Distinguishing Between Pointlike and "Extended" Dark Matter

Distinguishing between pointlike and "extended" dark matter will be possible in new experiments. Detecting the elusive dark matter that seems to pervade the universe is difficult enough, but it's an even bigger challenge to distinguish between the different kinds of the invisible matter.

Candidates for dark matter include pointlike particles such as neutralinos (predicted by supersymmetry theory) and extended objects such as "Q-balls," large bags of particles such as squarks and sleptons which are also predicted in supersymmetry. Numerous planned and ongoing experiments aim to detect directly dark matter, which is believed to possess energies in the minuscule range of 0.001-0.01 MeV.

To catch such novel forms of matter researchers place their detector deep underground, to shield their equipment from cosmic ray particles such as muons and electrons. A dark matter detector typically contains some transparent medium, such as a clear crystal or liquid made of specific atoms or molecules. A dark matter particle usually passes easily through ordinary matter, but occasionally it collides with an atomic or molecular nucleus in the medium. As a result, the nucleus recoils, radiating a flash of light with a specific energy that can provide information on the identity of the dark matter.

Pointlike and extended dark matter would strike their target in different ways, according to researchers (Alexander Kusenko, UCLA, 310-825-4814, kusenko@ucla.edu). Hitting a car with a hammer, they point out, produces different results from hitting a car with a pillow, even though the two may carry the same amount of energy. The pointlike (hammer-like) matter transfers its momentum instantaneously, while the extended (pillow-like) matter transfers its momentum more slowly. Such a difference can be detected in a plot of the number of collision events versus momentum. Extended dark matter would produce a greater amount of "softer" collisions in the lower-momentum range.

Current dark matter experiments are good enough in certain cases to tell the difference between the two types of dark matter, though a welcome improvement would be to detect even lower-momentum collisions than presently possible. (Gelmini et al., Physical Review Letters, 2 Sept 2002)