Most detailed survey of spaceborne particles yet opens new mysteries

An 11-year-long survey of the particles and antiparticles near our sun is unlocking our solar system’s history – and raising new mysteries about the particles themselves.

“It’s like when you walk into a dark room and see many, many new things,” says Samuel Ting at the Massachusetts Institute of Technology.

Space is filled with energetic particles, which travel in bursts called cosmic rays. When a cosmic ray enters the Alpha Magnetic Spectrometer (AMS) detector on the International Space Station (ISS), magnetic fields separate its particles based on their electric charge, and then the detector measures their masses and energies. This separation is crucial because it helps identify differences in the behaviour of a particle and its antiparticle, which is identical except with an opposite charge, says Ting.

He and his colleagues at the AMS Collaboration analysed more than 11 years of AMS data and found, surprisingly, that we don’t know as much about particle behaviour as we thought. For instance, the survey revealed trends in the number of particles over time, and in the ways different types of particles interacted with each other. There are more than 600 theoretical models that could possibly explain each of these trends – but none simultaneously explain both findings, says Ting.

And the survey’s results may matter for more than single particles. Researchers have been capturing cosmic rays with different detectors for more than a century because their changing properties could serve as records of the solar system’s history, says Jamie Rankin at Princeton University. But we never before had such a detailed understanding of how the solar cycle affects the rays, she says.

That’s because 11 years is the length of one solar cycle, so collecting data for that whole period captures all the repeating variations in the magnetic field of the sun, which change the behaviour of cosmic rays. Such a detailed survey may become a key that unlocks a way to use cosmic rays for “solar system archaeology”, she says.

But where cosmic rays themselves come from is still mysterious, says Gavin Rowell at the University of Adelaide in Australia. “The particles AMS measures are essentially coming from outside the solar system,” he says. The amount of detail present in the new analysis, including how different particle nuclei within cosmic rays behave, may help researchers zero in on a more definitive theory of cosmic rays’ origin.

And there are other unanswered cosmic questions. “We don’t see antimatter in our world, so the fact that the AMS can observe antiprotons, to me, that’s a great mystery,” says Ian Low at Northwestern University in Illinois. The origin of those antiparticles may be connected to mysterious dark matter or otherwise go beyond our current best understanding of the cosmos, he says.

Ting and his colleagues are now working on upgrading the AMS detector to be able to detect even more particles – and coordinating with the astronauts who will help install it.

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