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…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.”