Experiment measures excess of cosmic positrons

Physics : The first results from the Alpha Magnetic Spectrometer (AMS) experiment, a project 18 years in the making and orbiting aboard the International Space Station since May 2011, were announced yesterday. AMS counts the arrival of positrons, the electron’s antiparticle, as a function of their energy. Years ago the PAMELA and Fermi missions established that high-energy positrons were more copious than well-understood cosmic-ray theory predicted; AMS confirmed those results with much greater precision and extended the measured positron energy range. An exciting possibility is that the extra positrons result from interactions of the mysterious dark matter that makes 5/6 of the universe’s material stuff, but astrophysical objects such as pulsars also generate positrons. AMS has been collecting data for two years and should be collecting for another decade or two. The instrument will further extend the energy range of the positron spectrum and may provide the data that convincingly determine the source of the positron excess.