you see in the disk. Imagine that a disk-shaped, spiral galaxy had a invisible, spherical halo that completely surrounds it. We measure the gravitational effects of the halo, but we don’t actually `see' it. In the case of the Milky Way, we are immersed in it. There are several models that try to explain Dark Matter. The most accepted explanation is that it is made of particles, with a mass of about 100 times a proton, but with an extremely low probability of interacting with ordinary matter, which makes them extremely difficult, if not impossible, to detect. We call them WIMPS (Weakly Interacting Massive Particles).

Why are scientists so interested in dark matter?

Scientists are always interested in things they do not understand. They are interested in anomalies. When everything is understood, scientists get bored. That said, Dark Matter is one of the biggest mysteries of modern science.

Understanding its nature will lead to a deeper understanding of the Universe and of particle physics. So far, there is an accepted model that describes the behaviour of subatomic particles, the so called `Standard Model’, which works extremely well. The discovery of Dark Matter in form of new particles would represent a deviation from the Standard Model, and is therefore extremely exciting.

How is the scientific community's understanding of dark matter changing?

The theoretical work on Dark Matter has been extremely intense in the last years. It would be impossible to summarize it in a few lines. However, there is one aspect that is worth mentioning. The identification of Dark Matter with WIMPs was partially justified by one specific extension of the Standard Model, called Supersymmetry (SUSY). SUSY is a very nice model that theoretical physicists like for several reasons. Among its predictions is that there is the existence of a particle called “neutralino", with the right characteristics to be a WIMP. If SUSY is true, the big accelerator LHC at CERN should at some point `see’ something, but so far there hasn’t been any hint of it. The search is far from being over. LHC has just started its new run at full energy, but the likelihood of SUSY being a valid model has somewhat shrunk in the last couple of years. We have to wait and see, but it might be that SUSY is not the right explanation and another model will explain the Dark matter problem. For example, there is another candidate particle, called `axion’, that hasn’t been excluded so far.

It’s like looking in a very big and dark room with a weak, but increasingly stronger, light source. Once we illuminate one part of the room, and we find nothing, we exclude that part. But we need more powerful instruments to look in the remaining, further away, parts.

Francesco Arneodo, Visiting Professor of Physics

Do you think we need to keep learning more about what dark matter isn't, to find out what it is?

Absolutely. In fact, present Dark Matter experiments work by exclusion. We build instruments to be more and more sensitive, like the XENON project in which I am involved, or the LZ project in US, and others. So far, no experiment has a credible signal. Because of this, some models are wiped off the blackboard, and the mass and other characteristics of the WIMPS become more constrained. It’s like looking in a very big and dark room with a weak, but increasingly stronger, light source. Once we illuminate one part of the room, and we find nothing, we exclude that part. But we need more powerful instruments to look in the remaining, further away, parts. The problem is that at some point we will have to stop building bigger and more sensitive instruments, due to technical and economical constraints. And we might leave some parts of the `room’ unexplored.

What are some of the major constraints on dark matter research?

There are several ways to look for Dark Matter, and they fall mainly in two categories: direct and indirect. I'm involved in direct research and the main challenge is represented by one word only: background. Any kind of radioactive background or `noise’ would kill our search. This is why direct searches for Dark Matter have to be carried out in an underground environment because on the Earth’s surface there are hundreds of charged particles, the cosmic rays, that cross a square meter in any given second. It would be impossible to detect any rare signal, like the one given by the tiny interaction of one WIMP and a nucleus, in such a noisy environment. As one Italian scientist, the founder of the Gran Sasso laboratory, put it, it would be “like trying to hear a distant violin note in a crowded stadium where a goal has just been scored”. This is why we have to be underground; only kilometres of rock can protect us from cosmic rays. And it’s not even enough. We have to further shield our detectors with layers of lead, copper, polyethylene, and water to stop any residual radiation to penetrate the detector. Also, the materials that go into its construction have to be carefully chosen for their low radioactivity. To give an idea of the rarity of the signal, we want to be sensitive to even one single collision between a WIMP and a nucleus in one ton of target material in one year. It is incredibly difficult.

Why are scientists looking for Dark Matter from a lab under a mountain?

The Gran Sasso laboratory in Italy is the biggest underground laboratory in the world: about 200,000 cubic meters, divided in three big halls, under 1,500 meters of rock, beside a highway tunnel in the center of Italy. There are several experiments of astrophysics and particle physics there (actually there is a new term to describe our research, Astroparticle Physics). One of them is the XENON100 experiment, that has been running for several years now and has been the most sensitive instrument in the world for long time. It makes use of a target of about 160 kilograms of liquefied xenon, at the temperature of about -90 C. It is based on the fact that any interaction of particles inside the liquid, such as the collisions between WIMPs and xenon nuclei, give origin to a very tiny flash of light (scintillation), that is detected by hundreds of sensors called photomultipliers. By a very smart technique it is possible to disentangle the WIMPs interactions (if any) from the residual background that still survives after all the shielding and cleaning of the detector’s materials.

XENON100 is approaching its end and we are now about to commission its upgraded version, called XENON1T, that will begin taking data in 2016. That one will make use of three tons of liquid xenon, and its background will be 100 times less than its predecessor, making it the most sensitive Dark Matter experiment ever. Since few months ago, NYUAD is also part of this big international effort, thanks to the support given to my experimental research group. My group is involved in the data analysis, computing, and purification system that handles the 700 tons of water that surround the liquid xenon detector. I add with great pleasure that several NYUAD students have been and are involved in the project with great success.