An international team of scientists using the Alpha Magnetic Spectrometer aboard the International Space Station has announced the first results in the search for dark matter. The results are the most precise measurements to date of the ratio of positrons to electrons in cosmic rays.
The International Space Station taken during STS-134 mission’s fourth spacewalk. The Alpha Magnetic Spectrometer (AMS) was installed one week earlier during the mission’s first spacewalk (NASA)
The Alpha Magnetic Spectrometer (AMS) consists of seven instruments that monitor cosmic rays from space. Unprotected by Earth’s atmosphere the instruments receive a constant barrage of high-energy particles. As these particles pass through AMS, the instruments record their speed, energy and direction.
The project is one of the largest scientific collaborations of all time involving 56 institutes from 16 countries. The instrument was tested at ESA’s technical facility in the Netherlands before being shipped to the U.S. for launch on Space Shuttle Endeavour.
The first AMS results, published in the journal Physical Review Letters (free .pdf), are based on some 25 billion recorded events, including 400,000 positrons with energies between 0.5 GeV and 350 GeV, 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 20-250 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 Alpha Magnetic Spectrometer (AMS). 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,” said AMS project spokesperson Prof Samuel Ting.
Cosmic rays are charged high-energy particles that permeate space. The AMS experiment 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 can not yet rule out the alternative explanation that the positrons originate from pulsars distributed around the galactic plane. 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. 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 will be the first of many,” Prof Ting said. “AMS is the first experiment to measure to 1 per cent 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.”
“The AMS result is a great example of the complementarity of experiments on Earth and in space,” said CERN Director General Dr 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.”
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Bibliographic information: M. Aguilar et al. 2013. First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV. Phys. Rev. Lett., vol. 110, no. 14; doi: 10.1103/PhysRevLett.110.141102
