Space-station experiment measures arriving positrons with unprecedented precision

Positrons are copiously produced in interstellar space as a result of collisions between energetic cosmic-ray nuclei and ambient gas. Within the past few years, the PAMELA and Fermi missions have established that for energies above 10 GeV, the ratio of interstellar positrons to electrons is greater than predicted from that well-understood production mechanism and rises with increasing energy. In an experimental tour de force, the excess has now been confirmed with 1% precision by the Alpha Magnetic Spectrometer (AMS), which has been aboard the International Space Station (see the figure) since May 2011. Moreover, the AMS experiment, led by MIT’s Samuel Ting, has extended the energy range over which the positron-to-electron ratio is measured by more than 100 GeV, to 350 GeV; established that the ratio curve has no fine structure; and observed a flattening of the curve at the highest energies. As the AMS group continues its observations over the next decade or two, it will gather enough data to further extend the energy range over which it can make precise measurements and it may see the slope of the flux-ratio curve become negative. If so, the details of the turnover should help scientists distinguish between the exciting possibility that dark-matter annihilation is the source of the positron excess and the more mundane possibility that astrophysical sources such as pulsars are the culprits. Already the team has reported a feature of its positron measurements that is difficult, though hardly impossible, to reconcile with isolated astrophysical sources: The observed spectrum shows no evidence of anisotropy. (M. Aguilar et al., AMS collaboration, Phys. Rev. Lett. 110, 141102, 2013.)—Steven K. Blau