cdms-ii - CDMS Experiment - University of California, Berkeley

1.3. DIRECT DETECTION OF DARK MATTER 21
to be a featureless spectrum that falls off very rapidly. Putting in numbers gives event
rates of < 1 event kg −1 day −1 with typical recoil energy of a few to tens of keV.
1.3.2 Some Current Direct Detection Experiments
The extremely low rates and recoil energies place severe constraints on experiments
searching for neutralino dark matter through elastic scattering off of nuclei. The experiments
must have extremely low backgrounds and low thresholds. The primary
backgrounds for these experiments come from radioactive contaminants and cosmic
rays. In order to reduce these backgrounds, most experiments are operated in low
radioactivity environments deep underground. Additionally, the lack of any spectral
features makes it worthwhile for experiments to have an additional way of demonstrating
that a signal is due to interaction with galactic dark matter.
DAMA
The DAMA collaboration uses NaI as their target mass and measures the recoil energy
by measuring scintillation in the crystal. For this dark matter search, the signature of
interaction with dark matter is the annual modulation of the recoil spectrum. When
we include the motion of the earth through the galaxy, we find that the expected
recoil spectrum should be harder when the earth is moving with the solar system
through the galaxy and softer when the earth is moving against the solar system.
The DAMA collaboration has taken seven years of data for a total exposure of 58,000
kg days. There is clear evidence for an annual modulation of their signal which the
DAMA collaboration claims to be due entirely to dark matter. This experiment is the
only experiment to claim a dark matter detection [39]. Fig. 1.11 shows the allowed
region of neutralino mass and scalar nucleon cross section for their detection.
ZEPLIN
ZEPLIN uses liquid Xe as its target mass and discriminates between nuclear recoils
and electron recoils. Dark matter interactions will only scatter off of target nuclei
while the majority of known backgrounds will scatter off of electrons in the target