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article imageAMS experiment on ISS may have detected dark matter signature

By JohnThomas Didymus     Apr 4, 2013 in Science
Scientists have announced that the $2 billion particle detector, the Alpha Magnetic Spectrometer (AMS), mounted on the International Space Station (ISS), may have detected the signature of dark matter annihilation.
According to Digital Journal , the Alpha Magnetic Spectrometer (AMS) is a state-of-the-art particle detector which performs "accurate measurements of cosmic ray with unprecedented sensitivity."
The equipment was installed on the International Space Station (ISS) to detect positrons and electrons generated during dark matter particle-antiparticle interactions.
Since the Alpha Magnetic Spectrometer (AMS) was mounted on the space station by space shuttle Endeavor in May 2011, it has been detecting electrons and their positron anti-particles from deep space and carrying out measurements of their energies. The instrument which has detected billions of cosmic ray particles since it started operation recently recorded the potential signature of dark matter. Scientists believe that "dark matter" makes up more than 80 percent of all matter in the universe while the rest is made up of "baryonic matter."
According to the AMS scientists in a press release, AMS found about 400,000 positrons with energies that suggest they might have been created through collision of dark matter particles which annihilate each other.
Due to the fact that dark matter does not emit light, does not interact with electromagnetic radiation and thus cannot be detected by telescopes, scientific investigators are forced to rely on indirect methods such as the presence of possible products of particle interactions to detect its presence.
Digital Journal explains that according to physicists, the gravitational force of dark matter shapes the large scale structure of spacetime. Its existence was first suspected when calculations led scientists to suggest "missing mass" as explanation of the gravitational effects they observed.
Digital Journal reported: "...based solely on the gravitational force contribution of visible mass in the universe, the galaxies should not hold together. There must be an infusion of 'invisible mass' to explain the stability of large scale galactic structures. Scientists postulate dark matter as constituting 23% of cosmic mass-energy, dark energy 73%, and observable matter only 4%."
Scientists believe that dark matter is made up of what they call weakly interacting massive particles (WIMPs). WIMPs are believed to be their won antimatter particles. Thus when two WIMPs collide they annihilate each other resulting in creation of a positron and an electron.
According to Digital Journal, AMS is able to detect positrons and electrons produced by dark matter particle-antiparticle interactions. AMS has so far detected 25 billion particle, including 8 billion electrons and positrons.
Digital Journal reported that "the Alpha Magnetic Spectrometer (AMS) works on the theoretical notion that dark matter is made of WIMPs (weakly interacting massive particles), consisting of matter particle-antiparticle pairs that annihilate each other to release an electron and its positron antiparticle. Scientists hope that by measuring the ratio of electrons to positrons and the behavior of any excess of positrons across the energy spectrum, they may edge towards a better understanding of dark matter.
"According to physicists, a scenario in which the AMS experiment detects an excess of positrons peaking at a certain energy could indicate a detection of dark matter because while electrons are all around us, only very few processes are known to generate positrons."
Based on theory, scientists have worked out the energies of positrons needed to confirm WIMP annihilation events. According to Digital Journal, Michael Turner, director of the Kavlin Institute for Cosmological Physics at the University of Chicago, said: "The smoking gun signature is a rise and then a dramatic fall in the number of positrons with respect to energy, because the positrons produced by dark matter annihilation would have a very specific energy, depending on the mass of the WIMPs making up dark matter. That's the key signature that would arise." reports that study co-author Veronica Bindi, a physicist at the University of Hawaii, said scientists expect that positrons from dark matter would be detected at energy levels higher than 10 gigaelectron volts (GeV).
CERN said in its press release that the results of measurement of positron energies is consistent with the theory. CERN writes: "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."
AMS measurement of the particle energies are the most accurate so far. Other space-based experiments such as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instrument have detected dark matter signature. But their measurements were not sufficiently refined.
However, AMS is able to produce refined measurements of the spectrum of positron energies.
The evidence for dark matter WIMPs is further strengthened by the fact that the positrons appear to come from all directions in space as scientists expect if they were products of dark matter particle interactions.
However, scientists are being cautious because the evidence so far does not prove dark matter annihilation events conclusively. Experts acknowledge that fast-spinning neutron stars called pulsars could generate the same positron signal.
Nobel laureate Samuel Ting, of the Massachusetts Institute of Technology (MIT), who leads the international AMS team, said: "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. 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."
But Bindi told that even with more data, "we will still not be completely able to figure out if it's really a dark-matter source or a pulsar."
According to, scientists say they will need to detect WIMPs directly through underground experiments such as the Cryogenic Dark Matter Search and XENON Dark Matter projects to fully understand dark matter.
More about Dark matter, Iss, AMS, AMS experiment, Samuel Ting
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