Knocking on Heaven's Door - Lisa Randall [183]
The LHC’s potential to create dark matter is certainly intriguing. But most cosmology experiments don’t take place at accelerators. Experiments on Earth and in space that are dedicated to astronomical and dark matter searches are primarily responsible for addressing and advancing our understanding of potential solutions to cosmological questions.
Of course, dark matter’s interactions with matter are very weak, so current searches rely on a leap of faith that dark matter, despite its near invisibility, nonetheless interacts feebly—but not impossibly so—with matter we know (and can build detectors out of). This isn’t merely a wishful guess. It’s based on the same relic density calculation mentioned above that shows that if dark matter is related to models proposed to explain the hierarchy problem, then the density of particles that remains is the correct amount to account for dark matter observations. Many of the WIMP dark matter candidates suggested by this calculation interact with Standard Model particles at rates that might well be detectable with the current generation of dark matter detectors.
Even so, because of dark matter’s feeble interactions, the search requires either enormous detectors on the ground or, alternatively, very sensitive detectors that look for the products of dark matter meeting, annihilating, and creating new particles and their antiparticles on Earth or in space. You probably wouldn’t win the lottery if you bought only a single ticket, but if you could buy more than half of what’s available, then your odds would be pretty good. Similarly, very large detectors have a reasonable chance of finding dark matter, even though dark matter’s interaction with any single nucleon in the detector is extremely small.
The challenging task for dark matter detectors is to detect the neutral—uncharged—dark matter particles, and afterward distinguish them from cosmic rays or other background radiation. Particles with no charges don’t interact with detectors in conventional ways. The only trace of a dark matter particle passing through a detector would be the consequences of hitting nuclei in the detector and changing its energy by a minuscule amount. Because this is the only observable consequence, dark matter detectors have no choice but to search for evidence of the tiny amounts of heat or recoil energy that get created when dark matter particles pass through. Detectors are therefore designed to be either very cold or very sensitive in order to record the small heat or energy deposits from dark matter particles subtly ricocheting off.
The very cold devices, known as cryogenic detectors, detect the small amount of heat emitted when a dark matter particle enters the apparatus. A small amount of heat added to an already hot detector would be too difficult to notice, but with specially designed cold detectors, the tiny heat deposit can be absorbed and recorded. Cryogenic detectors are made with a crystalline absorber such as germanium. Experiments of this sort include the Cryogenic Dark Matter Search (CDMS), CRESST, and EDELWEISS.
The other class of direct detection experiments involves noble liquid detectors. Even though dark matter doesn’t directly interact with light, the energy added to an atom of xenon or argon when a dark matter particle hits it can lead to a flash of characteristic scintillation. Experiments with xenon include XENONIOO and LUX, and the other noble liquid experiments, ZEPLIN and ArDM.
Everyone in the theoretical and experimental communities is eager to know what the new results from these experiments will be. I was fortunate to be present at a dark matter conference at the KITP in Santa Barbara organized in December 2009, by two leading dark matter experts, Doug Finkbeiner and Neal Weiner, when CDMS, one of the most sensitive dark matter detection experiments, was about to release new results. In addition to being young and tall contemporaries who had done their PhDs together at Berkeley, Doug and Neal both had a great understanding of dark matter experiments and what their implications