Upper limits on the thermally averaged cross section of the WIMP as a function of its mass calculated using the VHE flux limits and DM density models for Draco (solid line), Ursa Minor (dashed line), and M15 (dotted line). Note that the limits derived from M15 represent a scenario in which the DM halo has been adibatically compressed by the infall of baryonic matter. Shown as open circles are MSSM models that fall within three standard deviations of the relic DM density measured in the three-year WMAP data set.

A Search for Dark Matter

One of the leading candidates for dark matter (DM) is a weakly interacting massive particle (WIMP) that was in thermal equilibrium with baryons and photons in the early universe and decoupled as the universe expanded.  The mass of the WIMP is expected to be on the scale of 50 GeV - 10 TeV based on the known relic density of DM and the results of accelerator searches.  Particle physics models such as the minimal supersymmetric extension to the standard model (MSSM) can naturally accomodate a particle with these properties, for example the lightest supersymmetric particle, usually the neutralino.  The self-annihilation of WIMPs in astrophysical environments is predicted to generate stable secondary particles including very-high-energy (VHE) gamma-rays with energies up to the mass of the WIMP particle. Ground- and satellite- based VHE gamma-ray observatories can potentially detect gamma-ray fluxes from DM annihilations in regions of especially high DM density such as the Galactic Center and the cores of nearby galaxies.  The detection of the gamma-ray emission from such regions with the unique spectral characteristics of DM annihilations would provide a compelling case for the identification of the particle physics counterpart to astrophysical DM.

The Whipple 10m telescope conducted a search for VHE gamma-ray emission from DM self-annihilations in five astrophysical targets: the dwarf galaxies Draco and Ursa Minor, the local group galaxies M32 and M33, and the globular cluster M15.  These data comprise approximately 50 hours of observations taken over the course of the 2002-2004 observing seasons.  No sources of significant VHE emission were detected, and upper limits were derived on the thermally averaged product of velocity and cross-section of the WIMP as a function of its mass (see Figure).  To interpret the VHE flux upper limits as constraints on properties of the WIMP, the DM distribution of each source modeled with input from stellar and gas kinematics as well as cold dark matter (CDM) simulations.  For some sources, such as dwarf galaxies, the present-day distribution of DM is likely well described by collisionless CDM simulations and can be directly inferred from existing kinematic data.  Sources in which baryonic matter dominates the central gravitational potential such as globular clusters and local group galaxies are more challenging to model but may have a significantly larger annihilation signal augmented by the adiabatic compression of DM in the core by infalling baryonic matter.  Future measurements by current generation instruments such as VERITAS should improve on the sensitivity of the limits presented here by at least an order of magnitude.

For more information on the science of this object please see M.Wood et al, The Astrophysical Journal, Volume 678, Issue 2, pp. 594-605 (arXiv:0801.1708 ).  You may also contact Matthew Wood.