By Ron Cowen
Of all the things worth arguing about in the universe, physicists are once again haggling over a bunch of WIMPS.
At a meeting in Venice on elementary particles, Rita Bernabei of the University of Rome announced that her team has found additional evidence for an exotic type of subatomic particle called a WIMP, for weakly interacting massive particle. The new findings, based on several years of experiments conducted beneath the Apennines east of Rome, are controversial. Despite years of searching, no other experiment has ever found evidence for the elusive particle. But the stakes are high, because proving the existence of WIMPS could in one fell swoop settle a 75-year-old puzzle about the identity of the dark matter in the cosmos. A true WIMP discovery would also provide a key clue to unifying the four fundamental forces of nature.
In the latest version of their experiment, known as DAMA/LIBRA, Bernabei and her colleagues analyzed faint flashes of light from 25 ultrasensitive sodium iodide detectors at the Gran Sasso National Laboratory beneath the Apennines. Like many other WIMP experiments, this one is conducted underground to provide shielding from stray cosmic rays that might confound the results.
For 11 years, using two different sets of detectors, the team has found an annual rise and fall in the number of flashes that the scientists say is consistent with Earth moving through a vast cloud, or halo, of WIMPS enveloping our galaxy.
“The data show, with very high confidence level, agreement with all the features expected for the presence of dark matter particles in the galactic halo,” Bernabei says. No known systematic errors or process related to known elementary particles can account for the annual modulation in flashes, she asserts. The researchers have posted their findings online (http://lanl.arxiv.org/abs/0804.2738, http://lanl.arxiv.org/abs/0804.2741),
“On the basis of their plots, there is no doubt that they do observe a modulation,” says Bernard Sadoulet of the University of California, Berkeley, who conducts his own search for WIMPS a half-mile underground at the Soudan mine in Minnesota.
Some of the objections physicists have raised in the past about the Italian experiments have been answered, but “the tension is increasing” between Bernabei’s results and the lack of signal found by his team and other groups, Sadoulet adds. “I don’t see how the two are compatible.”
Still, he notes, WIMPS have a strong appeal to both astronomers and physicists. For astronomers, WIMPS are one of the leading candidates for the invisible material, known as dark matter, believed to account for some 85 percent of all the mass in the universe. The tug of ordinary, visible matter isn’t nearly enough, theorists have calculated, to keep individual galaxies from flying apart and clusters of galaxies from breaking apart. Indeed, the cosmic tapestry of galaxy and galaxy clusters would never have formed without dark matter. And the unseen presence of dark matter has revealed itself by the way it bends light, distorting images of background objects — a key prediction of Einstein’s theory of general relativity.
Physicists like WIMPS because they may belong to a family of elementary particles that would allow scientists to unify all the known forces and particles in nature. In a proposed theory known as supersymmetry, every known elementary particle has a heavier, as yet undiscovered counterpart. WIMPS may be neutralinos, the lightest of these supersymmetric partners. The WIMPS that DAMA/LIBRA may have found could have a mass of about 50 times that of the proton.
The experiment relies on the assumed distribution and properties of WIMPS. If the universe is indeed chock-a-block with this hypothetical stuff, simulations show that it would clump into vast halos that extend hundreds of thousands of light-years beyond the visible outlines of galaxies like the Milky Way. And although the starlit, spiral arms of the galaxy rotate, the dark matter particles, immune to non-gravitational forces, would remain stationary.
As the Milky Way rotates, it carries our solar system along with it. In the summer, Earth moves around the sun in roughly the same direction as the sun moves through the galaxy. As a consequence, during the summer months Earth would travel through the stationary WIMP cloud faster and experience a stronger wind of these subatomic particles. In winter, when Earth moves in the opposite direction, the WIMP detection ought to fall. That’s just what Bernabei’s team says it has consistently found. Bernabei says it’s also possible the dark matter particles may turn out to be axions, which weigh much less than WIMPS.
One possible way to explain the discrepancy between the Italian experiment and the negative results of other teams is if the dark matter particles found by DAMA/LIBRA are extremely light, perhaps only a few times the mass of a proton, notes Graciela Gelmini of the University of California, Los Angeles. Some lightweight particles could be seen by DAMA/LIBRA but might not produce enough of a signal in the heavier germanium nuclei detectors used by some other teams, she says.
In the end, says Juan Collar of the University of Chicago, “there’s going to have to be information from particle accelerators, satellites and direct detections that all point to the same mass range and coupling. That’s when we’ll all start believing each other. I don’t think that anyone is going to tell you that DAMA/LIBRA on its own will be able to prove the existence of WIMPS, but certainly if they’re right, they were the first.”
