The search for light dark matter

The figure shows the rates for light WIMP interactions (technically, the spin-independent WIMP-nucleon cross section) versus WIMP mass from several different direct detection experiments. The closed contours show possible hints for light WIMP detections from a few experiments, while the open curves indicate the lack of WIMP signals from other experiments (upper limits). The two black curves denoted by crosses show results anticipated in the coming year from the data being taken by CDMS at Soudan. Other experiments, such as the DAMIC experiment led by Fermilab, also expect results on light WIMPs in 2013.

Astronomical observations have shown that there is roughly five times more dark matter than normal matter in our universe. Particle physics theories such as supersymmetry predict the existence of weakly interacting massive particles (WIMPs) that could explain the nature of this dark matter. WIMPS would have been formed in the big bang, and simulations indicate that their gravitational interactions would have provided the scaffolding for normal matter to form galaxies, including our own.

If these ideas are correct, our galaxy is immersed in a cloud of dark matter, and millions of WIMPS are streaming through you as you read this. Fortunately for you, they almost never interact with normal matter. On the rare occasion that they do, the result is a “billiard ball” scatter from an atomic nucleus, imparting a small recoil energy to the nucleus. Since normal-matter particles usually interact with the atomic electrons more than with the nucleus, it is possible to distinguish WIMP interactions from those created by normal matter. In practice, experiments such as the Cryogenic Dark Matter Search (CDMS) have to be extremely well shielded to avoid an overwhelming rate of normal-matter interactions.

Some theorists have begun to suggest that there may be a “dark sector” containing a whole spectrum of dark-matter particles and forces separate from those that make up normal matter. One consequence of such theories would be the existence of low-mass WIMPs, or “light dark matter”. The interaction of a light WIMP with a nucleus would be like a ping pong ball hitting a billiard ball; the latter would barely move. Thus experiments to search for light WIMPS must be sensitive to extremely small energy deposits.

CDMS experimenters mined their data to see if there is an excess of events at the lowest energies, compared with the predicted rate of events from normal matter interactions. Recently, the scientists also looked to see whether the rate of events varied over the course of a year as the WIMP rate is predicted to do, given the earth’s motion through our galaxy. No evidence for light WIMPS was seen, and the resulting limits appear to rule out hints from other experiments. Stay tuned, though: CDMS and other experiments are enhancing their techniques in order to shed new "light" on dark matter.

—Dan Bauer