FERMI LAT: Data Holds New Clues To Dark Matter

At left is a map of gamma rays with energies between 1 and 3.16 GeV detected in the galactic center by Fermi's LAT; red indicates the greatest number. 

Prominent pulsars are labeled. 

Removing all known gamma-ray sources (right) reveals excess emission that may arise from dark matter annihilations.

Image courtesy T. Linden, Univ. of Chicago.

A new study of gamma-ray light from the center of our galaxy makes the strongest case to date that some of this emission may arise from dark matter, an unknown substance making up most of the material universe.

Using publicly available data from NASA's Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory (Fermilab), the Harvard-Smithsonian Center for Astrophysics (CfA), the Massachusetts Institute of Technology (MIT) and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.

Dan Hooper

"The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it," said Dan Hooper, an astrophysicist at Fermilab in Batavia, Ill., and a lead author of the study.

"The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models."

The galactic center teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas.

It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity.

Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.

No one knows the true nature of dark matter, but Weakly Interacting Massive Particles (WIMPs), represent a leading class of candidates.

Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide.

Both of these pathways end with the production of gamma rays -- the most energetic form of light -- at energies within the detection range of Fermi's Large Area Telescope (LAT).

When astronomers carefully subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains.

This excess appears most prominent at energies between 1 and 3 billion electron volts (GeV) -- roughly a billion times greater than that of visible light -- and extends outward at least 5,000 light-years from the galactic center.

Hooper and his colleagues conclude that annihilations of dark matter particles with a mass between 31 and 40 GeV provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center, and its overall brightness.

Writing in a paper submitted to the journal Physical Review D, the researchers say that these features are difficult to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize.

"Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider (LHC), so if this is dark matter, we're already learning about its interactions from the lack of detection so far," said co-author Tracy Slatyer, a theoretical physicist at MIT in Cambridge, Mass.

"This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time."

More Information: "The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter" Authors: Tansu Daylan, Douglas P. Finkbeiner, Dan Hooper, Tim Linden, Stephen K. N. Portillo, Nicholas L. Rodd, Tracy R. Slatyer

DarkSide: Project aims to find particles of dark matter

The DarkSide-50 research team is made up of faculty, students and researchers from dozens of institutions around the world,

From left, Luca Grandi, an assistant professor at the University of Chicago, Richard Saldanha, an associate fellow in the Kavli Institute for Cosmological Physics at the University of Chicago, and Hanguo Wang, a researcher at the University of California-Los Angeles. 

While wearing protective clothing to keep the environment clean, they are working to assemble the core of the dark matter detector, an argon-filled tank with photodetectors at the top and bottom to spot the light from collisions, and copper coils to help determine where the collisions occur. 

Credit: Peter Meyers

In a laboratory under a mountain 80 miles east of Rome this fall, a Princeton-led international team switched on a new experiment aimed at finding a mysterious substance that makes up a quarter of the universe but has never been seen.

The experiment, known as DarkSide-50, is searching for particles of dark matter. For the last several decades, researchers have known that visible matter—the stuff we can see—makes up only 4 percent of the universe, while dark energy is thought to make up about 73 percent.

Dark matter is thought to make up the remaining 23 percent, and finding it, researchers say, will solidify our understanding of how the universe formed and shed light on its ultimate fate.

Peter Meyers

"This is like the search for the Higgs boson was 10 years ago," said Peter Meyers, a professor of physics at Princeton University and one of the lead scientists on the project.

"We have a good idea of what to look for, but we don't know exactly where or when we will find it."

Housed inside a cavernous chamber in Italy's Gran Sasso National Laboratory, the DarkSide-50 collaboration involves 17 American institutions as well the Italian Institute for Nuclear Physics (INFN) and other institutions in Italy, France, Poland, Ukraine, Russia and China.

The research team includes postdocs, staff researchers and several graduate and undergraduate students from Princeton.

The researchers spent last summer assembling the detector, which consists of three fluid-filled chambers nested one inside the other like Russian dolls.

Now that the experiment is up and running, the waiting begins. Unlike the massive Large Hadron Collider that discovered the Higgs, DarkSide-50 doesn't smash anything. Instead, it is designed to detect dark matter particles that drift through its chambers.

Looking for WIMPs 
The evidence for dark matter dates to the 1930s, when astronomers realized that the amount of matter we can see—as planets, stars and galaxies—falls far short of what must be out there to give galaxies their characteristic spiral shapes and clustering patterns.

Without this missing matter, the galaxies should have flown apart long ago. Matter provides the gravity that keeps the stars in rotation around the galaxy's center.

Unless our theories of gravity are wrong—and a minority of physicists think that is a possibility—dark matter must exist.

Cristiano Galbiati

"Finding dark matter particles would help confirm our understanding of the universe," said Cristiano Galbiati, an associate professor of physics at Princeton.

"And, whether or not we find it, we will have learned a great deal about how to go about looking for it. This is as exciting a moment in the search for dark matter as there has ever been."

Although no one knows for sure what dark matter is made of, the DarkSide-50 team and many other scientists think the most likely candidate is a particle so weak that it is called a WIMP, which is short for "weakly interacting massive particle."

As the name suggests, WIMPs barely interact with their surroundings. They simply drift through walls like ghosts.

If you cup your hands together, you will surround—but never trap—a few of these ethereal beings. Scientists suggest that a WIMP can be detected when it smacks into the nucleus of an atom such as argon, which is found in air.

When this happens in a chamber of densely packed argon atoms, the stricken atom recoils and creates a track of excited argon atoms in its wake.

This track appears as a fleeting trail of light, which can be detected by devices called photodetectors. But these collisions are rare—just a few WIMPs are detected each year.

Because other particles also give off light when they collide with argon, DarkSide-50 is located nearly a mile beneath Gran Sasso mountain ("gran sasso" is Italian for "great stone").

The rock shields out cosmic-ray particles that routinely bombard the Earth.

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