Recent experiments have revealed a striking feature of the high-energy cosmic rays coming from outer space: they contain an odd excess of high-energy positrons, the antiparticles of electrons. The result, obtained through years of balloon and space-borne measurements, is not compatible with current astrophysical models. The excess points at yet-to-be-identified sources of cosmic-ray positrons, which could be nearby cosmic bodies such as pulsars or, as many speculated, dark matter annihilation events.
Previous positron surveys, most recently the AMS experiment on the Earth-orbiting International Space Station, have focused on the accurate assessment of the positron-electron fraction (how many positrons are present for each electron). But a more complete characterization requires knowing the energy spectrum of positrons (the absolute number of positrons as a function of particle energy). This is more challenging, as it requires absolute calibration of the detection scheme over a wide range of energies.
Now, a large international collaboration running PAMELA—the satellite-based experiment that delivered the first conclusive evidence for the high-energy positron excess in 2009—has extended its previous surveys and analyzed about positrons collected over three years of measurements, with energies falling in the – giga-electron-volt range. The results, as reported in Physical Review Letters, offer the most accurate picture to date of the information-rich, high-energy part of the positron spectrum.
While confirming the positron abundance known from the previous positron-fraction measurements, the data provide new and complementary information: precise knowledge of the positron energy spectrum will pose further constraints on theories, helping vet the multitude of models, including dark-matter-based ones, that seek to pinpoint the mysterious sources of the most energetic positrons. – Matteo Rini