Nobel Laureate Samuel Ting laughed when I asked where all of the high energy electrons that hit his particle detector were coming from. “The data has just been published three days ago,” he told me, hinting at the depth of the mystery and the virtue of patience. “The most important thing is that none of our results can be explained by current models.”
|NASA Image: ISS028E016133 – Exterior view of the International Space Station (ISS) taken during a session of Extravehicular Activity (EVA) with a fisheye camera. The shuttle Atlantis is partially visible docked to the Node 2/Unity and the Alpha Magnetic Spectrometer – 02 (AMS-02) is visible in the foreground.|
That unexplained data is the energy distribution of 28 million electrons that were intercepted by AMS, the Alpha Magnetic Spectrometer, as they tore through the universe at nearly the speed of light. It’s the subject of research recently published in the American Physical Society’s journal Physical Review Letters by the AMS collaboration.
Ting has led the AMS experiment since it was nothing more than a wild idea 25 years ago, but he hasn’t seen the detector since 2011. At least not in person. That’s because it’s mounted to the outside of the International Space Station (ISS).
Why space? Although it may seem mostly empty, particles called cosmic rays are constantly zooming through space in all directions. Some are ejected by the sun and other stars. Many—the higher energy ones—are produced by supernovas and other cosmic engines. Some are the product of particle decays or collisions between particles. All of them contain information about the universe that is difficult to access in other ways.
Detecting particles in space is like viewing a room with the lights on for the first time, explains Ting. A room that could reveal the nature of dark matter, antimatter, and cosmic rays.
AMS isn’t the first space-based particle detector, but it is the largest and longest lasting. It’s also the only space-based detector that can distinguish between the electrons and their antiparticles, positrons, in cosmic rays. This is key: at a fundamental level, we still don’t know why there is more matter in the universe than antimatter. But some of the best places to find clues are in the distribution and origin of particles and their antiparticles.
AMS measures the charge, velocity, path, and energy of cosmic rays. Cosmic rays are notorious for colliding with atoms in the Earth’s upper atmosphere, so AMS was designed to intercept them before that happens. It can distinguish between electrons and positrons by how they react to a magnetic field, in this case produced by a cylindrically shaped magnet with a field 3,000 times more intense than that of the Earth. The detector weighs 7.5 tons. (Putting a strong magnet on the ISS is dangerous business and took 17 years of careful planning. That’s a fascinating story for another day.)
For the last eight years, data has been streaming from the ISS to AMS control room at CERN in Switzerland. Ting ponders the data from there or his office at the Massachusetts Institute of Technology. Hundreds of collaborators from 16 different countries do the same in their respective offices.
In January, the collaboration published the energy distribution of cosmic ray positrons detected over a 6.5-year period. They found that most low-energy positrons were produced by cosmic ray collisions and that high ones were produced by an unknown source up until a cutoff energy, above which no positrons were detected. This latest result is based on the cosmic ray electrons detected over the same time span. “What we found is that electrons have totally different spectra than positrons,” says Ting. Specifically, the AMS collaboration found that a much smaller fraction of the lower-energy electrons were produced by cosmic ray collisions than positrons. And there was another big difference—no evidence of an energy cutoff for electrons. Together, these results suggest that the positrons and electrons have different origins.
Before AMS, cosmic ray experiments comparing positrons to electrons had huge errors—around 30% or more. “When you have large errors, you can accommodate many theories,” Ting told me. With the precision of AMS, that field of workable theories is vanishingly small. That presents an enormous opportunity to learn and explore.
Where do the excess positrons come from? Dark matter collisions? A nearby pulsar? Above the cutoff, are there are really no positrons, or just very few? What about the high energy electrons? Are they produced in supernovas and/or some other cosmic event? Why are the two spectra so different?
As AMS continues to collect this unique data, new models will be proposed. New questions will arise. And maybe, if we’re patient, new answers will be found.