Discover Interview The Dark Hunter

Physicist Elena Aprile is certain that dark matter exists. She just hasn’t found it yet.

by Fred Guterl

Dark matter sounds like some physicist’s tall tale: There’s this invisible matter, see, and it has this powerful gravitational effect on galaxies. That’s why we know it exists. In fact, it outweighs ordinary matter by about five to one. Problem is, dark matter doesn’t reflect or absorb light, so we can’t see it. Oh, and it rarely interacts with conventional atoms, so we can’t feel it, either. However, we know it makes up a huge part of the universe, so we keep looking for it.

As mind-bending (and perhaps logic-challenging) as these ideas may seem, a lot of physicists are searching for this elusive matter. Elena Aprile is one of the leading lights in this dark business. She heads a prominent dark matter experiment called Xenon, which is based 5,000 feet underground in Italy’s Laboratori Nazionali del Gran Sasso, one of the world’s largest subterranean physics labs. Aprile, who is also a codirector of Columbia University’s Astrophysics Lab, started the project in 2007 with a detector called Xenon10. Since then she has upgraded to the more sensitive Xenon100. “I feel proud to have one of the best instruments in the field for detecting dark matter,” Aprile says of Xenon100. But the huge questions remain: What is dark matter, and how close are scientists to finding it? Aprile recently updated DISCOVER on how things are going in her search for the missing majority of the universe.

Seriously—what is dark matter? 

The best answer is that we have no idea. We know dark matter is there. We’ve known it for more than 70 years. There was a 1933 paper by the Swiss astronomer Fritz Zwicky showing that visible matter is only a small fraction of the universe. Just 18 percent of the matter in the universe is composed of the stuff we know. The remaining 82 percent is what we call dark matter. Other discoveries in astronomy have since reinforced this view that something is missing. We know dark matter is there, but only from its gravitational effects. For example, the presence of dark matter helps explain why our galaxy is stable. The Milky Way is a disk that rotates like a merry-go-round. The question is, what keeps it from flying apart? Gravity, of course, but there is not enough visible matter in the galaxy to account for the amount of gravity needed to hold it together. That’s why we know that there must be other matter there that we can’t see.

What is dark matter composed of? 

We think it’s made of a type of particle that doesn’t like to interact with normal matter [protons, neutrons, and other types of particles] very often. And it’s very heavy, very massive. Perhaps as heavy as an entire lead atom or even heavier. It’s probably a relic particle from the Big Bang, a member of a family of particles that we’ve named weakly interacting massive particles, or WIMPs.

How do we know dark matter consists of some new kind of particle? 

Actually, we might be on the wrong track in thinking that dark matter is composed of a fundamentally new type of particle. That’s why we call it “the WIMP miracle.” The so-called standard model of particle physics, which lays out the way physicists think the universe works, has deficiencies. A lot of things, a lot of data, don’t fit. We have theories, such as supersymmetry and extra dimensions, that have been put forward to explain the things that are missing from the standard model or that don’t fit with the data we get. Some of the particles predicted by those theories are natural candidates to be dark matter because they have all the right characteristics. A particle called the neu­tralino, for instance, is a type of WIMP that’s a perfect candidate for dark matter in part because it doesn’t interact with other particles much, and that would explain why nobody has yet detected it.

If WIMPS don’t interact much with other particles, how can you find them? 

The way we go about this search is to wait for a particle of dark matter to come into contact with our device, which is basically a pot of liquid xenon [an element that is used, in gas form, in the very bright headlights of many new cars] sandwiched between two detectors. We use xenon because it is one of the heaviest elements—meaning that each atom contains a lot of protons and neutrons—and that increases the odds that dark matter will interact with it. Whenever that happens, whenever a WIMP gets stuck in there, the xenon displays some remarkable properties. There will be a flash or scintillation of ultraviolet light. You can’t see it with the naked eye, so to detect this light, we have 178 extremely sensitive one-pixel cameras, called photomultipliers, above and below the liquid-xenon-filled detector. We also look for an ionization signal: If a dark matter particle rubs against a xenon atom, there will be electrons liberated and a charge produced. Those electrons drift upward through the liquid xenon to a positively charged anode [electric terminal], which produces a second flash of light that the cameras will detect.

That signal would tell you that a WIMP has finally made contact? 

Well, we can extract a wealth of information from those two signals, including the speed of the particle, the location of the interaction, and the type of particle it was—an electron, a neutron, or dark matter. The more gently the particle touches the xenon, the more likely that it’s a WIMP.

But you haven’t definitely found one yet? 

No. I mean, it constantly happens that you look at something and say, “Hey, what’s this?” and you think it could be dark matter. But we have always found an explanation for these events. Still, it’s important to consider the possibility that we might actually be looking right in the eye of dark matter.

How close are we to finding dark matter? Have others had hits, or possible hits? 

There was news in December (article; live-blog) that another group of researchers, the Cryogenic Dark Matter Search (CDMS), had detected dark matter in a mine in Minnesota, but they saw a very weak signal. They recorded two events that they cannot fully explain as background noise. One of the events is very close to the threshold of noise. It’s not a detection; the collaboration itself doesn’t call it that. It’s the hint of a detection. There was initially a lot of excitement, but that has died down.

As for our group, we have collected data for several months now with Xenon100 and will continue through the summer. This powerful detector has the lowest background noise ever measured for any dark matter detector, and it is the largest-scale detector in operation. If the signal CDMS found was truly from dark matter, we’ll easily be able to confirm it this year. At the same time, particle physicists are looking for dark matter with the Large Hadron Collider in Geneva. We’re hoping they can tell us more about these particles in the next few years.

What is it like to search for something that you may never find? 

It feels very exciting and almost like a duty. The fact that we don’t know if we will discover dark matter does not take away the necessity to try. The Italian particle physicist Carlo Rubbia, who was my doctoral thesis adviser at the University of Geneva, recently quoted Galileo at a conference on dark matter: Provando et riprovando—“Try and try again.” This is the basis of experimental science. We must try and try again to find the truth. If we stop because there is no guarantee that we will find anything, then we would never find anything again. In fact, in terms of dark matter, not finding anything is extremely important because it will make us find new roads to explore. We must keep searching for it with the best tools we have.