Fermi Gamma-ray Space Telescope

A Fermilab theorist and his colleague at NYU might have found clues to some of the universe's juiciest secrets at the center of the Milky Way.

In their analysis of public data from the Fermi Gamma-ray Space Telescope, Dan Hooper, Fermilab scientist, and Lisa Goodenough, a graduate student at New York University, report that very-high-energy gamma rays coming from the center of the Milky Way originate from dark-matter collisions.

“We went out of our way to consider all causes of backgrounds that mimic the signal, and we found no other plausible astrophysics sources or mechanics that can produce a signal like this,” Hooper said.

A recent paper, published on the pre-print server arXiv, outlines their findings.

Astrophysicists have long postulated a wide range of dark matter particles, including axions, super heavy particles and particles that fall in between: Weakly Interacting Massive Particles, or WIMPs.

Using the gamma ray data, Hooper and Goodenough identified the mass of a WIMP with a range of 7.3-9.2 GeV, about eight times heavier than the proton. It is the same mass derived from candidate dark matter particle events in two ground-based detectors, CoGeNT, a University of Chicago dark matter experiment in the Soudan mine in Minnesota, and DAMA, an Italian experiment located under the Gran Sasso Mountains near Rome.

If Hooper is right, then physicists now know the mass of these particles to within 10 percent.

This image shows the good fit of the data with a dark matter model. The lines with bars show the spectrum of the anomalous signal, while the solid black line shows a well-fitting dark matter model.

Basically, Hooper explained, he and Goodenough identified an anomalous flux in gamma rays coming from the innermost part of the Milky Way. The signal, which is highly concentrated in an area within the inner 100 light years around the Milky Way, didn't look like any conventional astrophysics source to the two experts.

However, when they plugged in a simple dark matter model, it fit.

Before publishing their paper, Hooper and Goodenough sent their findings around to their peers for unofficial review. No one disagreed with the fundamental results.

Fermilab's Craig Hogan, head of the laboratory's Center for Particle Astrophysics and a University of Chicago researcher, thinks that Hooper's analysis is spot on.

"Dan and Lisa's analysis is really quite straightforward," Hogan said. "What they've found is just what you'd expect annihilating dark matter to look like near the Galactic center. It's the simplest explanation of the data we have."

Hogan believes that there is no other obvious astrophysical explanation.

"It walks like a duck, it quacks like a duck, so maybe it’s a duck," he said.

Steve Ritz is the deputy principal investigator for the Large Area Telescope, Fermi Gamma-ray Space Telescope's main instrument and the co-coordinator for the collaboration's dark matter and new physics groups.

"I think that this is a very interesting paper," Ritz said. "They are certainly right that the galactic center is an important region and that the Large Area Telescope data should be used to look for signals for new physics."

The Milky Way's galactic center. A new paper reports that very-high-energy gamma rays coming from the center of the Milky Way originate from dark matter collisions. Image courtesy of: NASA/CXC/UMass/D. Wang et al.

However, Ritz said, this is the most complex area of the gamma ray sky and looking for signals of new physics in the galactic center is like looking at the heart of a city's downtown and quickly discerning what is happening.

Ritz said that the collaboration has long noted the excesses, what Hooper refers to as anomalous flux, and members continue to work hard at analyzing and interpreting the data.

"The burden of proof to claim new physics is high. One must show that the surprising features in the data are not explained by systematic uncertainties or other plausible astrophysics," Ritz said.

It will take time for other dark matter experiments to confirm or deny Hooper and Goodenough’s findings. Experiments such as CoGeNT and DAMA are well-suited to see the low mass WIMPS.

In the meantime, Hooper explained, when experiments such as the Fermi Gamma-ray Space Telescope look at other points of the sky, he hopes they'll do so with this dark matter particle in mind.