Knocking on Heaven's Door (59 page)

Because it was a two-hour informal presentation, those of us in attendance could interrupt whenever necessary to understand as much as possible. The talk nicely addressed questions that the audience, which consisted mostly of particle physicists, would find confusing. Harry, who was trained as a particle physicist—not as an astronomer—spoke the same language we did.

With these extraordinarily difficult dark matter experiments, the devil is in the details. Harry made that abundantly clear. The CDMS experiment is based on advanced low-energy physics technology—the kind more conventionally associated with so-called condensed matter or solid state physicists. Harry told us how before joining the collaboration he would never have believed such delicate detections could possibly work, joking that his experimental colleagues should be grateful he wasn’t a referee on the original proposal.

CDMS works very differently from scintillating xenon and sodium iodide detection experiments. It has hockey-puck-size pieces of germanium or silicon topped by a delicate recording device, which is a phonon sensor. The detector operates at very low temperature—low enough to be just at the border between superconducting and non-superconducting. If even a small amount of energy from phonons, the sound units that carry the energy through the germanium or silicon, much like photons are the units of light—hit the detector, it can be enough to make the device lose superconductivity and register a potential dark matter event through a device called a
superconducting quantum interference device (SQUID)
. These devices are extraordinarily sensitive and measure the energy deposition extremely well.

But recording an event isn’t the end of the story. The experimenters need to establish that the detector is recording dark matter—not just background radiation. The problem is that everything radiates. We radiate. The computer I’m typing on radiates. The book (or electronic device) you’re reading radiates. The sweat from a single experimentalist’s finger is enough to swamp any dark matter signal. And that doesn’t even take into account all the primordial and man-made radioactive substances. The environment and the air as well as the detector itself carry radiation. Cosmic rays can hit the detector. Low-energy neutrons in the rock can mimic dark matter. Cosmic ray muons can hit rock and create a splash of material, including neutrons that can mimic dark matter too. There are about 1,000 times as many background electromagnetic events as predicted signal events, even with reasonably optimistic assumptions about the mass and interaction strength of the dark matter particles.

So the name of the game for dark matter experiments is
shielding and discrimination
. (This is the astrophysicists’ term. Particle physicists use the more PC term
particle ID
, though these days I’m not sure that’s so great either.) Experimenters need to shield their detector as much as possible to keep radiation out and discriminate potential dark matter events from uninteresting radiation scattering in the detectors. Shielding is ac-complished in part by performing the experiments deep in mines. The idea is that cosmic rays will hit the rock surrounding the detector before they hit the detector itself. Dark matter, which has far fewer interactions, will make it to the detector unimpeded.

Fortunately for dark matter detection, plenty of mines and tunnels exist. The DAMA experiment, along with experiments called XENONIO and the bigger version XENONIOO—as well as CRESST, a detector that uses tungsten—take place in the Gran Sasso laboratory, situated in a tunnel in Italy about 3,000 meters underground. A 1,500-meter-deep cavern in the Homestake mine in South Dakota, originally built for gold excavation, will be home to another xenon-based experiment known as LUX. This experiment will take place in the very same cavity where Ray Davis discovered neutrinos from nuclear reactions taking place in the Sun. The CDMS experiment is in the Soudan mine, about 750 meters underground.

Still, all that rock above the mines and tunnels is not enough to guarantee that the detectors are radiation-free. The experiments further shield the actual detectors in a variety of ways. CDMS has a layer of surrounding polyethylene that will light up if something too strongly interacting to be dark matter comes through from the outside. Even more memorable is the surrounding lead from an eighteenth-century sunken French galleon. Older lead that has been underwater for centuries has had time to shed its radioactivity. It is a dense absorbing material that is perfect for shielding the detector from incoming radiation.

Even with all these precautions, a lot of electromagnetic radiation still survives. Distinguishing radiation from potential dark matter candidates requires further discrimination. Dark matter interactions resemble nuclear reactions that occur when a neutron hits the target. So opposite the phonon readout system is a more conventional particle physics detector that measures the ionization created when the alleged dark matter particle passed through the germanium or silicon. Together, the two measurements, ionization and phonon energy, distinguish nuclear events—the good processes that might be the result of dark matter—from events due to electrons, which are just radioactivity induced.

Other beautiful features of the CDMS experiment include the excellent position and timing measurements that it can make. This is nice because although the position is only directly measured in two directions, the timing of the phonons gives the position in the third coordinate. So experimenters can locate exactly where the event happened and discard background surface events. Another nice feature is that the experiment is segmented into the stacked hockey-puck-size detectors. A true event will occur in only one of these detectors. Locally induced radiation, on the other hand, won’t necessarily be confined to a single detector. With all these features and an even better design to come, CDMS has a good chance of finding dark matter.

Nonetheless, impressive as it is, CDMS is not the only dark matter detector and cryogenic devices are not the only type. Later on in the week, Elena Aprile, one of the xenon experiment pioneers, gave comparable details about her experiments (XEN0N10 and XEN0N100), as well as other experiments performed with noble liquids. Since these would soon be the most sensitive detectors for dark matter, the audience paid rapt attention to her talk too.

Xenon experiments record dark matter events through their scintillation. Liquid xenon is dense and homogenous, has a large mass per atom (enhancing the dark matter interaction rate), scintillates well, ionizes fairly readily when energy is deposited so that the two types of signals described above can efficiently discriminate against electromagnetic events, and is relatively cheap compared to other potential materials—although the price had fluctuated by a factor of six in the course of the decade. Noble gas experiments of this type have become a lot better as they have gotten bigger, and they should continue to do so. With more material, not only is detection more likely, but the outer part of the detector can shield the inner part of the detector more efficiently, helping assure the significance of a result.

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 nonetheless occasionally annihilates with itself or with its antiparticle. This doesn’t happen enough to significantly affect the overall density, but it might be enough to produce a measurable signal. That’s because when dark matter particles annihilate, new particles get produced that carry away their energy. Depending on its nature, dark matter annihilation could sometimes yield detectable Standard Model particles and antiparticles, such as electrons and positrons, or pairs of photons. Astrophysical detectors that measure antiparticles or photons might then see signs of these annihilations.

The instruments that search for these Standard Model products of dark matter annihilation weren’t initially designed with this goal. They were conceived as telescopes or detectors out in space or on the ground to detect light or particles in order to better understand what is in the sky. By looking at what types of stuff gets emitted by stars and galaxies and exotic objects that lie within them, astronomers can learn about the chemical composition of astronomical objects and deduce the properties and nature of stars.

The philosopher Auguste Comte in 1835 mistakenly said about stars, “We can never by any means investigate their chemical composition,” which he thought beyond the boundary of attainable knowledge. Yet not too long after he said those words, the discovery and interpretation of the spectra of the Sun—the light that was emitted or absorbed—taught us about the composition of the Sun and proved him decidedly wrong.

Experiments today continue this mission when they try to deduce the composition of other celestial objects. Today’s telescopes are very sensitive, and every few months we learn more about what is out there.

Fortunately for dark matter searches, the observations of light and particles that these experiments are already engaged in might also illuminate the nature of dark matter. Since antiparticles are relatively rare in the universe and the distribution of photon energies could exhibit distinctive and identifiable properties, such detection could eventually be associated with dark matter. The spatial distribution of these particles might also help distinguish such annihilation products from more common astrophysical backgrounds

HESS, the High Energy Stereoscopic System located in Namibia, and VERITAS, the Very Energetic Radiation Imaging Telescopic Array System in Arizona, are large arrays of telescopes on Earth that look for high-energy photons from the center of the galaxy. And the next generation of very high-energy gamma-ray observatory, the Cherenkov Telescope Array (CTA), promises to be even more sensitive. The Fermi Gamma-ray Space Telescope, on the other hand, orbits the sky 550 kilometers above the Earth every 95 minutes on a satellite that was launched at the beginning of 2008. Photon detectors on Earth have the advantage of having enormous collecting areas, whereas the very precise instruments on the Fermi satellite have better energy resolution and directional information, are sensitive to photons with lower energies, and have about 200 times the field of view.