Knocking on Heaven's Door - Lisa Randall [186]
By measuring both ionization and the initial scintillation, experimenters distinguish signal from background radiation. The XEN0N100 experiment uses very special phototubes that were designed to work in the low-temperature, high-pressure environment of the detector to measure the scintillation. Argon detectors might provide even better scintillation information in the future through their use of the detailed shape of the scintillation pulse as a function of time, and that will also help separate the wheat from the chaff.
The strange state of affairs today (although this might soon change) is that one scintillation experiment—the DAMA experiment in the Gran Sasso Laboratory in Italy—has actually seen a signal. DAMA, unlike the experiments I just described, has no internal discrimination between signal and background. Instead it relies on identifying dark matter signal events solely by their time dependence, using the distinctive velocity dependence coming from the Earth’s orbit around the Sun.
The reason the velocity of incoming dark matter particles is relevant is that it determines how much energy is deposited in the detector. If the energy is too low, the experiment won’t be sensitive enough to know if anything was there. More energy means the experiment is more likely to record the event. Due to the Earth’s orbital velocity, the speed of dark matter relative to us (and hence the energy deposited) depends on the time of year—making it easier to see a signal at some times of year (summer) than at others (winter). The DAMA experiment looks for an annual modulation in the event rate that accords with this prediction. And their data indicates they have found such a signal. (See Figure 79 for the oscillating DAMA data.)
[ FIGURE 79 ] Data from the DAMA experiment showing the modulation of the signal over time.
No one yet knows for sure whether the DAMA signal represents dark matter or is due to some possible misunderstanding about the detector or its environment. People are skeptical because no other experiment has yet seen anything. This absence of other signals is inconsistent with the predictions of most dark matter models.
Although confusing for the time being, this is the sort of thing that makes science interesting. The result encourages us to think about what different types of dark matter might exist and whether dark matter might have properties that make it easier for DAMA to see it than other dark matter detection experiments. Such results also force us to better understand the detectors so that we can identify spurious signals and tell if the data mean what the experimenters claim.
Other experiments all over the globe are working to achieve greater sensitivity. They could either rule out or confirm the DAMA dark matter discovery. Or they might independently discover a different type of dark matter on their own. Everyone would agree that dark matter had been discovered if even one other experiment confirms what DAMA has seen, but this has not yet occurred. Nonetheless, answers should be available soon. Even if the results just presented are out of date when you read this, the nature of the experiments most likely won’t be.
INDIRECT DARK MATTER DETECTION
LHC experiments and ground-based cryogenic or noble liquid detectors are two ways to determine the nature of dark matter. The third and final way is through indirect detection of dark matter in the sky or on Earth.
Dark matter is dilute, but