Scientific American (USA): dark matter may lurk in the low-energy frontiers — is a proof

Even after a decade of painstaking search, scientists have not managed to find any dark matter particles. Scientists lead the almost “iron” evidence of the existence of this form of matter, but has so far failed to determine what it is. For several decades physicists have supported the hypothesis that dark matter is heavy and consists of so-called weakly interacting massive particles — vinow, which are easy to detect in the laboratory.

However, despite many years of painstaking research, scientists have not been able to detect wimpy. And physics took up the search with more vigor. To the extent that, as researchers conduct new, more accurate experiments, accumulating more data, there is a reassessment of the hypotheses that shed light on how it would be possible by using detectors to detect dark matter particles, which mass is lighter than the proton. And in the beginning of this year on the arXiv pre-print server. org was published two works, which became a symbol for change in physics. In these articles the authors first propose to focus on the search of plasmons (collective movement of electrons in a substance) produced by dark matter.

The first works were written by a group of scientists specializing in the study of dark matter in the National accelerator laboratory. Enrico Fermi (Fermilab) in Batavia, Illinois, as well as experts from the University of Illinois at Urbana-Champaign and University of Chicago. Scientists have put forward the hypothesis that generate plasmons capable of dark matter is small mass, and these particles can be caught with the help of some detectors. Inspired by this innovative of its kind article, physicists from the University of California in San Diego Tongyan Lin (Lin Tongyan) and Jonathan Kozachuk (Jonathan Kozaczuk) has estimated the probability with which the detectors are able to detect dark matter low-mass.

“We scream, “Plasmon, plasmon, plasmon!”, after all, it is an intriguing phenomenon, in our opinion, will help us to explain the experiments with dark matter,” said co-author of the first articles and an expert on dark matter krnjaic of Gordan (Gordan Krnjaic) from Fermilab and the Institute for cosmological physics. Kavli at the University of Chicago. Experts in the field of particle physics together with astrophysicists over the last decade reflect on the problem of detecting dark matter low-mass. But so far none of them were in search of plasmons (and surface plasmons familiar, rather, chemists and material scientists), which are identifiers identifying marks, dark matter.

“I think it’s great,” says Yonit Hochberg (Hochberg Yonit), a theoretical physicist from the Hebrew University in Jerusalem, commented on the results obtained by the team of Krnjaca (though Yonit not directly involved in any of the mentioned articles). “The fact that there are [plasmons], which are able to act in some unknown way, is, in my opinion, an extremely important result, which indeed requires further study.”

Some scholars with great skepticism look at the results of the first published articles. As expressed, for example, Katherine Zurek (Kathryn Zurek), a specialist in the study of dark matter from the California Institute of technology, the article “not quite convinces me,” and added: “I just don’t understand how it works.” (We will add that Turek also did not participate in the writing of these articles).

In turn, one of the authors of the first article Kurinsky Noah (Noah Kurinsky), which is engaged in experimental activity in the study of dark matter at Fermilab, and the Institute of cosmological physics. Kavli, believes that the very fact of criticism from experts, there is nothing unusual. “We gave them the task: to prove that we are wrong. And this, I believe, at the highest level will benefit the research conducted in this field of physics. This is what they should be trying to do,” says Kurinsky.

To combine efforts

The hunt for invisible matter which almost leaves no traces, usually happens like this: to detect dark matter particles, physicists take a piece of a certain material, place it somewhere deep under the ground, connect to hardware, and then wait in hope to capture the signal. In particular, scientists hope that the particle of dark matter would hit the detector, which will appear electrons, photons or even stand out heat, which can be fixed equipment.

Theoretical approaches to the discovery of dark matter were described in the article back to 1985; it told about how the detector of neutrinos can be reshaped for the search of dark matter particles. As shown in that article, inflected particle dark matter is able to get on the atomic nucleus of the substance from which made the detector, and give it momentum, just as one billiard ball colliding with another, giving an impetus to the last of them. As a result of the collision of dark matter, is quite striking to the core, said to the momentum, resulting in the fly electron or photon.

At high energies all turns out great. The atoms in the detector can be considered as free particles, discrete and not related to each other. However, at lower energies, the picture changes.

“But the detectors do not consist of loose particles, — says the co-author of the first article Jonathan Kahn (Kahn Yonatan) University of Illinois at Urbana-Champaign, engaged in theoretical studies of dark matter. They are simply made of a definite material. And you should therefore have all the information about this stuff, if you want to understand how it works in reality your detector”.

Inside the detector the particle dark matter low-mass still will transmit an impulse, but as a result of hitting other particles do not scatter the balls in a Billiards, and begins to oscillate. In other words, here is more appropriate the analogy of a ball for ping-pong.

“As soon as we move to the dark matter of lower mass, then you begin to show and others are more subtle effects,” explains Lin. These subtle effects is what physicists like to call an expression of “collective excitation”. And the point here is this: if some particles move simultaneously with each other, they prefer to describe as a whole — say, like a sound wave consisting of a plurality of vibrating atoms.

If the electrons start to behave in a similar way, in this case just occur and plasmons. If it starts to vibrate a group of atomic nuclei, their collective excitation called a phonon. This phenomenon is commonly faced astrophysicists and experts in the field of high energy physics, studying dark matter; however, they regard it as irrelevant.

But, as once said the late Nobel prize-winning physicist Philip Anderson (Philip Anderson), “more means different” — we are talking about recognition of the fact that the growth of the system it can appear absolutely other laws of conduct [referring to the article by Philip Anderson, 1972 “More means different”, i.e. More is different — approx. transl.]. For example, a drop of water behaves differently than a single molecule of water (H2O). “I completely soaked in this concept,” says Jonathan Kahn.

Approaches to the production of plasmons used in both articles are somewhat different from each other. However, the authors come to the same conclusion: we really need to look for signals that indicate the generation of plasmons. In particular, according to the calculations of Lin and Kozachuk, the rate of plasmon formation of dark matter low-mass would amount to about one ten-thousandth of the speed of emergence of an electron or photon. This value may seem unlikely, but for physicists it is quite accurate.

Energy pulse in the dark

Until recently, the most sensitive detectors designed for dark matter detection, used giant tanks with liquid xenon. However, in the last few years they’ve been replaced by a new generation of solid-state detectors of smaller size. They are known under the acronyms EDELWEISS III, SENSEI and CRESST-III and constructed of such materials as germanium, silicon, and scheelite. These detectors are sensitive to collisions with dark matter, which may be only one electron.

But all detectors, regardless of the degree of its protection, sensitive to outside noise, the sources of which may be, for example, background radiation. And now for the past year, scientists working with multiple detectors, dark matter, suddenly began to record an increase, or excess amounts of low-energy impacts, but that they had remained silent.

The article Kurinsky and his colleagues first noted a remarkable similarity between such low-energy “surpluses” that have been observed in various experiments with dark matter. It seems that some of these exceedances are concentrated around a figure of 10 Hertz per kilogram of mass of the detector. And because detectors are made of different materials located in different places and work in different from each other, it is unlikely that you can name some other universal cause for this strange consistency, in addition to the subtle influence of dark matter. The ensuing scientific discussion has attracted the attention of other physicists, such as Lin, who quickly began to work on mathematical calculations related to the plasmon. But even Lin doubts: what if the results of the ongoing experiments suggests that the plasmon is not generated by dark matter, and something else? “I’m not saying that the reason is not dark matter. I’m just saying that dark matter seems to me while unconvincing factor,” Lin says.

This hypothesis will be repeatedly checking and re-checking upon receipt of fresh data from the latest detectors of dark matter. But no matter, notice that the detectors in the present, a mysterious substance or not. Now scientists working in the field of physics, studying plasmons and other behaviors of dark matter low-mass. Research is ongoing.

“I do not exclude that we made a lot of mistakes, but they in themselves are of interest,” says Grgic.