Earth's atmosphere protects us from the continuous barrage of high-energy cosmic rays that reach our planet from space. It keeps us from living in a giant microwave, in a sense. While these rays have many sources—black holes, supernovae, quasars, gamma-ray bursts, the Big Bang itself—one is of particular interest: dark matter. And hanging off the side of the International Space Station one finds the Alpha Magnetic Spectrometer, part of an experiment designed to hunt for dark matter by observing the production of antimatter in our atmosphere.
Indeed, newly crunched results from the AMS detector are now showing even more promising hints of dark matter from these showers of cosmic rays.
In the dark matter hunt, physicists are particularly interested in the pairs of matter and antimatter particles that are created when superfast-moving particles collide with bits of matter or energy in the atmosphere. Antimatter is a natural result of the particle creation that occurs when two massless particles (like photons) collide to form two massive particles (an electron and positron).
Normally, the antimatter part of the equation is annihilated immediately when it meets some bit of normal matter, resulting in nature's most efficient, near-perfect release of energy. But up in orbit, at the very edge of the atmosphere, it's possible to count antimatter particles before they blow up. Crucially, it's possible to determine if there are extra antimatter particles beyond what should be expected from detectable cosmic rays. This would indicate that there is some additional source of rays so far unaccounted for.
In 2013, physicists made the discovery (explained in the video above) that above a very high-energy threshold (8 billion electronvolts), the numbers of positrons (the antimatter version of electrons) shot way up. This was a sure sign that something else is indeed bombing our atmosphere, possibly dark matter—but by no means definitively dark matter.
The new AMS results suggest the energy level at which this promising positron spike ceases and things go back to normal: 275 billion electronvolts. (For reference, the Large Hadron Collider in its most recent setup was able to smash particles at 2.36 trillion electronvolts.) So, between 8 billion and 275 billion eV is where we find this unexplained extra contribution to the cosmic ray blast zone.
"Scientists have been measuring this ratio [between antimatter and matter] since 1964," Jim Siegrist, an associate director at the US Department of Energy's Office of High-Energy Physics, told Fermilab's Symmetry Breaking. "This is the first time anyone has observed this turning point." What's more, the positron overpopulated region shows a smooth distribution of those positrons across the entire energy gap, rather than sudden jolts, eliminating some non-dark matter possibilities.
Energy is but the flipside to mass, so by knowing this range of energies we can put some limits on the potential masses of our new dark matter candidates. By projecting a curve of possible masses from the observed collision energies of these excess positrons, it should be possible to eliminate some other potential non-dark matter causes, like pulsars or some other non-massive source. A massive particle, like dark matter, would necessarily show a very steep drop-off at either energy limit because of this mass constraint. A photon sailing across the cosmos from some distant black hole doesn't have the same speed limit that results from having mass.
Describing this curve will take some more time. For one thing, researchers still have about 10 billion collision events to analyze out of the total 54 billion collected. In a few years time, it's hoped that we'll have the statistics to narrow things down further. Fortunately, the ISS experiment is just one of many dark matter probes currently underway, so it's likely we won't have to wait quite that long for our next bit of dark matter news.