Research

The best place to search for dark matter can be the bowels of the Earth.

Nearly two dozen underground laboratories,Scattered around the world, laden with vats of liquid or blocks of metal and semiconductors, scientists are looking for traces of dark matter. Their experiments are becoming more and more difficult, and the search is becoming more accurate, but so far no one has found direct evidence for the existence of a mysterious substance, of which 84% of all matter in the Universe is made. According to a new study, we must ripen to the root, that is, even deeper.

Dark matter is different from ordinary baryonicmatter - the substance of which the stars, galaxies, dogs, people and everything else - that does not interact with anything in any way, except through gravity (and, perhaps, a weak nuclear force). We do not see this, but physicists are almost certain that it exists and how the sculptor sculpts galaxies on their way through space.

For many decadespreferred candidates for dark matter particles were hypothetical modest particles - WIMPs (WIMP), or weakly interacting massive particles. Many experiments are trying to find Wimps in the wake of their collision with ordinary matter. In such a scenario, wimp should touch the atomic nucleus through a weak force. The frightened core bounces off and emits energy in some form, a flash of light or a sound wave. Detection of such barely noticeable phenomena requires sensitive instruments, which are usually laid deep underground. This is mainly due to the fact that the instruments will be protected from cosmic rays, which can also cause nuclear reactions.

After a decade of searching for these weak signals,scientists have found almost nothing. And so, a team of physicists from Poland, Sweden and the United States proposed another idea. They believe that you need to look not at germanium, xenon and scintillators in the detectors under the earth's crust. They believe that you need to look at the very crust. In the chronicles of rocks, where they are recorded and covered with layers of the history of our solar system, we could find fossilized records of disturbed atomic nuclei, frozen traces of WIMPs.

“We are always looking for alternative approaches,” says Kathryn Friese, a theoretical physicist at the University of Michigan and the developer of the ideas that formed the basis of existing detectors.

Underground paleo detector will work similarlymodern methods of direct detection. Instead of equipping a laboratory with a large volume of liquid or metal to observe real-time WIMP flashes, you can search for fossilized WIMP traces crashing into atomic nuclei. Some classes of minerals could capture such traces.

If the core bounces off with enough energy, andif the perturbed atoms are then deep underground (to protect the sample from cosmic rays, which can entangle data), the bounce trace can be saved. If so, then scientists can unearth a stone, disassemble it in layers of time, and investigate past events using sophisticated nano-visualization techniques, such as atomic force microscopy. The end result will be the fossil trail: the trail of a zauropod during his flight from a predator, only in the terminology of dark matter.

Tiny footprints

About five years ago, Freese started looking for ideas forNew types of detectors along with Andrzej Drukier, a physicist from Stockholm University, who began his career with the study of the detection of dark matter, before going into biophysics. One of their ideas, developed together with biologist George Church, concerned dark matter detectors based on DNA and enzyme reactions.

In 2015, Drukier went to the RussianNovosibirsk, to work on a prototype biological detector, which will be placed under the earth's surface. In Russia, he learned about wells drilled during the Cold War, some of which go down 12 kilometers. No cosmic rays can penetrate this far. Drukier was intrigued.

Conventional dark matter detectors are relativelylarge and very sensitive to sudden events. They have been conducting their searches for several years, but for the most part they are looking for WIMP signals in real time. Minerals, although relatively small and less sensitive to interactions, can personify a search that lasted hundreds of millions of years.

"These pieces of rock, extracted from very, verydeep cores, almost a billion years old, ”says Drukier. “The deeper you go, the older they are. No need to build a detector. There is already a detector in the ground. ”

But the earth has its problems. The planet is full of radioactive uranium, which produces neutrons as it decays. These neutrons can also knock out nuclei. Frieze says that the original work of scientists describing paleo-detectors did not take into account the noise created by the decay of uranium, but many comments from other interested scientists forced them to return and revise the document. The team spent two months studying thousands of minerals in order to understand which of them are isolated from the decay of uranium. They claim that the best paleo-detectors will consist of marine evaporites — essentially, rock salt — or rocks containing very little silica, called ultrabasic rocks. In addition, they search for minerals that contain a lot of hydrogen, since hydrogen effectively blocks neutrons that arise from the decay of uranium.

Finding traces in the soil can lead us to low-mass wimps, says Trace Slatier, a theoretical physicist at the Massachusetts Institute of Technology, who did not take part in research.

"You are looking for a core that seems to be for no reasonjumps, but it must jump a certain amount to be noticed. If a ping-pong ball collides with a bowling ball, you will not notice a particular displacement of the latter - unless you can register the smallest change in the movement of the bowling ball. ”

The most difficult experiment

Work in the field will not be easy. Research will have to be carried out deep underground, where core samples will be protected from cosmic and solar radiation. And to detect evidence of pitted kernels, modern nano-visualization techniques will be required.

According to Slatier, even if WIMP leaves visiblescar, the main problem of paleo-detectors will be the proof that fossil traces are really born of dark matter particles. Researchers will have to spend a lot of time to convince themselves that interactions with nuclei are not the work of neutrons, the neutrinos of the sun, or something else.

"We'll have to go quite deep toprotect against cosmic rays. But this is not a laboratory. These are not controlled conditions. You may not know the complete history of rock deposits. Even if you find a signal in them, you have to do a lot more work to make sure you don’t see any background. ”

Drukier and Freese believe that the power of paleo-detectorsmay be in numbers. The breed contains many minerals, each of which contains atomic nuclei, which bounce off of marauding wimps in different ways. Therefore, different elements will serve as different detectors, but they will all be enclosed in a single core sample. In the future, the paleodetector could even provide records of wimps over time, just as fossils allow paleontologists to reconstruct the history of life on Earth.

According to Slatier, a long chronicle couldto offer a unique view of the dark matter halo of the Milky Way, a cloud of invisible material through which the Earth floats when the solar system moves in a 250 million-year orbit around the center of the galaxy. Understanding the halo distribution of dark matter in the Milky Way can give an idea of ​​its physical behavior, says Slatier. Perhaps it will also demonstrate whether dark matter can interact in ways that go beyond gravity.

"It is here that theory and modeling are instages of active development, ”she says. "Will we find dark matter," asks Drukier. “I spent thirty-five years looking for her. Perhaps this is the most difficult experiment in the world, so we may not be lucky. But it's cool. ”

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