Another clue in the case for dark matter

DENVER — Using a sensitive detector to survey the abundance of high-energy electrons and positrons in nearby reaches of space, the Fermi Gamma-ray Space Telescope has found new evidence that may hint at the existence of dark matter, the exotic invisible material believed to make up 85 percent of the mass of the universe.

The measurements, reported May 2 at a meeting of the American Physical Society and also online May 4 in Physical Review Letters, bolster the possibility that another orbiting observatory called PAMELA (for Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) did indeed see indirect signs of dark matter (SN: 9/27/08, p. 8), which has eluded detection ever since astronomers first proposed the material more than 75 years ago.

But it’s also possible that many of the energetic electrons and positrons Fermi recorded might instead come from a more mundane astrophysical source — dense, rapidly rotating stars called pulsars — cautions Fermi researcher Peter Michelson of Stanford University. Fermi also scouts for high-energy gamma rays and may provide observations that could soon resolve the ambiguity, he adds.

“The research is preliminary,” comments Savas Dimopoulos, a particle physicist also at Stanford but not part of the Fermi team. “The case [for dark matter] is far from being proven.”

It’s the comparison between the Fermi results and those of PAMELA that suggest the possible detection was real, Michelson notes. In September, researchers reported that PAMELA had found a puzzling excess of energetic positrons, compared with what a well accepted model of particle acceleration suggests the Milky Way could produce. The model suggests that when protons revved up to high energies by blast waves from exploded stars collide with other protons in interstellar space, the particles produce positrons. But the abundance of positrons seemed too high.

PAMELA directly measured the abundance of positrons, but so far its team has only reported the ratio of positrons to the total number of electrons and positrons the craft recorded, notes Michelson. That means that if, for some unknown reason, the number of electrons at higher energies had declined sharply, the data reported so far from PAMELA might document a positron excess when no excess actually exists.

Fermi records the abundance of positrons and electrons of much higher energies, and with greater accuracies, than PAMELA does. Fermi’s new observations reveal that the abundance of electrons doesn’t decline sharply enough at higher energies to explain away the PAMELA results. PAMELA has truly found an unexplained positron excess, Michelson says.

A preliminary analysis of the abundance of electrons recorded by PAMELA at energies between 1 and 100 gigaelectronvolts also shows that the positron excess is real, PAMELA researcher Mark Pearce of the Royal Institute of Technology in Stockholm reported at the meeting on May 4.

There’s two ways that dark matter could account for the positrons, he adds. One proposed type of dark matter particle, known as a WIMP (for weakly interacting massive particle), may annihilate when it collides with another WIMP. The annihilation produces a cascade of ordinary particles, including positrons and electrons.

It’s also possible that dark matter particles, while long-lived, may not last forever. Enough of these individual dark matter particles could by now have decayed into positrons to account for the excess that PAMELA detected, Michelson says.

For the time being, however, astronomers aren’t forced to go over to the dark side. Michelson emphasizes. The whirling dervishes known as pulsars could explain the current observations, although such an explanation would need some tinkering, Michelson adds.

Fermi’s continuing all-sky survey of energetic gamma rays, which also could be produced by pulsars or the decay of dark matter particles, will be critical to distinguishing between the dark and light scenarios. Unlike charged particles such as positrons, which are bent or deflected from their original paths by galactic magnetic fields, particles of light such as gamma rays have no such deflection and can therefore be traced directly to their source.

Pulsars concentrate along the plane of the Milky Way’s disk, while particles of dark matter would have a much more uniform distribution. Therefore, determining whether or not any observed excess of gamma rays is distributed uniformly across the sky could fingerprint the source.

Also, if dark matter is the source, the gamma rays would not only show a uniform distribution but would also be relatively more abundant in the energy range of 100 billion electronvolts to a few trillion electronvolts, Michelson adds.

In a related finding, a balloon-borne experiment called ATIC last year had found a dramatic rise in the abundance of the otherwise smoothly decreasing number of cosmic ray electrons of unusually high energies pouring into Earth’s atmosphere. That unexpected rise could have been attributed either to the decay or annihilation of dark matter, or to acceleration of the electrons by pulsars.

Fermi, which has a higher resolution and collecting power, ought to have easily detected such a spike. But the craft, scanning for electrons in the range of 300 to 800 gigaelectronvolts, measured no such increase, Fermi researcher Alexander Moiseev of NASA’s Goddard Space Flight Center and the University of Maryland at College Park reported at the meeting May 4.

“This is a real detective story, and we already have some clues,” says Michelson. “It’s possible that within a year we may know whether or not we’ve got dark matter, or at least the kind of dark matter we thought we might have.”

“We are all looking forward to seeing the analysis of the gamma-ray data,” says Dimopoulos.

This story was updated May 4.