A $2-billion particle detector mounted on the International Space Station has registered an excess of antimatter particles in space, the experiment’s lead scientist announced April 3. That excess could come from fast-spinning stellar remnants known as pulsars and other exotic, but visible sources within the Milky Way galaxy. Or the antiparticles might have originated from the long-sought dark matter, the hypothetical massive particles that constitute some 27 percent of the universe.
Dark matter makes its presence felt by its gravitational pull, but exactly what it is has remained a puzzle. Some popular explanations for dark matter’s identity suggest that when two dark-matter particles collide, they annihilate to produce antimatter electrons, or positrons. The Alpha Magnetic Spectrometer (AMS), delivered to the space station in 2011 during the penultimate space shuttle mission, was built to detect positrons and other high-energy particles streaming through space, in part to investigate the nature of dark matter. The detector has now collected some 25 billion cosmic-ray particles, including 6.8 million electrons and positrons. The fraction of positrons in the particle mix exceeds what would be naively expected in the absence of dark matter or other unaccounted sources, but the new data lack a distinctive feature predicted of dark matter annihilations.
Dark matter collisions would produce relatively more high- than moderate-energy positrons. But the rise in positrons with increasing energy would continue only up to a point. Beyond a certain energy level, the number of positrons would fall off steeply, AMS spokesperson and Nobel laureate Samuel Ting of the Massachusetts Institute of Technology explained in a seminar at CERN, the European laboratory for particle physics. “The positrons could also come from nearby pulsars, and in such a case the positrons will have a slow drop-off” at higher energies, Ting said. “So the way they drop off tells you everything.”