Signal of Destroyed Dark Matter Seen in Space Telescope’s Data
In 2008, the Italian satellite PAMELA picked up an unusual signal: a spike in antimatter particles whizzing through space. The discovery, controversial at the time, hinted that physicists might be coming close to detecting dark matter, an enigmatic substance thought to account for 85% of the matter in the universe. Now, new data from NASA’s Fermi Gamma-ray Space Telescope confirm the spike. Alas, they also undermine its interpretation as a sign of dark energy.
Physicists hope they might use the accumulating data on antimatter to home in on the mass of the weakly interacting massive particle (WIMP), which is thought to be the fundamental dark matter particle. The expected signal would be a steady rise of positrons over a given range of energies, followed by a sudden drop-off. Noting the energy level—physicists measure this in billions of electron volts—at which the positron signal drops off would allow physicists to calculate the WIMP’s mass.
After the excitement generated by the PAMELA result, Stanford University physicists Stefan Funk and Justin Vandenbroucke wanted to zero in on the positron signal. They found a way to do so, using Earth itself as a particle filter. “You can basically look in certain directions from which only electrons or only positrons will get through the Earth’s magnetic field,” Vandenbroucke says.
Funk and Vandenbroucke’s method, which has been submitted to Physical Review Letters, confirmed the Italian result. That is, that the relative abundance of positrons seems to rise from 20 billion to 100 billion electron volts. And for the first time, the researchers showed that the signal continues to get stronger up to 200 billion electron volts. If what they’re seeing are remnants of dark matter deaths, then the mass of the WIMP would have to be at least 100 times that of a proton, which is within many theoretical predictions.