AMS measures antimatter excess in space

The international team running the Alpha Magnetic Spectrometer (AMS) has announced the first results in its search for dark matter. They indicate the observation of an excess of positrons in the cosmic-ray flux. The results were presented by Samuel Ting, the spokesperson of AMS, in a seminar at CERN on 3 April, the date of publication in Physical Review Letters.

The AMS results are based on an analysis of some 2.5 × 10¹⁰ events, recorded over a year and a half. Cuts to reject protons, as well as electrons and positrons produced in the interactions of cosmic rays in the Earth’s atmosphere, reduce this to around 6.8 × 10⁶ positron and electron events, including 400,000 positrons with energies between 0.5 GeV and 350 GeV. This represents the largest collection of antimatter particles detected in space.

The data reveal that the fraction of positrons increases from 10 GeV to 250 GeV, with the slope of the increase reducing by an order of magnitude over the range 20–250 GeV. The data also show no significant variation over time, or any preferred incoming direction. These results are consistent with the positrons’ origin in the annihilation of dark-matter particles in space but they are not yet sufficiently conclusive to rule out other explanations.

The AMS detector is operated by a large international collaboration led by Nobel laureate Samuel Ting. The collaboration involves some 600 researchers from China, Denmark, Finland, France, Germany, Italy, Korea, Mexico, the Netherlands, Portugal, Spain, Switzerland, Taiwan and the US. The detector was assembled at CERN, tested at ESA’s ESTEC centre in the Netherlands and launched into space on 16 May 2011 on board NASA’s Space Shuttle Endeavour. Designed to study cosmic rays before they interact with the Earth’s atmosphere, the experiment is installed on the International Space Station. It tracks incoming charged particles such as protons and electrons, as well as antimatter particles such as positrons, mapping the flux of cosmic rays with unprecedented precision.

An excess of antimatter within the cosmic-ray flux was first observed around two decades ago in experiments flown on high-altitude balloons and has since been seen by the PAMELA detector in space and the Large Area Telescope on the Fermi Gamma-ray Space Telescope. The origin of the excess, however, remains unexplained.

One possibility, predicted by theories involving supersymmetry, is that positrons could be produced when two particles of dark matter collide and annihilate. Assuming an isotropic distribution of dark-matter particles, these theories predict the observations made by AMS. However, the measurement by AMS does not yet rule out the alternative explanation that the positrons originate from pulsars distributed around the galactic plane. Moreover, supersymmetry theories also predict a cut-off at higher energies above the mass range of dark-matter particles and this has not yet been observed.

AMS is the first experiment to measure to 1% accuracy in space – a level of precision that should allow it to discover whether the positron observation has an origin in dark matter or in pulsars. The experiment will further refine the measurement’s precision over the coming years and clarify the behaviour of the positron fraction at energies above 250 GeV.