The international team running the Alpha Magnetic Spectrometer (AMS) today announces the first results in its search for dark matter. The AMS paper, to be published in the journal Physical Review Letters, reports the observation of an excess of positrons in the cosmic ray flux.
The AMS results are based on some 25 billion recorded events, including 400,000 positrons with energies between 0.5 GeV and 350 GeV (gigaelectronvolts, a unit of energy equal to one billion electron volts), recorded over a year and a half. This represents the largest collection of antimatter particles recorded in space. The positron fraction increases from 10 GeV to 250 GeV, with the data showing the slope of the increase reducing by an order of magnitude over the range 20 to 250 GeV. The data also show no significant variation over time, or any preferred incoming direction. These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations.
“As the most precise measurement of the cosmic ray positron flux to date, these results show clearly the power and capabilities of the AMS detector,” AMS spokesperson Samuel Ting says. “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”
Cosmic rays are charged high-energy particles that permeate space. The AMS experiment, installed on the International Space Station, is designed to study them before they have a chance to interact with the Earth’s atmosphere. An excess of antimatter within the cosmic ray flux was first observed around two decades ago. The origin of the excess, however, remains unexplained. One possibility, predicted by a theory known as 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 AMS measurement cannot yet rule out the alternative explanation that the positrons originate from pulsars distributed around the galactic plane. Supersymmetry theories also predict a cutoff at higher energies above the mass range of dark matter particles, and this has not yet been observed. Over the coming years, AMS will further refine the measurement’s precision, and clarify the behavior of the positron fraction at energies above 250 GeV.
“When you take a new precision instrument into a new regime, you tend to see many new results, and we hope this this will be the first of many,” Ting says. “AMS is the first experiment to measure to 1 percent accuracy in space. It is this level of precision that will allow us to tell whether our current positron observation has a dark matter or pulsar origin.”
Dark matter is one of the most important mysteries of physics today. Accounting for more than a quarter of the universe’s mass-energy balance, it can be observed indirectly through its interaction with visible matter but has yet to be directly detected. Searches for dark matter are carried out in space-borne experiments such as AMS, as well as on the Earth at the Large Hadron Collider and a range of experiments installed in deep underground laboratories.
“The AMS result is a great example of the complementarity of experiments on Earth and in space,” said CERN Director General Rolf Heuer. “Working in tandem, I think we can be confident of a resolution to the dark matter enigma sometime in the next few years.”