In the realm of dark matter research, like in a china shop, there is an elephant: a claim that is hard to believe, impossible to confirm, and surprisingly difficult to explain. But now we have four instruments with the same type of detector as the collaboration that owns this controversial claim. Over the next three years, these experiments will either confirm the existence of dark matter or disprove the claim once and for all, the physicists working on them say.
"Everything will work out," says Frank Kalaprice of Princeton University in New Jersey, who is leading one of the experiments.
The initial announcement was made by the DAMA collaboration, whose detector is located in a laboratory deep under the Gran Sasso mountain range east of Rome. Over a decade ago, she provided amazing evidence for dark matter, an invisible substance believed to bind galaxies together by its gravitational pull. The first of the new detectors will start operating in South Korea in just a few weeks. The rest will be launched in the next few years in Spain, Australia and Gran Sasso. All of them will use sodium iodide crystals to search for dark matter, which no large-scale experiment has done, except for DAMA.
Scientists are very confident in the existence of dark matter and that it is at least five times larger than normal matter. But its nature remains mysterious. The leading hypothesis is that at least part of its mass consists of weakly interacting particles (WIMPs) that must occasionally collide with atomic nuclei on Earth.
Sodium iodide crystals should emit a flash of light if this happens in the detector. While natural radioactivity also produces such flares, DAMA announced the discovery of WIMPs back in 1998, citing the fact that the number of flares produced per day fluctuated with the seasons.
This is what would be expected if the signal is generated by WIMPs that spill onto Earth as the solar system moves through the dark matter halo of the Milky Way. In this case, the number of particles crossing the Earth should peak when the orbital motion of the planet aligns with the movement of the Sun, in early June, and decrease when the movement goes against the Sun, in early December.
But there is one big problem. “If it was really dark matter, many other experiments would have seen it by now,” says Thomas Schwetz-Mangold, a theoretical physicist at the Karlsruhe Institute of Technology in Germany - and no one has seen it yet. But at the same time, all attempts to find weaknesses in the DAMA experiment, including environmental effects, which scientists did not take into account, have not been successful. "There is a modulation signal," says Kaijuan Ni of the University of California, San Diego, who is working on the XENON1T dark matter experiment. "But how to interpret this signal - in favor of dark matter or something else - is not clear."
No other full-scale experiment used sodium iodide in its detector, although KIMS in South Korea used cesium iodide. Thus, the possibility remains that dark matter interacts with sodium in some way differently than with other elements. "Until someone fires a detector on the same element that left the hint, you can't be sure of anything," says Juan Collar of the University of Chicago in Illinois, who has worked on several dark matter experiments.
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Many have faced the difficulty of growing sodium iodide crystals with the required purity. Potassium contamination, which has a naturally occurring radioactive isotope, also stands out.
But three teams of scientists - KIMS, DM-Ice from Yale University in New Haven and ANAIS from the University of Zaragoza in Spain - managed to obtain crystals with a background level of radioactivity half that of DAMA. It's clean enough to test her results, scientists say.
Scientists from KIMS and DM-Ice have built a sodium iodide detector in conjunction with the Yangyang underground laboratory, 160 kilometers east of Seoul. The tool uses an “active veto” sensor that will better distinguish dark matter from background noise than DAMA, says Yengduk Kim, director of South Korea's Center for Underground Physics in Daejeon, which runs KIMS.
ANAIS is building a similar detector at the Canfranc underground laboratory in the Spanish Pyrenees. Together, KIMS, DM-Ice and ANAIS will carry over 200 kilograms of sodium iodide and exchange data. Compared to the 250 kilograms that DAMA had, the scientists expect to catch a similar number of WIMPs. And while the new detectors will have higher levels of background noise, they will be able to either fake or reproduce the strongest DAMA signal, says Reina Maruyama of Yale University, who runs DM-Ice.
But Kalapris argues that high purity is more important than mass. Together with colleagues, he developed a way to reduce pollution, and in January announced the receipt of crystals that are cleaner than DAMA crystals. He hopes to reduce the background level further, down to one-tenth of the DAMA.
His SABER (Active Background Rejection Sodium Iodide) project will house one detector in Gran Sasso and another in the Stowell Underground Physics Laboratory, which is being built in a gold mine in Victoria, Australia. SABER will use a detector that separates the dark matter signal from the noise, and will weigh about 50 kilograms.
SABER will complete its research and development in about a year and begin building its detectors shortly thereafter, Kalapris says. Then the technology will become available to other laboratories - something DAMA did not do. And if you have two identical detectors in the northern and southern hemispheres, you can understand whether environmental effects could fake the seasonality of dark matter in the DAMA results - if the signal came from WIMPs, both detectors will see peaks at the same time.
DAMA is very confident in its results, says Rita Bernabei of the University of Rome. She is not particularly excited about the upcoming launch of sodium iodide detectors. “Our results have been validated many times over 14 years, so we have no reason to look forward to seeing what others will do,” she says. If other experiments don't see the annual modulations, the collaboration will simply assume they weren't sensitive enough.
But what if the DAMA scientists are right? “At first, I didn't want to believe the DAMA results, I didn't even take them seriously,” says Katherine Freese, an astro-particle theorist at the University of Michigan at Ann Arbor who proposed the DAMA's seasonal modulation method. But since no other explanation for their signal was found, she was reassured. "The more a person studies their experiment, the more he realizes how well it is done."
Ilya Khel