The Mysterious Case of the Excess Positrons

In 2008, the European satellite PAMELA detected a surprisingly large concentration of high energy positrons above our atmosphere. The presence of so many positrons, the anti-matter counterpart of electrons, goes against theoretical predictions but has been verified by other detectors. In new research published earlier this month by the AAAS journal Science, a team of researchers from Germany, Mexico, Poland, and the United States now cast doubt on one of the leading explanations for the mysterious excess—leaving its origin still unknown.

PAMELA, which stands for the Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics, collects data on high energy particles called cosmic rays produced by astronomical events. As astronomers well know, a small fraction of these particles are antimatter particles—positrons (also called antielectrons) and antiprotons. The detection of positrons by PAMELA was therefore no surprise, but it was surprising when data analysis revealed that the ratio of positrons to electrons increased at high energies instead of decreasing, as predicted. This implies that there is a another, currently unknown source of positrons.

Since 2008 a number of possible sources have been suggested, but none confirmed. The two leading hypotheses are that the extra positrons are produced by (1) interactions involving dark matter particles or (2) nearby spinning, collapsing stars called pulsars. There are a few pulsars close and old enough to be possible candidates.

Unfortunately, we can’t just point a detector at a possible source to see if it ejects positrons in numbers that match the data. Positrons are charged particles, so their paths are strongly influenced by magnetic fields and we can’t trace them back to their origin. However, we can make indirect measurements using gamma rays. These energetic photons are produced by interactions between positrons and photons left over from the early universe—interactions made possible by extremely hot, energetic astronomical objects and events like pulsars. Gamma rays aren’t influenced by magnetic fields, so we can trace their paths back to their origins.

In this new research, scientists used gamma ray data from the HAWC Observatory, which stands for the High-Altitude Water Cherenkov Gamma-Ray Observatory, to explore two pulsar candidates: a relatively nearby pulsar called Geminga, and its poetically named sister, PSR B0656+14. The stars are 800 and 900 light years from us, respectively.

An image of the HAWC detector consisting of 300 large water tanks.
Image Credit: Jordan A. Goodman.

HAWC is an array of 300 massive water tanks sitting at an altitude of about 13,500 ft in Mexico. When gamma rays hit our atmosphere, they produce showers of particles that travel extremely fast. Not faster than the speed of light, but faster than the speed of light in water. Because of their immense speed, when these particles hit the HAWC water tanks they create flashes of blue light. By recording the flashes simultaneously in an array of tanks, astronomers can detect showers and then reconstruct the energy and path of the gamma ray that started it all. (For more on HAWC, see our earlier story Exploring Cosmic Rays Through the Shadows.)

The team studied the gamma rays HAWC detected from Geminga and PSR B0656+14 over a 17-month span, paying special attention to how they were distributed in location and energy. From this, the researchers extracted information on the area around the pulsars and how matter diffuses through this space. By incorporating the details of pulsar emissions and our current understanding of how electrons and positrons diffuse through the space between the pulsars and the Earth, the researchers estimated about how many positrons originating from these pulsars could have reached the Earth by now.

A HAWC map of the nearby pulsars Geminga and PSR B0656+14.
Image Credit: John Pretz.

The result? Not enough. The data shows that even though these two pulsars are old enough and close enough to account for the excess in positrons, the positrons don’t diffuse away from pulsars quickly enough to have reached the Earth in the numbers detected by PAMELA.

This doesn’t mean that the positrons definitely come from dark matter particle interactions, although you could say it increases the odds. We need more evidence to know for sure. It may be that positrons undergo a much more complicated diffusion process than we know about and get here faster than the HAWC model shows. Or, perhaps the positrons are coming from another source altogether…

“This new measurement is tantalizing because it strongly disfavors the idea that these extra positrons are coming to Earth from two nearby pulsars, at least when you assume a relatively simple model for how positrons diffuse away from these spinning stars,” said Jordan Goodman from the University of Maryland in a recent statement. Goodman is the lead investigator and U.S. spokesperson for the HAWC collaboration. “Our measurement doesn’t decide the question in favor of dark matter, but any new theory that attempts to explain the excess using pulsars will need to account for what we’ve found,” he says.

For now, the mystery remains, but Physics Buzz will keep you updated as new developments emerge!

Kendra Redmond

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