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When Will We Find Dark Matter?

One of the most fundamental but elusive constituents of the cosmos could soon be cornered
 


This Fermi map of the Milky Way center shows an overabundance of gamma-rays (red indicates the greatest number) that cannot be explained by conventional sources.
T. Linden, Univ. of Chicago

Scientists are playing a dogged game of hide-and-seek with one of the universe’s most plentiful components: dark matter. So far, dark matter continues to hide as scientists still seek. No one knows what comprises this invisible form of matter, but a leading candidate is a type of particle called a WIMP (weakly interacting massive particle). WIMPs are appealing because although they themselves neither radiate nor reflect light, they might produce other particles that do.
 
WIMPs might be the same as anti-WIMPs, meaning that they are their own antimatter counterparts. If this is the case, when two WIMPs collide they would destroy each other, as matter and antimatter do on contact. Such annihilations would probably produce detectable evidence in the form of high-energy photons called gamma rays. By identifying these gamma rays, scientists might be able to indirectly detect dark matter and find out what it’s made of.
 
Taking this tack is NASA’s Fermi Gamma-Ray Space Telescope, an Earth-orbiting satellite that launched in 2008. Scientific American spoke to Fermi collaboration member Stefan Funk, an astrophysicist at the SLAC National Accelerator Laboratory in California, about the trials of searching for dark matter and the prospects of finding it anytime soon.
 
[An edited transcript of the conversation follows.]
 
 
Have we seen any signs of WIMP annihilation yet?
Indeed, there are some signs that we are seeing gamma rays from dark matter annihilation, although it should be said that all these signs have to be taken with a grain of salt. The fact that we see gamma rays from a particular place doesn’t necessarily mean they are produced by dark matter. Other processes can also produce gamma rays. We have to be very careful when we look at these signals to be able to rule out all other explanations.
 
But still I would say there are two quite exciting signals that have basically come out within the last two or three years, both from the Fermi Large Area Telescope. Both come from the region which surrounds the center of our galaxy. One is what we call a line—a signal of gamma-rays that all have a certain characteristic energy, 130 GeV [giga-electron volts]. This is what we saw at the galactic center with a somewhat low significance. Unfortunately, with more data this seems to have been a combination of statistical fluctuations and some systematic effect in our detector. If that signal had increased, this would have been pretty clear evidence that it was a true signal and would have been very clear evidence for dark matter annihilation.
 
The second thing that people are very excited about is also from the galactic center region. At low energies, there is not a monoenergetic line but a broader spectral feature. There’s a range of energies within that signal but this signal is hard to explain with anything we know from the center region of our galaxy in terms of traditional astrophysical sources. This signal is not a statistical fluke—it’s a very significant signal.
 
Is it a problem that this isn’t a line signal, but a range of energies?
Depending on the way these particles annihilate, they either produce a monoenergetic signal or they produce a range of energies. Of course it would be much nicer to see a signal that is a monoenergetic line, because that’s more easy to rule out astrophysical sources for, but nature might not be so kind. There’s also a way for these particles to annihilate that produces a range of energies.
 
There is certainly an excitement about this second signal now but of course there’s also caution, because it might have a different origin than dark matter annihilation.
 
Besides the galactic center, where else do you search for signs of dark matter annihilation?
We look at certain objects that we know have a high dark matter density, for example satellite galaxies to our own galaxy. So far, there’s not a signal that we see. There’s a small signal at low energies that has a similar energy to the range from the galactic center but I would say this is certainly still within our systematic uncertainties. There’s a small deviation from what we expect if there was no signal but I wouldn’t make a big deal out of it.
 
What’s needed to know for sure if what Fermi’s seeing is dark matter?
More data is certainly needed. We also are significantly improving our reconstruction of the gamma rays within our detector. This will clarify the signal in the dwarfs [satellite galaxies], and will also help us better understand the properties of the signal in the galactic center.
 
In the galactic center there’s the complication that there’s a lot of gas—and gas also creates gamma rays. So there’s always the problem of foreground emissions. To improve that just means we need to improve our understanding of the galactic center region in general, and that is a hard thing to do. More data will help, but only up to a certain point.
 
What about other indirect dark matter searches, like the Alpha Magnetic Spectrometer (AMS) experiment on the International Space Station?
When dark matter particles annihilate, there are various ways to annihilate, and one is to produce charged particles such as positrons [the antimatter counterparts to electrons]. So looking for gamma rays is not the only way to look for dark matter annihilation. Gamma rays travel in straight lines once they are produced, so by determining the direction from which they come we can know where the interaction that produced the gamma rays took place. This is very helpful.
 
With charged particles, which is what AMS is looking for, it is much more difficult because you don’t know where these particles come from, because they are deflected by magnetic fields on their way to us. Nevertheless, instruments like AMS and PAMELA [for Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics] have shown that there is an anomalous positron abundance arriving here at Earth. But it’s a little bit similar to the case of the second gamma-ray signal in the galactic center—it’s very, very hard to rule out an astrophysical origin for the positrons.
 
How can we tell if this is a true sign of dark matter?
What AMS wants to do is measure exactly how does this positron fraction cut off at the highest energies. That might hold some clues and might allow us to distinguish between astrophysical models and dark matter models. But that is a very hard thing to do.
 
There are also other people who are trying to use particles called antideuterons, which also would get produced in dark matter annihilations but wouldn’t get produced typically in astrophysical sources. This is done with balloon experiments but people haven’t been able to detect any so far.
 
Do you think we’re getting close to detecting dark matter for sure?
If the dark matter is really a WIMP, then I think the chances are pretty good that we’ll see it soon either through direct detection experiments or through indirect detection—or if we start to see supersymmetry with the new run at the Large Hadron Collider. If dark matter is not a WIMP, then this could be going on for a long time. Then the prospect of detecting it is much more difficult to evaluate.
 
It’s absolutely an exciting time to be doing physics. The exciting thing is that we know so much about the universe. We know dark energy exists, we know dark matter exists. We know that it comprises, in the case of dark matter, 80 percent or so of all the matter that exists—but we still don’t know what it is. What I also like is that different communities are coming together to try to tackle the problem in different ways. I hope that within my lifetime all of these efforts will bear fruit.
 
 

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