Direct Detection of WIMP Dark Matter



Essay Series

Ingredients of the Universe

Why Dark Matter?

Dark Matter Candidates

The Search for Dark Matter

Frequently Asked Questions

The Science of CDMS

Direct Detection of WIMPs

CDMS II Detectors

Soudan Installation

Physics Goals


WIMP Dark Matter

Expectations for non-baryonic dark matter are founded principally in Big Bang nucleosynthesis calculations, which indicate that the missing mass of the universe is not likely to be baryonic. The supersymmetric standard model (SUSY) offers a promising framework for expectations of particle species which could satisfy the observed properties of dark matter. The most likely SUSY candidate for a dark matter particle is the lightest supersymmetric particle (LSP), the neutralino χ01, which is a superposition of the fermionic superpartners of the Higgs and neutral gauge bosons. In order to be consistent with an early-universe annihilation rate leaving proper relic abundances, such a particle should have a small but measureable interaction cross section with ordinary matter. Specifically, a cross section for interaction between a neutralino and a nucleon in ordinary matter of the order of the electro-weak scale would be consistent with a meaningful cosmological role for the particle. This expectation of a weak interaction together with the expected mass range of the neutralino, 10 to 1000 GeV, produce the acronym "WIMP": Weakly Interacting Massive Particle. By virtue of their weak-scale interaction, WIMPs should be able to be observed by directly detecting their interactions with ordinary matter. Note that this detection procedure is not specific to neutralinos - any WIMP with comparable mass and scattering cross section is detectable in this manner.

Direct Detection of WIMPs

According to models of cosmological structure formation, the luminous matter of galaxies is gravitationally bound to a more massive, sprawling halo of dark matter. Should the dark matter of the universe consist of unidentified particles, our solar system and our planet would be passing through a flux of these dark matter particles which constitute the dark halo of the Milky Way galaxy. WIMP dark matter could then be detected directly as the Earth (and some detection apparatus beneath its surface) pass through our galaxy's DM halo. Given the expected weak interaction scale of the neutralino-nucleon scattering, galactic WIMPs should deposit a measurable amount of energy in an appropriately sensitive detector apparatus. This can occur through elastic scattering between an incident WIMP and a nucleus in the fiducial volume of some monitored detector material. The CDMS experiments (and many others) aim to measure the recoil energy imparted to detector nuclei through neutralino-nucleon collisions by employing sensitive phonon detection equipment coupled to arrays of cryogenic germanium and silicon crystals. The phonon signals generated within the crystal detectors can be processed and interpreted with information about known background rates. If found, a confidently identified above-background event rate would be analyzed to determine the nature of the responsible interaction -- perhaps enabling an identification of WIMPs. Conversely, a null WIMP-nucleon scattering find can be used to improve greatly current limits on the possible neutralino interaction cross section.

Last updated April 28, 2007