The Picasso Experiment



    An artists view of our galaxy. Our Milky Way measures 100 000 Light years (Ly) in diameter. The sun lies 20 000 Ly from the center in the Orion arm. The very bright center coincides with the location of a supermassive black hole. Not shown in the picture is the spherical halo of dark matter which surrounds our galaxy.
Credit: Mark A. Garlick / uk

    The most favored candidate for Cold Dark Matter is the Neutralino. This hypothetical particle is neutral, more than 100x heavier than the proton and interacts very weakly with ordinary matter. Therefore this particle is often also called a "WIMP" particle, i.e. a Weakly Interacting Massive Particle!

    If today's Dark Matter paradigm is correct, our galaxy is surrounded by a spherical halo of self-gravitating WIMP particles, which means that these particles travel undisturbed in Keplerian orbits around the galactic centre. The rotation curve of the Milky Way, i.e. the speed of the stars as a function of distance from the center indicates that the halo particle density should fall off with distance from the galactic centre like 1/r2 and in the vicinity of the solar system the mass-energy density should be around 0.3 GeV/cm3. The velocity distribution of the halo WIMPS is a bell shape curve with a dispersion of v=230 km/s. To this so-called Maxwellian velocity distribution we still have to add the relative velocity of the solar system, 244 km/s, with respect to the halo, which gives us a WIMP distribution with a mean velocity of 270 km/s and with a cut-off at 600 km/sec, the WIMP escape velocity. 

    How many WIMP particles traverse each of us per second? Well, it comes to about 109 or a billion particles per second!  But no worries: since WIMPs interact so weakly, they cannot do any harm!

More about the hunt for Neutralinos


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