The lightness of dark matter

This figure shows the new CDMSlite result (black solid line with salmon-shaded band), compared with some other recent results on low-mass dark matter. The curves indicate that dark matter WIMPS were not found with masses and interactions with normal matter above and to the right of the curves. The shaded regions show areas where there have been experimental hints of a dark matter signal; these are mostly ruled out by this result and others.

This figure shows the new CDMSlite result (black solid line with salmon-shaded band), compared with some other recent results on low-mass dark matter. The curves indicate that dark matter WIMPS were not found with masses and interactions with normal matter above and to the right of the curves. The shaded regions show areas where there have been experimental hints of a dark matter signal; these are mostly ruled out by this result and others.

Astronomical observations have established the existence of dark matter in the universe, including our own galaxy. However, as yet there is little understanding of the nature of this dark matter. Many particle physics theories predict the existence of weakly interacting massive particles (WIMPs) that could constitute dark matter, but these particles have proven elusive.

Experiments such as the Cryogenic Dark Matter Search (CDMS) try to directly detect dark matter particles by searching for the rare interaction of such particles with those that make up normal matter, particularly with atomic nuclei. These experiments must be carefully designed to separate such interactions from the vastly more numerous backgrounds due to normal-matter particles interacting with each other.

Recent theories have postulated 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 have recently refined their experimental technique, called CDMSlite, to be even more sensitive than before to low-energy scatters. The main improvement was a deeper understanding of where normal matter backgrounds would appear in the CDMSlite germanium detector and the development of a way to eliminate those regions from the search. Additional improvements came from better operating stability, improved rejection of electronic and vibrational noise, and extensive calibration to understand the energy response of the detector.

Alas, no WIMP signal is yet visible in the data. But the experiment has considerably narrowed the region where light WIMPS might be hiding. The top figure shows the new territory explored with this result.

This is by no means the end of the story. The collaboration is designing a new experiment, SuperCDMS SNOLAB, with advanced detectors that will have far better sensitivity to light WIMPS. While the ultimate goal is to find dark matter, SuperCDMS SNOLAB will be capable of finding another particle more familiar to Fermilab. Neutrinos can interact in a similar “billiard ball” style as WIMPs do, and actually form a small part of the dark matter in the universe. SuperCDMS SNOLAB will be sensitive enough to see neutrinos from the sun, which, in a way, is another form of lightness from dark matter.

See other science results from Fermilab.