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2014 Annual Meeting: Finding Dark Matter with Light

It has long been hypothesized that a large portion of the universe is comprised of dark matter, although it remains elusive and unseen. Through its gravitational interactions with stars and gas, we can deduce that galaxies are swaddled in vast clouds of the stuff. Unfortunately, its tendency to interact very weakly, if at all, with ordinary matter makes it extremely difficult to examine experimentally. It's never been observed any way other than through its gravitational pull. 

Many of the more popular theories about the detailed nature of dark matter do offer some hope that we can learn more about it. Very rarely, individual dark-matter particles floating in the vast spaces between stars may produce light by decaying or colliding with other particles of dark matter. By observing the energy of these extremely high-energy photons, called gamma rays, we can gain information about the fundamental nature of dark matter, such as its particle mass and interactions with ordinary matter.

AAASMC's Chris Spitzer spoke with Karen Byrum of Argonne National Laboratory, about her Annual Meeting talk, titled, "New Results from Cosmic Gamma Rays," which was part of the symposium on Extremities of the Cosmos: New Experimental Results in Particle Astrophysics, on Saturday, Feb. 15. Byrum works on the VERITAS gamma ray telescope.

AAASMC: What are the current approaches to studying dark matter?
Karen Byrum, Kavli Institute for Cosmological Physics (KICP) associate, Argonne National Laboratory:
We've seen dark matter gravitationally, but we really don't know what it is. We have different ways to try to measure its particle properties, and connect it to what we do know—the Standard Model of Particle Physics. There are three complementary ways you can search for dark matter. You can try to make it in the laboratory, like at the Large Hadron Collider. You can try to look for it directly—as we kind of swim through the dark matter halo of our galaxy, particles of dark matter can interact in big underground detectors, which would be a direct detection. [Direct detection experiments are designed to look for rare interactions between dark matter particles and the ordinary atoms that make up the detector.] The last way to look for it is indirectly. We have telescopes pointed to areas in the cosmos that are known to be dark matter dominated, and we look for a signature of the dark matter in the gamma rays.

If the dark matter decays directly to gamma rays, that would be the smoking gun signal because it would give you a peek at the mass of the WIMP—a Weakly Interacting Massive Particle (a theory of dark matter's particle properties). Or the dark matter could decay into other particles, which then decay into gamma rays, so you get gamma rays regardless. Gamma rays are a really nice tool in that they don't have a charge, so they're not deflected by magnetic fields in the cosmos. They point directly back to their source.

AAASMC: Why do we need all three approaches?
They're complementary to each other in many ways. Since we don't really know what dark matter is, having many tools to understand it is important. Let's say that our direct detector observed a signal that looks like dark matter. To convince yourself, you're going to want to make it in a lab or use one of these telescopes to know that dark matter really is what we're seeing. There are key features that need to be consistent across different methods of looking at the signal. If you look at parameter space for studying dark matter based on the theory that it is a WIMP, the three methods probe it in different ways. They're all accessing a different part of that parameter space.

AAASMC: The light you're searching for is at tera-electron-Volt (TeV) energy scales, far above the typical eV energies of visible light. What sorts of measurements do you make with VERITAS, a ground-based telescope that can measure photons at these energies?
The study of TeV gamma rays is actually quite young. We only observed our first source in 1989, and even in 2003 there were only a handful of sources that had been detected. Now there are close to 150. That really opens up a new window to view the sky and understand what is out there. For VERITAS, in 2012 we had our most recent paper on deep exposure of Segue, which is a dwarf spheroidal galaxy. Dwarf galaxies are the most dark matter-dominated objects, with no known gamma ray backgrounds. These are ideal targets.

AAASMC: What are some other recent experiments you might touch on in your talk?
MAGIC, which is the other northern hemisphere experiment, has recently just come out with a new publication with a deep exposure on Segue. It's deeper than the previous VERITAS measurement. So that is a new result that I will talk about. 

You can also probe from space with the Fermi-GLAST telescope. It looks at lower energies, but there is some overlap with VERITAS with an energy spectra as it goes from GeV to the TeV scale, so I have a little bit of that. Also, HESS [a ground-based gamma ray telescope] has looked for photon-like line signatures within in the last year.

There are some new results where people are getting creative. The next trick is instead of looking at just one dwarf, we're beginning to find ways to combine them together in what we call a stacking analysis to increase our sensitivity. Fermi has done a Milky Way dwarf stacking-type analysis and also a line search. These experiments have only determined limits on the properties of dark matter. No one has seen dark matter.

AAASMC: What are the next steps? How close do you think we are to discovering the detailed particle nature of dark matter?
For direct detection, the generation-two direct detectors should really probe down. For the indirect detection, the next generation experiment is called the Cerenkov Telescope Array (CTA). Current ground-based experiments have on the order of four telescopes. When the gamma ray comes into the atmosphere it creates this huge pool of Cerenkov light that is about the size of a football field. Four telescopes capture some part of that shower, but you don't have enough coverage to catch the whole shower. The next generation of ground-based experiments aims to have enough telescopes to capture the whole shower, which improves your sensitivity.