the dark side of the force


Luca Mazzucato


An interview with Itay Yavin. The Israeli physicist, now at McMaster University in Canada, obtained his PhD at Harvard under the supervision of Nima Arkani-Hamed. He is a leading expert in physics beyond the Standard Model and dark matter.

Four-fifths of all the matter in the Universe has never been observed: it is the dark matter. Physicists are hunting this new form of matter and the current experiments give seemingly contradictory results. According to Itay Yavin, of the Center for Cosmology and Particle Physics, NYU, we can find the dark matter just where we least expect it: in the Sun!

You work on the physics of dark matter. Could you explain what dark matter is and which convincing clues we have about it?

In the same way that our Earth rotates around the Sun, our solar system rotates around the center of our galaxy. We can also observe that movement in the rotation of other stars around our galaxy. As we know since the times of Newton, the force responsible for this rotation is gravity. The force is stronger when the center of rotation has more mass. In the case of the Earth motion around the Sun, the Earth orbit allows us to estimate the mass of the Sun.

Similarly, by observing the motion of stars around our galaxy astronomers can weigh our galaxy, that is to say, they can determine the mass of the galaxy. But, and that’s when things get interesting, they can also weigh the galaxy by essentially counting all the stars inside the galaxy from their light output. Since they have a good notion of how much a star weighs depending on its luminosity which they can observe, they can figure out what is the total mass.

The surprising fact is that these two independent ways of measuring the mass of the galaxy do not agree. Counting the stars results in a lower estimate for the mass of the galaxy. Furthermore, astronomers are very capable people and they measured the mass of other galaxies using these two methods, and they find that same disagreement in many places.

One possibility, which after many years of research seems the most consistent possibility, is that there is some amount of massive matter out there in the galaxy which we cannot see, i.e. dark matter. This will resolve the paradox because when counting the stars astronomers will fail to account for dark matter since they cannot see it directly with their telescopes. But, since it is massive, it influences the motion of stars through its gravitational pull. This has been known for some decades now, but the nature of dark matter is still a mystery. Our physical theory of fundamental matter and forces does an incredibly good job in explaining all the phenomena we see on the Earth and most of the phenomena we see in the universe, but fail to account for the existence of dark matter. This is a singular failure, which has triggered a lot of research trying to understand what is dark matter.

In recent years several new pieces of evidence were found that corroborate the existence of dark matter in the universe. They provide us with good measurements of the amount of dark matter both locally in our galaxy and globally in the observable universe which includes billions of other galaxies. Knowing how much dark matter exist, both locally and globally, is extremely useful when trying to unravel its nature.

Dark matter has been an astronomy issue so far. Why do particle physicists look at it now?

Particle physicists are interested in the basic constituents of matter and the forces that govern these building blocks. There are good reasons to believe that dark matter is some new basic form of matter and as such is of interest to particle physicists. One of the most basic questions we would like to understand is whether or not dark matter interacts with any of the forces we are already familiar with. We know that it certainly interacts with gravity, but does it interact with the electromagnetic force, or the weak force? It cannot interact very strongly with the electromagnetic force or it wouldn’t be dark and we would see it. But, it can interact with the weak force for example, which is a short range force felt by protons, neutrons, electrons, and many of the other particles we know of.

As particle physicists we like to collide elementary matter together in an attempt to probe its interactions. So, for example, we will construct an electron beam, accelerate it to high energy and then direct the beam towards some target made of protons. By observing collisions, we try to learn about the interactions between electrons and protons. This teaches a lot about the electromagnetic forces between these particles. Ultimately, such experiments lead to the discovery that the proton has a substructure and is actually made of what we call quarks. So, colliding things is very useful if you want to learn about them.

Unfortunately, we can neither produce a beam of dark matter nor use it as a target. So what can we do? As I mentioned above, we know something about the amount of dark matter mass in our local surroundings. So we can place some amount of regular matter in the lab and monitor it very carefully and patiently. If dark matter interacts with normal matter sufficiently strongly it will collide with the nuclei in the target every once in a while. Experimentalists are very clever and they can observe nuclear recoil events, although it is not easy. If you now use a lot of matter, say 100 kg, and wait for many days, you may begin to observe sufficient number of such events. Now, this may sound crazy, but this is in fact how we observe neutrinos coming from the Sun. Such measurements revealed, among other things, that neutrinos have mass. That was one of the most important discoveries in the past decade. So, you can learn a lot from such experiments, even if they sound difficult.

You wrote a paper about the effects of dark matter in the sun. Isn’t this a paradox?

That sounds paradoxical because the Sun is so bright and dark matter is, well, dark. So, where is the connection? We just saw that if we had a large enough target, we can hope to observe dark matter colliding with the nuclei in that target. Well, one very large target near us is the Sun. Our Sun moves through the Milky Way galaxy and every once in a while, dark matter moving through the Sun will collide with nuclei inside it. Since the Sun is so large the collision rate is actually sizable. But, clearly, we cannot observe these collisions even if they did happen, so what good is this? The important realization that people had was that there is a good chance for dark matter to become gravitationally bound to the Sun after such a collision. It will keep orbiting the Sun crossing its interior many times over the Sun’s lifetime. With every such passage it might collide further with matter and keep losing energy. After many such collisions dark matter loses enough energy to sink into the center of the Sun where it accumulates. What happens next is related to another property of dark matter which we are still not sure about, but we can make some educated guesses.

If it accumulates sufficiently, dark matter in the core of the Sun may begin to annihilate. Let me explain. Normal matter, even at very high densities, does not annihilate. But, if you densely pack matter together with anti-matter they will annihilate each other. We even use this fact in medical instruments now where we observe the annihilation of electrons and positrons into light rays in PET scans. Dark matter could very well be made of equal quantities of matter and anti-matter. In the same way that we place detectors around patients in PET scans to detect the electron-positron annihilation products (namely light), we can place detectors on Earth to try and measure the annihilation products of dark matter in the Sun. There is no guarantee that such a search will yield any positive signal. First, dark matter may not be captured at sufficient rates by the Sun. Second, dark matter may not annihilate. Third, even if it is captured and annihilates, the annihilation products may not be detectable on the Earth. This is a long shot, but if such a signal is observed, it will be an unequivocal evidence for dark matter interacting and annihilating in the Sun.

What are the future frontiers of dark matter physics?

First, it is important not to lose sight of the fact that we continue to learn about the properties of dark matter through astronomical observations. Traditional observations of the gravitational effects of dark matter on galactic rotations are now available for many more galaxies and have taught has a tremendous amount about the existence of dark matter in many different types of galaxies. New observations, such as the Bullet Cluster observation, has helped us to further constrain alternative theories to the dark matter paradigm. The Bullet Cluster is a system of two colliding clusters of galaxies and astronomers have succeeded in mapping out both the luminous matter distribution as well as the total matter distribution. It shows a clear separation of the two distributions. Luminous matter, mostly made out of gas, is attenuated in the collision, while the non-luminous component simply sails through. Now, my language implies dynamics, but really it is a frozen image and the dynamics is simply inferred. Nevertheless, it lends credence to our conception of dark matter as weakly interacting particles.

The astronomical observations, as varied as they may be, have only taught us about the interaction of dark matter with normal matter through the gravitational force. To better understand what it is made of, we would like to observe other forces it may share with normal matter. There are essentially three different ways in which we might do it and there are now experiments operating worldwide exploring these approaches. The first one, which I already mentioned above, is to try and observe collisions of dark matter with normal matter in the lab. This happens very rarely, if at all, but if you place enough matter there is a decent chance you might just see such an event. Such experiments are usually referred to as direct detection experiments because they aim at directly detecting dark matter in the lab.

These experiments are difficult because many mundane things, like radioactivity, which have little to do with dark matter, may mimic such collisions. These are what we call background events. Like the background in a family photo, it might be interesting, but it’s not the reason why we took the photo. We want to make sure we can see the family, so to speak. The experimentalist task then is to make sure background events are kept at a minimum and events of dark matter collision with normal matter can be clearly identified.

So far, the results are ambiguous with one experiment claiming detection and other experiments claiming exclusion (i.e. these experiments have not seen anything and so they can place limits on the strength of the forces between dark matter and normal matter). A healthy motto to adhere to is that “extraordinary claims demand extraordinary evidence” and so we should wait with the final verdict until more data is collected and the picture clarifies. In the meanwhile we try to understand whether these different results may still be consistent with each other and what it might tell us about the nature of dark matter. This is what makes this field interesting, there are still questions to be answered and puzzles to solve.

The second avenue, known as indirect search for dark matter, was also mentioned above when we discussed the possibility of observing the annihilation of dark matter in the Sun. If observed, such annihilations will constitute evidence for dark matter interactions with forces other than gravity (for example the electroweak force), since gravity is simply too weak to result in efficient annihilation. Above, we explored the possibility of dark matter annihilations in the Sun, however, this is not the only possibility. People have long realized that if dark matter does indeed self-annihilate it can do so in space, in our nearby galactic neighborhood. Such annihilations are extremely rare events per unit volume, but since we can observe huge volumes of space there is a good chance to observe the annihilation products in the form of intense gamma radiation, high-energy electrons and positrons as well as protons and anti-protons.

Dedicated satellites have been launched to search for such radiation and recent results suggest that such energetic particles are indeed observed above what was expected from known astrophysical sources. The problem is that our “expectation” suffers from large uncertainties and it is not clear that the excess observed is really due to dark matter annihilation. Again, we’ll need more data and more experiments before the dust settles.

The third avenue people are exploring is the possibility of dark matter production in particle accelerators. In the same way that dark matter might annihilate into matter, so can matter annihilate into dark matter. This is the business of high energy particle accelerators. This is how we discovered much of the zoo of elementary particles known today. We collided beams of electrons and positrons to produce quarks, muons, taus and all these interesting particles that make our Standard Model of particle physics. We have learned a great deal about the universe through such experiments. We have the hope that we can similarly learn a great deal about dark matter through such experiments. So far there is no evidence for the production of dark matter in accelerators, but the new Large Hadron Collider accelerator in Geneva might change this state of affairs soon.

There are many “maybes” in the last few paragraphs. This is unavoidable since we know so little about the nature of dark matter. We nevertheless try to use educated guesses to guide our searches. But, an educated guess is still a guess and we recognize that surprises may be in store for us. That is what makes fundamental science an adventure.

What is the timetable for discoveries?

That is difficult to answer because there is no guarantee that dark matter is affected by any of the forces we know of. Its interaction with normal matter, like electrons and protons may simply be too weak to observe. That would be unfortunate because it would mean that we will not learn anything about it beyond its gravitational effects on stars and galaxies. We will not know how it fits in with the other elementary particles we are familiar with. That will be a sad state of affairs. Since most of the matter in the universe is dark and only about 20% is normal matter it would mean that we will stay ignorant about most of what the universe is composed of.

That said, the experiments described above push into regions that have never been explored before. There are good reasons to believe, although we cannot be certain, that dark matter interacts strongly enough to begin revealing itself in direct detection experiments in the next few years. If seen, it will be an incredible triumph for many of our ideas about dark matter, not the least of which is the fact that it is a particle. It will enable us to measure its mass and allow us to learn something about the forces it feels.

What is the dark sector of the universe?

Before answering these questions, let’s try to give a simple answer to the question, what could dark matter be? It could be a single massive particle which interacts with normal matter only through the weak force. We already have one example of such a particle in nature, namely the neutrino. The neutrino is a very light particle with weak interactions to electrons and protons whose existence was experimentally established in the 1950’s. Can the neutrino be dark matter? We do not think that neutrinos constitute dark matter, because from everything we can tell about the role of dark matter in the formation of structures in the universe we can conclude that neutrinos are not massive enough to serve that role. But, dark matter could be some heavy version of neutrinos, a single particle with very weak interactions to normal matter. Much research has gone into this possibility of dark matter being a Weakly Interacting Massive Particle, or in short a WIMP. Unfortunately, as simple as this idea is, it does not enjoy any experimental support. There is certainly no experiment ruling out this possibility yet, but all the recent new observations mentioned above, if connected to dark matter at all, point us towards a somewhat more complicated picture.

The dark sector is the idea that the WIMP we just discussed could be a part of a larger set of particles, all with very weak interactions to normal matter. For example, there could be an additional force associated with the dark sector, a force that the WIMP feels. Such a force, if it exists, has many interesting ramifications and it is now being searched for on several fronts. The theories describing such a dark sector are fairly simple and result in concrete predictions for all the different type of experiments we mentioned above. It prompted us to think more carefully about the results of these experiments and helped stir fruitful discussions and collaborations. This is a healthy and enjoyable situation where experiments trigger us to reexamine and modify our theories which in turn motivates new experiments and searches.

As for expectations, it is difficult to state confidently because expectations arise from experience and we have little experience with dark matter. But, from our acquaintance with normal matter it is fair to state that having additional forces associated with dark matter is a reasonable expectation. Almost every particle we discovered led to new insights about the forces that govern its dynamics. I am making an artificial distinction between particles and forces which is not really appropriate when discussing elementary particles. But, our intuitive understanding of forces as capable of pushing particles together and pulling them apart are sufficient. So, if our experience with normal matter has any relevance for understanding dark matter, I’d say we could expect to discover new forces associated with dark matter. But, there are no guarantees and that is what makes scientific research so interesting.