CDMS looks for finger prints of axions

The finger print that CDMS is looking for: the expected solar axion event rate in a germanium detector depends on the energy of the axions and the position of the sun in the sky. The position of the sun is plotted as time of day.

The theory of strong interactions, known as quantum chromodynamics, predicts that matter and antimatter behave slightly differently, a phenomenon known as CP violation. However, CP violation has never been observed in strong interactions.

In order to save QCD from this dilemma, theorists predict the existence of a particle known as the axion, which barely interacts with matter. While the particle fixes the CP violation problem, experiments have not yet detected any axions.

According to theory, an axion could emerge when a photon traverses a very strong electric or magnetic field. The core of the sun would be a perfect region for the creation of axions. The particles would immediately escape the sun and some of them would travel through Earth.

The Cryogenic Dark Matter Search, which takes place deep underground in the Soudan Underground Laboratory in Minnesota, has searched for axions and set new limits on the properties of these particles. The result made the cover of the Oct. 1 issue of Physical Review Letters.

The primary goal of the CDMS collaboration is the search for weakly interacting massive particles, which are candidates for dark matter particles. But its germanium and silicon detectors, which operate at 40 milliKelvin, are also extremely sensitive to low-energy X-ray photons and hence serve as axion detectors as well. Solar axions that traverse the CDMS detectors would coherently scatter off crystals in the detectors, akin to X-ray Bragg scattering off crystal planes. The interaction probability depends on the energy and the incident angle of the axions.

Determining the incident angle required the precise knowledge of the orientation of the detector crystal planes, which are located a half mile underground, with respect to the location of the sun — a daunting task. Fortuitously, in 1999 the Fermilab Alignment Group had measured the absolute geodesic true North in the Soudan mine to within a few millidegrees of accuracy. The directions of the CDMS crystal planes are also precisely known.

Still, CDMS scientists had to correlate the two measurements, a challenge since the detectors are located inside a vacuum vessel and buried within a massive shield to protect the detectors from background noise. Ultimately, CDMS scientists determined the direction of their detectors relative to the sun to within three degrees of accuracy.


Source: http://www.fnal.gov/pub/today/