PAMELA satellite

The European collaboration PAMELA has put an end to months of speculation by claiming that its Earth-orbiting satellite could have found the remnants of dark-matter annihilation.

In a preprint uploaded to the arXiv server last week, the collaboration presents data suggesting that cosmic rays above the Earth’s atmosphere contain an excess of high-energy positrons. This excess, the authors say, “may constitute the first indirect evidence of dark-matter particle annihilations” — although they add that there could yet be other explanations, such as the presence of a nearby pulsar (arXiv:0810.4995).

The PAMELA data briefly saw the light of day on a presentation slide at a high-energy physics conference in Philadelphia, US, at the start of August. At the time scientists in the audience spoke of the implicit significance of a positron excess for searches of dark matter, an unknown entity that is believed to make up 23% of the universe’s energy budget. But because the PAMELA researchers were planning to submit their work to Nature, which has a strict embargo policy, they avoided making any comments of their own.

Given that our preliminary conference data are starting to be used by people, we felt this was a necessary step Mirko Boezio, PAMELA collaboration

In an unusual twist, however, it emerged that at least one conference attendee had taken photos of the fleeting slide when a month later other preprints began appearing on arXiv making analyses of the data.

“We wanted to make our final results available to the scientific community once the data analysis was finalized,” PAMELA member Mirko Boezio told physicsworld.com, pointing out that Nature permits its authors to upload preprints to arXiv. “Given that our preliminary conference data are starting to be used by people, we felt this was a necessary step — not least because it provides a proper reference that correctly acknowledges the whole PAMELA collaboration and is available to the scientific community at large.”

Lots of data

Launched in June 2006, the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite was designed by institutions in Italy, Russia, Germany and Sweden to examine the nature of antiparticles in cosmic rays. Unlike other cosmic-ray missions, which mostly consist of short balloon flights, PAMELA operates in space where there is little noise from antimatter generated in the atmosphere. Moreover, the satellite has been working non-stop since its launch and will continue to do so until the mission ends after December 2009.

The result of this so far is a large body of cosmic-ray data which is both statistically robust and which extends high energies of about 80 GeV. It indicates that the fraction of positrons to electrons does not decrease steadily with energy, as would be suggested by theoretical “baseline” predictions (that is, predictions that only take into account the production of positrons from interactions between cosmic rays and interstellar gas). Rather, the positron fraction appears to rise after 10 GeV, implying that there is another, unknown source of positrons.

The PAMELA results are interesting, and they deserve the attention they are receiving Stéphane Coutu, HEAT collaboration

The PAMELA collaboration says that possibilities for this source could be a nearby pulsar or, more interestingly, dark-matter particle annihilations.

“The PAMELA results are interesting, and they deserve the attention they are receiving,” says Stéphane Coutu, an experimental high-energy physicist at Penn State University who has worked on the NASA-supported HEAT mission to study cosmic rays from balloons. But, Coutu adds, “the exact interpretation of what the PAMELA high-energy excess truly means will remain uncertain for some time, and the subject of much debate, I’m afraid.”

Positrons or protons?

Part of the debate over the PAMELA data will likely revolve around how effectively the satellite overlooks protons, which have the same charge as positrons and which are a thousand times more abundant in the cosmos. While a magnet spectrometer can effectively distinguish between an electron and positron by the sign of the charge, it is the length of the electromagnetic showers in an adjacent calorimeter that has to determine whether each positron is not in fact a proton.

The PAMELA collaboration says it has performed tests at the CERN lab near Geneva that showed that just one proton in 100,000 fools the calorimeter into believing it is a positron. But Coutu says that the satellite could have benefited from a “transition radiation detector” (TRD), which would have helped rule-out protons unambiguously. “If the discrimination capabilities of their instrument are any less than they think, as one could imagine in the absence of a TRD, then the outcome could be exactly what they observe: a rise with increasing energy in the fraction of positively charged particles, which could be due to something other than positrons themselves.”