The European collaboration PAMELA has published new data concerning a mysterious excess of positrons that permeate outer space. The data, which describe the collaboration’s first detection of absolute rather than relative positron numbers, could help refine theoretical models – including those that explain the excess as a footprint of dark matter.
PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) is a satellite that was launched in 2006 by institutions in Germany, Italy, Russia and Sweden to examine the nature of antiparticles in cosmic rays. The first results, published in 2008, revealed a surprising feature: a steady rise in the ratio of positrons to electrons above an energy of about 10 GeV. This is contrary to basic theoretical calculations, which predict that the positron fraction should have decreased.
Theorists have put forward several explanations for the positron excess. The most exciting involves dark matter, an elusive substance that interacts with gravity, but not light and tht is estimated to make up nearly 85% of the total amount of matter in the universe. According to many theoretical models, dark-matter particles can annihilate one another, thereby generating electrons and positrons. At low energies, electrons from other astrophysical sources would easily outnumber these positrons, but with increasing energy such electrons would dwindle, leaving the numbers of electrons and positrons to start balancing out, as PAMELA observed.
Another explanation is that positrons are being generated in spinning stars known as pulsars. And yet another possibility is that the PAMELA team has misunderstood its experiment. Misinterpretation now seems almost totally unlikely because early last year physicists using NASA’s Fermi Gamma-ray Space Telescope confirmed that they had also detected a rise in the positron fraction at energies of between 20 and 100 GeV. Then in April this year, data from the Alpha Magnetic Spectrometer (AMS) experiment aboard the International Space Station became the third to reveal a positron excess.
The latest PAMELA data, obtained between July 2006 and December 2009, include three times the number of positron events as in the previous sample. Perhaps more importantly, however, the data include absolute numbers of positrons, not just their fraction of the positron and electron total. “If you want to understand the mechanisms that could be associated with the [positron excess], then it is interesting to get the amount of positrons by themselves,” says Mirko Boezio, leader of the PAMELA analysis who is based at the National Institute for Nuclear Physics at Trieste University, Italy.
Measuring absolute numbers of positrons is not easy. Crucially, the measurement requires precise estimations of the number of positrons that are lost because of inefficiencies in detection – unlike a measurement of the relative positron-to-electron number, for which such inefficiencies cancel out. PAMELA is not the first collaboration to make an absolute measurement: it has been performed for several balloon-borne experiments and for the Fermi Gamma-ray Space Telescope. However, the PAMELA measurement is more precise and may, according to Boezio, help theorists rule out competing models.
A shadow over dark matter
Particle theorist Subir Sarkar of the University of Oxford in the UK is glad to see that PAMELA’s latest measurement of the positron fraction so closely matches the recent measurement by AMS. He points out, however, that the relationship between the positron fraction and energy appears to be disfavouring the dark-matter scenario. “An explanation in terms of dark matter is rather contrived as it requires a TeV/c2 mass particle with a hugely enhanced annihilation rate over the usual expectation, [and] which does not produce any antiprotons!” he says.
Sarkar thinks that an astrophysical source of positrons is more likely – but not pulsars. Instead, he believes that cosmic rays might be generating positrons during their interaction with ambient matter in supernova shock waves, which goes on to accelerate the positrons themselves. If so, he says, the ratios of certain nuclei in cosmic rays will be different from expectations, too. For instance, carbon will break up into boron, leading to an enhanced boron-to-carbon ratio at high energies.
Whatever the eventual understanding of the positron excess, though, it is clear that the PAMELA collaboration has given particle theorists a lot to play with. Theorist Marco Cirelli of the Institute of Theoretical Physics at CEA/Saclay, near Paris pays tribute to the results. “They are a great achievement of an experiment and a collaboration that has performed really well, and has delivered really well,” he says.
The results are published in Physical Review Letters.