An Anomaly in the Cosmic Ray Flux
Standard physics models predict a small, steady stream of antimatter particles, called positrons, raining down on Earth as a byproduct of cosmic ray collisions in deep space. But something is producing more than expected. In 2008, the satellite-based PAMELA experiment detected a surprising surplus of high-energy positrons. This "positron excess" begins at energies above 10 gigaelectronvolts (GeV), precisely where theories predicted the numbers should decline.
This finding has since been confirmed with much greater precision by the Alpha Magnetic Spectrometer (AMS-02), a 7.5-tonne particle detector installed on the International Space Station in 2011. The AMS-02 has collected data on billions of cosmic ray events, identifying hundreds of thousands of positrons in an energy range from 0.5 to over 500 GeV. Its data shows the positron fraction increases from 10 GeV up to around 275 GeV before starting to decline.
The physical instrument is in orbit, but its brain is on the ground in Geneva. The AMS Payload Operations Control Centre at CERN is where scientists and engineers monitor the detector and analyze the torrent of data it sends back to Earth 24 hours a day. The detector was also assembled and tested at CERN before its launch aboard the Space Shuttle Endeavour.
Cosmic Suspects: Pulsars or Dark Matter?
The source of these extra positrons remains an open question, with two primary suspects. One possibility is nearby astrophysical objects. Pulsars, rapidly spinning neutron stars, with powerful magnetic fields, are known to accelerate particles and can eject electron-positron pairs into the galaxy. If a pulsar like Geminga, located roughly 800 light-years away, is close enough, its ejected positrons could reach our solar system, contributing to the observed excess.
A more exotic explanation involves the annihilation or decay of dark matter. Many theories predict that when dark matter particles, perhaps a type called WIMPs (Weakly Interacting Massive Particles), collide, they annihilate and produce a shower of familiar particles, including electrons and positrons. Finding a definitive dark matter signature in the positron excess would be a historic discovery.
Scientists are looking for clues to distinguish between these possibilities. A signal coming from a specific direction (anisotropy) would point toward a nearby source like a pulsar. To date, the AMS-02 data shows the positrons arriving from all directions, a finding consistent with a diffuse cloud of dark matter. However, this isotropic signal could also be produced by multiple, more distant pulsars. Further analysis of data from experiments like AMS-02 and the High-Altitude Water Cherenkov (HAWC) Observatory continues to narrow the possibilities, pushing physics closer to solving this antimatter mystery.