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An international experiment aboard the International Space Station (ISS) today reported the observation of an excess of positrons in the cosmic ray flux, the source of which could be the elusive dark matter.
This forms the most important part of the first results from the experiment, called the Alpha Magnetic Spectrometer (AMS), which were reported by the experiment’s spokesperson, Nobel Laureate Samuel Ting of the Massachusetts Institute of Technology (MIT) at the meeting of the American Association for Advancement of Science (AAAS) at Boston, Massachusetts.
The mysterious dark matter, which is believed to account for a quarter of the universe’s mass-energy balance and is distributed isotropically — invariant with respect to direction — in the space, can be observed indirectly through its gravitational interaction with visible matter but is yet to be directly detected.
The search for dark matter is one of the objectives of this space-borne AMS even as it is being actively searched for in ground-based experiments such as the Large Hadron Collider (LHC) and other experiments in deep underground experiments.
The instrument is basically a giant magnet and an antimatter detector attached to the outside of the ISS. It is the most powerful and sensitive particle spectrometer ever deployed in space. It is designed to study the cosmic ray particles, which are charged high-energy particles that permeate space, before they have a chance to interact with the Earth’s atmosphere.
The first AMS results are based on the analysis of about 25 billion recorded primary cosmic ray events. The events were recorded between May 19, 2011 and December 10, 2012. Of these, an unprecedented 6.8 million were unambiguously identified as electrons and their antimatter counterparts, positrons, observed in the energy range 0.5 Giga electron-Volt (GeV) to 350 GeV. Of these 6.8 million particles, more than 400,000 were positrons. This is the largest number of energetic antimatter particles directly measured and analysed from space.
The data shows that the number of positrons, as a fraction of the total number of electrons and positrons, first decreases from 0.5 GeV to 10 GeV and then steadily increases from 10 GeV to 250 GeV. This is not the first time that positron excess in cosmic ray flux has been seen in space. At least two space missions, the Payload for Antimatter Exploration and Light-nuclei Astrophysics (PAMELA) and the Fermi space telescope, have already seen hints of such an antimatter excess, but they did not go up to the energy levels that AMS has done, and that too with great precision, which seems to provide some hints of the origin of this excess.
That hint, according to the researchers, comes from the fact that the rate of growth of the positron fraction indicates a decrease by an order of magnitude from 20 GeV to 250 GeV and then the fraction seems to flatten at energies above 250 GeV. Ting said that more data was needed to conclusively establish that flattening of the curve.
The other interesting thing is that the positron fraction shows no anisotropy, which means that the positrons are not coming from a preferred direction in space. “Together, these features show evidence of a new physics phenomenon,” the researchers have contended.
The positron ratio is a key parameter in the search for dark matter. Some theories, such as supersymmetry, predict that dark mater particles will collide and annihilate in space, producing an excess of positrons that detectors such as the AMS would be able to detect. The AMS data also seems to fit perfectly the predictions of models based on such theories.
Supersymmetric theories also predict a cut-off of positrons at higher energies, basically limited by the mass of dark matter particles, and this is yet to be observed. As AMS probes higher energies up to tera electron-Volt (Tev) scale, which it is capable of, it is to be seen whether the apparent flattening of positron fraction above 250 GeV continues and eventually leads to a cut-off.
As of now, however, the AMS experiment can not yet conclusively rule out the other possible explanation for this excess, which are the pulsars distributed around the galactic plane. Pulsars are rotating neutron stars with very high magnetic fields, which can occur in remnants of supernova explosions as isolated objects or as binary systems.
“As the most precise measurement of the cosmic ray positron flux to date, …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.”