Section 6: The Search for Particle Dark Matter
Starting in the late 1980s with the idea that dark matter could be a new kind of particle, nuclear and particle physicists began experiments to detect dark matter in the event that it interacts directly with normal matter. There are two main ideas about what these particles could be. One views the dark matter as a very light particle known as the axion. Hypothesized to explain a confusing property of the strong force that binds quarks together (see Unit 2), an axion would weigh about one-trillionth as much as a proton. The other idea comes from a very broad class of theories that predicts an electrically neutral particle weighing between 100 and 1,000 times as much as a proton. The general name of this kind of particle is a "weakly interacting massive particle" or WIMP. Physicists first introduced this concept to explain the problem of solar neutrinos that we met in Section 5.
Figure 13: If dark matter consists of axions, the Axion Dark Matter Experiment shown here could detect them in the next decade.
Source: © ADMX. More info
So far, physicists have found no evidence that axions or WIMPs actually exist; both particles remain in the realm of hypothesis. However, the physics community found the theoretical reasoning that led to the hypotheses were compelling enough to mount experimental searches for them. Some of their experiments have provided fascinating hints of the presence of these peculiar particles.
The types of experiments differ considerably, based on which particle they aim to detect. In each case, they rely on the specific physical properties of the two proposed particles. Because axions are hypothesized to have no electric charge or spin, extremely small masses, and minimal interaction with ordinary matter, experimenters must use indirect methods to detect them. In contrast, theorists see WIMPs as not only possessing large masses but also interacting—although infrequently—with ordinary matter. Thus, it may be possible to detect them directly as well as indirectly.
The quest for axions
The concept of the axion emerged as a solution to the so-called strong-CP problem. We first encountered CP, the product of charge conjugation and parity, in Unit 1. There we discovered that CP violation occurs in weak interactions, but does not appear to occur in strong interactions. In 1977, theorists Roberto Peccei and Helen Quinn suggested that this difference between the strong and the weak force was due to a broken symmetry. In Unit 2, we learned that symmetry breaking is accompanied by a new particle called a "Nambu-Goldstone boson." The new particle associated with the broken Peccei-Quinn symmetry would interact with ordinary matter so weakly as to be virtually undetectable. MIT theorist Frank Wilczek named it the axion after a laundry detergent because, he said, it cleaned up the strong-CP problem. Later, the weakness of its interactions made it a strong candidate for dark matter.
Figure 14: Axion hunters: two Fermilab physicists with their experiment designed to detect axions.
Source: © Fermilab. More info
Experimentalists who want to detect the particle can choose either to make their own axions or to search for those that already exist. Many of these experiments attempt to detect axions as they interact with photons. The basic idea is that when an axion collides with a photon, two photons are produced in the collision that have an energy proportional to the axion mass. Dark matter axions do not move very fast and are very light. Therefore, the photons produced would be low energy, with a wavelength roughly corresponding to radio waves. Axions are expected to interact with photons very weakly—much more weakly than electrons or protons—so the trick to detecting axions is to build a very sensitive radio antenna.
Trapping radio waves to identify axions
The process starts with a magnetic field about 200,000 times more powerful than Earth's field. When an axion interacts with the magnetic field, radio waves are generated. To capture the radio waves, experimentalists use a hollow superconducting cylinder called a "resonant cavity." The size and shape of the cavity are carefully selected to amplify radio waves of a particular frequency.
Figure 15: SQUID technology boosts the ability of the Axion Dark Matter Experiment to detect the faint signals that would indicate the presence of axions.
Source: © Lawrence Livermore National Laboratory. More info
For a typical mass of 2µeV, roughly 1030 axions would stream through the detector each second. Over time, the trapped radio waves would build up to a detectable amount. The radio waves built up in the resonant cavity are measured using a tool called a SQUID, for superconducting quantum interference device, which greatly improves the experiment's ability to detect faint signals. Since physicists do not know the mass of the hypothetical axion, they would have to adjust the radio frequency of the cavity in small steps, like tuning a radio, to scan for a signal from dark matter axions.
The best-known experiment of this type, the Axion Dark Matter Experiment (ADMX), has operated since 1995 without detecting a signal. Physicists at Lawrence Livermore National Laboratory and collaborating institutions improved ADMX in 2008 by adding sensitive amplifiers to the apparatus. Further enhancements include adding a cooling system that will improve the system’s sensitivity. The team will add more improvements and will continue to operate the experiment for many years before exhausting all its potential to hunt for axions.
Other searches for axions have started in recent years. A Japanese project, the Cosmic Axion Research with Rydberg Atoms in a Resonant Cavity (CARRAC) experiment, seeks axions in a range of masses similar to that sought by ADMX. An Italian group's PVLAS (for Polarizzazione del Vuoto con LASer) experiment looks for minute changes in the polarization of light that might stem from axions. And in contrast to those earthbound methods, the European Nuclear Research Center’s Axion Solar Telescope (CAST) searches for axions produced in the Sun.
Seeking the elusive WIMPs
As theorized, WIMPs interact with normal matter in the simplest way, by colliding with it. They don't do that very often; they easily penetrate the Earth or Sun without interacting at all. But very occasionally a WIMP will hit an atomic nucleus and cause it to recoil. Theorists believe that 5 million dark matter particles will pass through a 2 kilogram piece of normal matter, containing roughly 1025 atoms, every second. In rough numbers, just one of the WIMPs will hit a nucleus in an entire year. The nucleus will recoil and deposit its energy in the surrounding matter in the form of ionization electrons, which can attach to ions to create neutral atoms, or heat. The amount of energy deposited in this way resembles that of an x-ray photon. Physicists searching for dark matter face the twin challenge of collecting this deposited energy and ensuring that the energy they collect came from a dark matter interaction and not from a conventional physics process.
Distinguishing between dark matter interactions and conventional interactions proves to be very difficult. At sea level, 100 cosmic rays pass through each square meter of the Earth's surface each second, along with 28 neutrons from cosmic ray interactions in the atmosphere and 10,000 x-rays from low-level contamination in normal materials. In addition, everything contains trace amounts of uranium and thorium, both of which give rise to sequential radioactive decays. All these processes can mimic the scattering of dark matter off a nucleus.
Underground searches for WIMPs
Figure 16: The Large Underground Xenon detector will have 100 times more sensitivity to WIMPs than previous detection methods.
Source: © The LUX Experiment. More info
Dark matter recoil experiments address these problems in several ways. Since few cosmic rays penetrate deep underground, experiments placed in tunnels and mines under a kilometer of rock remove that source of interference. The Large Underground Xenon (LUX) detector, which will operate 1,463 meters deep in the familiar Homestake Gold Mine in South Dakota, exemplifies this approach. As its detector, LUX will use a cylinder containing 350 kilograms of liquid and gaseous xenon, which scintillates and becomes ionized when struck by particles, including WIMPs. Several precautions will minimize the number of non-WIMP particles likely to impact the detector. Up to a meter of high-purity lead or copper shielding will absorb x-rays and gamma rays emitted by the walls of the mine. In future experiments, a meter or so of water will absorb neutrons from both cosmic rays and the cavern's walls. Finally, experimenters will use only tested, low-radioactivity materials to build the detector.
Other groups are also undertaking the underground route to detecting WIMPs. The international Xenon Dark Matter Project uses a xenon detector in a laboratory under Italy's Gran Sasso Mountain. The second Cryogenic Dark Matter Search (CDMSII) project relies on cryogenic germanium and silicon detectors in Minnesota's Soudan Mine, another location well used by scientists; the original experiment had taken place in a tunnel under the Stanford University campus. And, the Italian-American WIMP Argon Program (WARP) uses argon in place of the more expensive xenon in its detector.
To clarify their results, the dark matter detectors measure the energy of the recoiling nucleus in two different ways. A neutron or dark matter interaction will divide its energy between heat and ionization electrons, while other radioactive decays will put virtually all their energy into ionization electrons. In the late 1980s, the first dark matter experiments were able to exclude neutrinos as dark matter by measuring the energy only one way. The two energy measurement techniques developed since then have led to an improvement of 10 million in sensitivity to dark matter interactions. Future detectors will have even greater sensitivity.
Monitoring the direction of dark matter
If dark matter WIMPs exist, we could learn more about them by measuring the direction from which they come toward Earth from space. A directional measurement would use gas molecules at about one-twentieth of an atmosphere pressure as targets for the dark matter particles to hit. Each nucleus struck by a WIMP would travel about 1 millimeter. That's a long enough distance for physicists to measure by collecting the ionization electrons created by the collisions directly or by converting them to scintillation light and using a charge-coupled device (CCD) camera to create an image. Since each struck nucleus will generally travel in the same direction as that in which the dark matter particle traveled before it hit the nucleus, measuring the direction of the recoiling nuclei will give experimenters critical details about dark matter in our galaxy.
Figure 17: If they exist, WIMPs could stream toward Earth in a specific direction in a "WIMP wind" that might be experimentally detectable.
In the simplest picture, the normal matter in our Milky Way galaxy rotates through a stationary halo of dark matter. If we could easily detect dark matter on Earth, we would see a "wind" of dark matter coming from the direction in which our solar system is moving through the Milky Way. Since the constellation Cygnus orbits around the galactic center ahead of our solar system, the dark matter would appear to be streaming at us from Cygnus. Thus, a directional experiment would see nuclei recoiling away from Cygnus. Measuring direction in this way not only would yield information about dark matter, but it also would make the experiment more sensitive, since no background source of radiation would follow the trajectory of Cygnus. In addition, a detector able to measure direction would begin to explore the velocity distribution of dark matter in the Milky Way much more directly than ever before. A directional detector would work, in effect, as a dark matter telescope.
Collider and satellite searches for dark matter
If WIMPs comprise dark matter, high-energy collisions may also shed light on their nature. Both the Tevatron and the Large Hadron Collider (LHC) may be able to produce WIMPs by colliding protons and antiprotons or protons and protons at energies high enough to fuse the quarks inside those particles into WIMPs. Teams at both the Tevatron and LHC will continue sifting through vast amounts of data, hoping to find evidence of WIMPs in their detectors.
Figure 18: NASA's Fermi Gamma-ray Space Telescope has spotted an excess of normal matter particles that may have arisen when WIMPs annihilated each other.
Source: © NASA/Fermi Gamma-ray Space Telescope. More info
Finally, it may be that WIMP dark matter particles annihilate each other in the galaxy to produce extra amounts of normal matter (such as protons, electrons, antiprotons, positrons, neutrinos, or gamma rays), which could be detected from Earth or in space-borne experiments. Separating these extra normal particles from cosmic rays is difficult. But in the last year, two satellite experiments may have observed some hints of dark matter. NASA's Fermi Gamma-ray Space Telescope, launched in 2008, discovered evidence of more high-energy electrons and their antimatter positrons than anticipated. The excess could stem from WIMP annihilations. About the same time, the European Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite, launched in 2006, detected more positrons than expected. However, it is much too early to tell whether either satellite has actually seen dark matter.