Sunday, May 12, 2013

Dark Matter

This topic is a bit late, but is too important to be left out altogether.The  Alpha Magnetic Spectrometer experiment in the International  space station has seen the excess of positrons at high energies that is supposed to be one of the characteristic signatures of dark matter. Dark matter is supposed to neither emit nor absorb light. Its presence is inferred from its gravitational effects on normal matter. Dark matter is supposed to account for about  85 percent of the total matter in  the universe. The current experiments carried out by the AMS experiment measured the excess of high energy positrons in earth bound cosmic rays, as predicted due to the presence of dark matter.

Well, dark matter is supposed to be made up of some hitherto undetected types of elementary particles, e.g. weakly-interacting massive particles, or WIMP-s, as well as many others. Particles of this type would be produced thermally in the early universe and are predicted by many theoretical extensions to the Standard Model of particle physics. If dark matter is made from such particles, and they encounter their own antiparticle, they may annihilate each other and produce some of the particles we are familiar with, e.g. protons and electrons. Some possible dark matter particles, are expected to be their own antiparticles, (e.g. photons, Z particles and Higgs bosons are examples of particles which are their own antiparticles), they are also expected to be rather heavy. If such dark matter particles encounter each other and annihilate to give lighter weight familiar particles, then these light particles would have high energy. This is a consequence of energy momentum conservation, by which pairs of heavy, slow moving dark matter particles would convert their energy into pairs of light, fast moving, familiar particles, as seen in cosmic rays, which the AMS measures. What the AMS does, is to count the number of electrons and positrons at a certain energy, and measure the fraction of positrons. At high energies, in the absence of dark matter annihilation, the number of electrons is expected to be much larger than the number of positrons. This is because, electrons are common in the universe, and can be easily accelerated to high energies, whereas positrons are produced out of collision processes of electrons, and consequently come out with lower energies than those of the electrons that produced them. However, if the electrons and positrons came out of dark matter particle annihilations, they would be equally energetic. In this case, the fraction of positrons would increase at large energies. The  typical energy of such electron positron pairs would be a little less than  the mass energy of the annihilating dark matter particles, and hence there would be a bump in the energy distribution of the positrons at this energy.

Experts say that the AMS result only confirms what other experiments, like, the measurements made by the Pamela and Fermi satellites, as well as terrestrial experiments. However, it is all to the good that all the experiments are pointing in the same direction. The AMS also has far more accurate measurements, and has narrowed the error bars. The AMS also finds that the distribution of positrons is isotropic, which was not quite expected from astronomical sources. All said and done, there are exciting days ahead in this direction. As for dark energy, may be some other day.

This blog post is by Neelima Gupte and Sumathi Rao.

For a more technical description of dark matter, and all the other experiments, see  this blog.


Two positrons walked into a bar.  `I'm much higher than you on the GeV scale' , bragged the high energy positron.  `And how did you do it' said the low energy positron, wistfully.  `By dark and annihilatory deeds', said the  high energy positron.

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