Advancing Basic Science for Humanity
Blowing in the Wind
The next time you walk outside on a calm summer night and look up at the constellation Cygnus, also known as the Northern Cross, consider what is coming at you. According to one widely accepted model for the makeup of our galaxy, you are facing straight into a cosmic wind thousands of times faster than the gusts of a Category 5 hurricane.
But you don’t feel a thing, because the stuff coming at you is dark matter. It does not interact with photons or electrons, so our normal means of perception, light and electromagnetism, miss it. And most of it passes through ordinary matter (made up of the “baryonic” particles such as protons, neutrons and electrons) without leaving a trace. If the leading hypothesis is correct, dark matter is moving through you constantly at about 250 kilometers a second, or nearly 560,000 miles an hour. You don’t notice it, but a sensitive enough instrument just might.
Or at least that is the hope of scientists who are in the dark matter hunt.
The Search for Proof
They assume, first, that dark matter is real. Among other things, gravitational theory requires dark matter to explain “why our galaxy looks the way it does,” says Peter Fisher, an MIT professor of Physics and member of the MIT Kavli Institute for Astrophysics and Space Research. Without dark matter in and beyond the galaxy’s visible regions, centrifugal force would push rotating stars out of their galactic orbits and off into space. Second, scientists assume that dark matter is plentiful. Again, if gravitational theory is correct, there should be about five or six times more dark matter in the universe than the matter we can now detect. Finally, they figure that dark matter must be made up of particles that obey at least some known laws of physics. When a piece of dark matter scores a direct hit on a detectable object such as an atomic nucleus, for instance, the momentum of the dark matter particle should budge the nucleus at least a little, like a billiard ball.
In dozens of projects all over the world, researchers are trying to detect dark matter particles by looking for evidence of those collisions. Most of these efforts try to record the vibrations from the “nuclear recoil” that is supposed to occur when a dark matter particle (called a weakly interacting massive particle, or WIMP) jostles an atom. A few others take a slightly different tact and try to map the course of detectable particles after they are struck by WIMPs. In theory, this can tell them the direction in which the WIMPs were traveling -- a crucial item of data for determining if it was really dark matter or something else.
The dark matter particle (blue) comes in form the left and strikes a flourine nucleus (green). The fluorine nucleus then recoils through the gas knocking electrons (yellow) from the gas atoms. A very large electric field between the grid and the anode causes the electrons to collide with other electrons, thus emitting visible light. A CCD camera (bottom) collects the light to form an image
Researchers at MIT Kavli are involved in dark-matter searches of both types. Tali Figueroa, an assistant professor of Physics at MIT and a Kavli Institute member, works on one of the longest-running efforts, the Cryogenic Dark Matter Search (CDMS). This project aims to detect nuclear recoil in germanium and silicon crystals cooled to near absolute zero and stationed a half mile underground in a Minnesota mine. (CDMS has another link to the Kavli network: One of its leaders is Blas Cabrera, Figueroa’s former thesis advisor and a member of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford).
Fisher is engaged in a new project based on directional detection. He and his principal collaborator, Steve Ahlen of Boston University, are building a detector that uses a camera in a chamber of carbon tetraflouride gas to trace the path of electrons emitted by particle collisions. Fisher’s team at MIT Kavli includes research scientist Roland Vanderspek, an expert on charge-coupled device (CCD) imaging technology. With funding in hand from the Kavli Foundation, the National Science Foundation and the U.S. Department of Energy, Fisher hopes to have a 250-1000 liter chamber completed in 2009 and deployed (like the CDMS sensors) deep underground. In this case, the system will sit about a mile deep in the Homestake Mine in Lead, South Dakota.
Whatever the method used, confirming the existence of dark matter particles has proved enormously difficult, mainly because WIMPs are difficult to tell from fakes. “The hard part isn’t coming up with something that detects dark matter,” says Fisher. The trick is to discriminate dark matter from everything else. Burying detectors deep underground helps shield detectors from cosmic rays, but nearby radioactive minerals such as uranium or thorium can still confuse the data with photons, electrons and neutrons. And neutrinos from the sun can pass through anything, including solid rock.
Creating a Dark Matter Detector
CDMS scientists can fingerprint particle interactions with their detectors into two broad classes: electronic interactions (photons and electrons), and nuclear interactions (neutrons, neutrinos, and WIMPS). Thus photons and electrons, which are the biggest background, can be thrown out by looking at their fingerprint. Neutrino interactions don’t leave enough energy to be detected, so that leaves neutrons to worry about. Figueroa says one neutron is expected to hit one of the 30 CDMS detectors every two to three years (the system is well-shielded, after all). “If we see a nuclear interaction rate higher than that, we would be detecting dark matter particles,” he says.
Fisher’s new system, if it works as planned, will be able to determine the direction and force of incoming particles by imaging the tracks of particles thrown off, billiard-style, by collisions. Real dark matter can be identified by its direction of approach. According to gravitational theory, the sun and its solar system are plowing through a “halo” of dark matter – part of the structure holding the galaxy together – at about 250 kilometers per second. And because we are moving toward Cygnus, the dark matter seems to come from that constellation. A dark-matter detector sensitive to direction thus acts like a super-sensitive weather vane, detecting the dark-matter “wind” coming from a particular place in the sky. It is able to discriminate between WIMPs – from Cygnus – and everything else, such as neutrinos from the sun or a stray neutron kicked up by a gamma ray hitting a nearby rock.
This isn’t the first attempt to create a directional detector, but Fisher says his device improves on other designs, which use electronic technology that “barely shows directional sensitivity” and is “very, very expensive.” He and Ahlen rely on visual imaging with sensitive CCD cameras and amplification that boosts the signal of electrons by a factor of 100,000. The camera records the tracks (about 2 millimeters long) created by collisions of particles in a gas that has been thinned to about one-twentieth atmospheric pressure to allow free movement of the emitted particles. The system has been tested at Fisher’s lab, where the camera distinctly recorded the direction of fluorine nuclei propelled by a collision with a neutron beam.
Will it work down in the mine? Collisions between dark matter and ordinary particles are rare – “maybe once a month in a few kilograms of material,” Fisher says (his initial experiment will use only about 50 grams). So it may take some time before enough hits are recorded to draw any conclusions. But just suppose that the detector records two hits, and the particles both come from Cygnus. The chances of random background radiation producing such a result would be almost nil. “If you’re sitting down there and see two clicks, and they’re coming from Cygnus,” Fisher says, “you have to ask how this ‘background’ knows where the constellation Cygnus is.”
That would be one version of the smoking gun. Another might come from space – where scientists are looking for evidence of dark matter in radiation produced by the mutual annihilation of dark matter particles and antiparticles. Or, as Figueroa suggests, the compelling evidence may emerge less dramatically, in readings from projects that all point to the same conclusion. “In order to believe a dark matter signal, several groups will have to see it,” he says.