1. Nuclear Physics Seminar

    "Searching for Dark Matter"

    Presented by John D. Vergados, University of Ioannina

    Friday, May 24, 2013, 10 am
    Large Seminar Room, Bldg. 510

    Hosted by: Nick Simos

    The recent WMAP data not only have confirmed the previously existing gravitational evidence for the presence of dark matter in galactic halos, but they have also shown that this type is the dominant form of mater in the Universe. Modern particle theories provide viable cold dark matter candidates with masses in the GeV-TeV region. All such candidates are called WIMPs (Weakly Interacting Massive Particles). The nature of dark matter, however, can only be unraveled by its direct detection in the laboratory. Since the WIMPS have an average energy in the keV region, they cannot excite the nucleus and their detection consists of measuring the nuclear recoil energy. We will discuss all factors contributing to the event rate, namely : i) the nucleon cross section, which depends on particle physics information, ii) the WIMP velocity distribution and iii) the relevant nuclear structure. The experiments are hard, since the expected cross sections are small and the signal spectrum cannot be easily distinguished from the background. None of the experiments has yet detected dark matter, but the impressive limit on the coherent nucleon cross section of  10'44cm2 = 10'8pb has been reached, assuming that it is dominant. Limits which are 4 orders of magnitude larger have also been reached for the spin induced cross sections. The two modes cannot be separated in a single target experiment. We will show, however, that given a judicious choice of at least three odd mass targets one may be able to extract from experiments all three nucleon cross sections (coherent as well as the proton and neutron spin induced). For an unambiguous detection of dark matter one would like exploit some characteristic signatures of the reaction. These are a) The modulation effect, i.e. the time dependence of the rate due to the Earth's annual motion and b) rhe correlation of the event rates with sun's direction of motion in directional experiments, i.e. those that