Mysterious effects in a new generation of dark matter detectors could herald a revolutionary discovery. Over the past year, scientists working with these detectors have suddenly noticed an increase, or excess, in the amount of low-energy exposures.
Even after decades of painstaking search, scientists have not been able to find a single particle of dark matter. Scientists give almost "iron" evidence of the existence of this form of matter, but to date it has not been possible to determine what, in fact, it consists of. For several decades, physicists have held the hypothesis that dark matter is heavy and consists of so-called weakly interacting massive particles - WIMPs, which supposedly can be easily detected in laboratory conditions.
However, despite many years of painstaking research, scientists have not yet managed to find WIMPs. And physicists took up the search with even greater enthusiasm. As researchers conduct more and more precise experiments, accumulating more and more data, there is a reassessment of hypotheses that shed light on how detectors could use detectors to capture dark matter particles that are lighter in mass than a proton. And at the beginning of this year on the arXiv preprint server. org, two papers were published that became a symbol of change in physics. In these articles, the authors for the first time propose to focus efforts on the search for plasmons (collective movements of electrons in matter) produced by dark matter.
The first of the papers was written by a group of scientists specializing in the study of dark matter at the National Accelerator Laboratory. Enrico Fermi (Fermilab) in Batavia, Illinois, as well as specialists from the University of Illinois at Urbana-Champaign and from the University of Chicago. Scientists have hypothesized that low-mass dark matter is capable of generating plasmons, and these particles can be captured with the help of some detectors. Inspired by this groundbreaking paper, UC San Diego physicists Tongyan Lin and Jonathan Kozaczuk calculated the likelihood that detectors are capable of detecting low-mass dark matter.
“We are shouting 'Plasmon, plasmon, plasmon!' Because this intriguing phenomenon, in our opinion, will help us explain the experiments with dark matter,” said co-author of the first of the articles and expert on dark matter Gordan Krnjaic from Fermilab and the Kavli Institute of Cosmological Physics at the University of Chicago. Particle physicists, together with astrophysicists, have been pondering the problem of detecting low-mass dark matter for a decade. rather, chemists and materials scientists), which are identifiers, markings, dark matter.
“I think this is great,” exclaims Yonit Hochberg, a theoretical physicist at the Hebrew University of Jerusalem, who commented on the results obtained by Krnjajc's team (although Yonit was not directly involved in any of the articles mentioned). "The fact that there are [plasmons] that are capable of acting in some unknown way is, in my opinion, an extremely important result that really requires further study."
Some scientists view the results of the first published article with great skepticism. As Kathryn Zurek, a dark matter researcher at the California Institute of Technology, put it, for example, the article "doesn't quite convince me," and added, "I just don't understand how it works." (We add that Zurek also did not take part in the writing of these articles).
In turn, one of the co-authors of the first article Noah Kurinsky, who is engaged in experimental activities in the field of dark matter studies at Fermilab and at the Institute of Cosmological Physics. Kavli, believes that the very fact of criticism from experts is not unusual at all. “We set a task for them: to prove that we are wrong. And this, I believe, will greatly benefit the research that is being carried out in this area of physics. This is what they should be trying to do,”says Kurinski.
The hunt for invisible matter, which leaves almost no trace, usually goes something like this: in order to detect dark matter particles, physicists take a piece of some material, place it somewhere deep underground, connect it to equipment, and then wait in the hope of fixing a signal. In particular, scientists hope that a dark matter particle will strike directly into the detector, resulting in electrons, photons, or even heat that can be detected by the equipment.
Theoretical approaches to detecting dark matter were outlined in an article dating back to 1985; it discussed how a neutrino detector can be repurposed to search for dark matter particles. As shown in that article, an inbound dark matter particle can hit the atomic nucleus of the substance from which the detector is made, and give it an impulse, just as one billiard ball, colliding with another, imparts an impulse to the last of them. As a result of this collision, dark matter, hitting the nucleus hard enough, would impart a momentum, as a result of which an electron or photon would fly out.
Everything turns out great at high energies. The atoms in the detector can be viewed as free particles, discrete and unrelated to each other. However, at lower energies, the picture changes.
"But detectors are not made of free particles," notes co-author of the first article, Yonatan Kahn, of the University of Illinois at Urbana-Champaign, who does dark matter theoretical research. “They’re just made of a very specific material. And therefore you must have all the information about this material if you want to understand exactly how your detector actually works."
Inside the detector, a dark matter particle of small mass will still transmit momentum, but as a result of the impact, the rest of the particles will not scatter like balls in a billiard, but will begin to vibrate. In other words, the analogy of a ping-pong ball is more appropriate here.
"As soon as we move on to dark matter of lower mass, then other - more subtle - effects begin to appear here," explains Lin. These subtle effects mean what physicists like to call "collective excitations." And the meaning here is this: if several particles move simultaneously with each other, then it is more convenient to describe them as a single whole, say, as a sound wave consisting of many vibrating atoms.
If electrons begin to behave in this way, then in this case plasmons arise. If a group of atomic nuclei begins to vibrate, then their collective excitation is called a phonon. This phenomenon is commonly encountered by astrophysicists and high-energy physicists studying dark matter; however, they see it as irrelevant.
But, as the late Nobel laureate in physics Philip Anderson once remarked, “more means differently,” that is, we are talking about recognizing the fact that as the system grows, it can have completely different laws of behavior [meaning article by Philip Anderson, 1972. "More is different", that is, More is different, - approx. transl.]. For example, a drop of water behaves very differently than a single water molecule (H2O). “I am completely imbued with this concept,” says Yonathan Kahn.
The approaches to plasmon production used in both papers are somewhat different from each other. However, the authors come to the same conclusion: we really have to look for signals that indicate the production of plasmons. In particular, according to the calculations of Lin and Kozachuk, the rate of formation of a plasmon by dark matter of low mass would be approximately one ten-thousandth of the rate of appearance of an electron or a photon. This value may seem unlikely, but for physicists it is quite accurate.
Energy boost in the dark
Until recently, the most sensitive detectors for detecting dark matter used giant reservoirs of liquid xenon. However, in the past few years they have been replaced by a new generation of smaller solid-state detectors. They are known by the acronyms EDELWEISS III, SENSEI and CRESST-III and are constructed from materials such as germanium, silicon and scheelite. Such detectors are sensitive to collisions with dark matter, which can result in only one electron.
But all detectors, regardless of their degree of protection, are sensitive to external noise, the sources of which can be, for example, background radiation. And so over the past year, scientists working with several detectors of dark matter suddenly began to record an increase, or excess, in the number of low-energy impacts, but they passed over this fact in silence.
The paper by Kurinski and his colleagues noted for the first time a remarkable similarity between such low-energy "excesses" that have been observed in various experiments with dark matter. Some of these exceedances seem to be concentrated around 10 hertz per kilogram of detector mass. And since the detectors are made of different materials, are located in completely different places and operate in different conditions from each other, then there is hardly any other universal reason for this strange consistency, other than the subtle influence of dark matter. The ensuing scientific debate attracted the attention of other physicists, such as Lin, who quickly set to work on the mathematics related to plasmon. But even Lin doubts: what if the results of current experiments indicate that plasmons are generated not by dark matter, but by something else? “I’m not saying that dark matter is not the cause. I just say that dark matter seems to me so far an unconvincing factor,”says Lin.
This hypothesis will be repeatedly tested and rechecked as new data comes from the latest dark matter detectors. But it doesn't matter if the detectors are currently detecting the mysterious substance or not. Now scientists working in this area of physics are studying plasmons and other ways of behavior of low-mass dark matter. Research is ongoing.
“I don’t exclude that we made many mistakes, but they all arouse interest in themselves,” says Krzaych.