By Eric Hand
By invoking the effects of starlight, theorists have created a model of the behaviour of galactic electrons, casting doubt on a signal that some had hoped pointed to a detection of dark matter.
Within the past two years, several experiments — in space, on the ground, and in a balloon — have reported detecting more high-energy electrons than were expected to be whirling around the galaxy. Many theorists attributed the surplus electrons as either the effect of a nearby pulsars, or, more provocatively, dark matter — the elusive stuff thought to make up as much as 85% of the matter in the Universe (see 'Dark matter intrigue deepens').
A paper in the 10 February issue of the Astrophysical Journal1says that both explanations are wrong. The high-energy electrons can be produced naturally when the starlight they pass through is accounted for more correctly, says one of the paper's authors — Vah Petrosian, a theorist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California. "We have to put dark matter on the shelf," he says.
Galactic electrons are thought to originate in the explosion of supernovae, and conventional models predict that they lose energy as they pass through the Milky Way's magnetic field. The annihilation of proposed dark-matter particles would also create electrons, and some theorists had interpreted the recent experimental detections of surplus high-energy electrons as evidence for this process.
But starlight also scatters the electrons. Petrosian says that starlight suppresses the energy of most electrons in a way that makes it seem as if there is an excess of certain high-energy electrons. The Stanford group's models show an excess that is similar to that reported by NASA's Fermi Gamma-ray Space Telescope; the High Energy Stereoscopic System (HESS), a ground-based detector in Namibia; and the Advanced Thin Ionization Calorimeter (ATIC), a balloon-borne detector that flew over Antarctica.
HESS spokesperson Werner Hofmann says that the Stanford group's models are "quite possible" and would make it very difficult to make a strong case for dark matter in the high-energy electron signal. "I would say there's no compelling reason to invoke exotic explanations," says Hofmann, an astrophysicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
Need for ideas
More challenging for the group was explaining a signal arising from an Italian satellite, PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), which measures the ratio of electrons to positrons, their antimatter partners. A rising fraction of high-energy positrons has also been interpreted as a possible dark-matter signal. (See'Physicists await dark-matter confirmation')
But by tweaking parameters in their model, the Stanford group can also mimic the PAMELA results. Like the electrons, the positrons are also thought to originate near supernovae — although through secondary collisions of protons. By increasing the density of gas and the number of photons near these supernovae — both possible scenarios given that supernovae occur in gas-rich star-forming regions near lots of stars — the model predicts high-energy positrons similar to those reported by PAMELA. "It's a new possibility to consider and a new way to get 100 GeV [gigaelectronvolt] positrons to our Solar System," says Dan Hooper, a dark-matter theorist at Fermi National Accelerator Laboratory in Batavia, Illinois. "We need all the ideas we can get."
Using the high-energy electrons as a proxy for dark matter is just one of many approaches in the hunt. The Large Hadron Collider at CERN, the European particle-physics laboratory near Geneva, Switzerland, may create dark matter as it smashes high-energy protons together. And experiments underground use quiet environments to watch for the rare recoils of atomic nuclei that dark-matter particles ought to cause occasionally. In December, an underground detection group reported that it had seen two events that may have been dark-matter collisions — enough to get attention, but not yet enough to claim a definitive detection (see 'Two direct hits in dark matter hunt')