World’s most sensitive dark matter experiment announces surprise: or 40-year-old new particle

In the early hours of June 18, Beijing time, XENON1T, the world’s most sensitive dark matter experiment at the Gran Saso underground laboratory in Italy, announced that it had observed unexpected results of unknown origin, possibly a new particle called Axion. The axis, predicted by physicists in 1977, is an existence beyond the standard model of particle physics, named after Frank Wilczek, the Nobel Laureate.

However, XENON International Cooperation has not ruled out other more conventional possibilities, such as errors caused by trace amounts of radon impurities.

World's most sensitive dark matter experiment announces surprise: or 40-year-old new particle

XENON Experiment

Why did the dark matter detection experiment not find dark matter, but instead came out of an “axis”?

What is an axis?

Forty-three years ago, the “axis” hypothesis was proposed to explain the symmetry problem in modern physics in depth.

In 1956, Li Zhengdao, Yang Zhenning and Wu Jianxiong proposed and experimentally verified that Yu said P was not conserved in weak interactions. It was later found that the combined transformation CP of both positive and negative particle conjugate (C) and Yuji (P) in weak interactions was also not conserved. The C transformation is about to turn a particle into its antiparticle, and the P transformation is the inversion of spatial coordinates.

So, in the strong interaction cp transformation is not conserved?

In 1977, Nobel Laureate Steven Weinberg and Wilczek proposed the shaft model. Among the solutions to the strong CP problem, the shaft is the most natural and elegant, which has attracted a lot of attention.

According to the model, the shaft is a small mass particle, probably only one trillionth of the mass of electrons. Its interaction with other substances is so weak that it is difficult to find high-energy physics experiments such as colliders.

However, according to the theory, shafts and photons convert to each other in strong electromagnetic fields. The sun should therefore be a natural axial source.

The Solar Axis Telescope (CAST), located at the European Nuclear Center, a sacred site of high-energy physics, uses accelerator magnets to convert theoretically existing axes into photons to detect the presence of axial streams originating in the sun.

Dark matter candidate?

Interestingly, the axial model proposed to solve the theoretical problem of particle physics may also solve the dark matter mystery that astrophysicists are searching for. Although it is not the most mainstream candidate for dark matter, it cannot be ruled out.

The so-called dark matter refers to its non-interaction with light and cannot be “seen”, but its existence actually affects the movement of visible matter, as can be verified from galaxies to observations on cosmological scales.

For example, the actual speed at which our solar system rotates around the center of the Milky Way is significantly faster than the result sized by Newton’s law of gravity. This means that the presence of a large amount of invisible mysterious matter in the Milky Way provides additional gravity. If not, the solar system should have been “dumped” out. The mass of these invisible matter should be as high as 85% of the total amount of matter in the universe, far greater than the matter we know.

The most mainstream model before it was a particle called a weakly interacting large mass particle (WIMP). It is involved in gravity and weak nuclear forces and has a relatively large mass. According to the calculations, there are countless dark matter particles penetrating the Earth every second.

Therefore, the XENON experiment, the PandaX experiment in the underground tunnel of Jinping Mountain in Sichuan, China, and the LUX experiment under the abandoned gold mine in South Dakota, usa, have adopted the idea of keeping the rabbit in line: hoarding tons of liquid radon, waiting for a collision with dark matter particles for an “electric light stone fire”.

Once the WIMPs collide with one of the niobium atoms, their punch energy is converted into a photoelectric signal, which is recorded by a sensitive phototube in the detector. The laboratory is located in a kilometer deep underground to better shield from outside noise.

53 extra signals.

While models are popular, the probability of WIMPs being present is becoming increasingly remote.

In the international team cooperation competition, the accuracy of the liquid radon experiment is constantly refreshed, the scope of exclusion is getting bigger and bigger, the proposed restrictions are getting higher and higher, human beings are still nothing. This means that the theoretical existence of WIMPs is becoming more and more narrow.

The last experimental group to put up the latest limitations on the quality of WIMPs was XENON1T.

XENON International Cooperation is composed of 163 researchers from 10 countries and 28 research institutions, mainly in Europe, the United States and Japan.

Although the name is “one ton”, the experiment is currently used 3.2 tons of ultrapure liquid radon, of which 2 tons were used for the collision.

Of course, scientists have considered that there are known conventional particles that break into it. They expect the particles to produce 232 signals.

Unexpectedly, there were 285 signals in the final experiment. Since it does not conform to the characteristics of WIMPs, this out of 53, where come from?

3 Possibilities

At an online seminar on the same day, XENON scientists offered three possible explanations. The papers were also uploaded to the preprinted website ArXiv.

The first, and the most “normal” and new physics, is the isotope thorium mixed with trace amounts of hydrogen in the detector (a thorium atom consists of a proton and two neutrons). In fact, as long as every 10 s25 (1000000000000000000000000000000000000000000000000000000000000) A liquid thorium atom mixed into a few thorium atoms can cause these 53 redundant events. At present, no measurements can confirm or exclude this.

The second and most exciting one is that the solar axis produces a photoelectric signal. Although these atoms produced by the sun may not be the source of dark matter in the universe, proving the existence of new particles is itself a heavy discovery.

Third, it is another very small mass, a large number of particles that penetrate the Earth every moment – neutrinos. However, if you want to generate an excess signal of this magnitude, neutrinos need to have some previously unknown physical properties. Therefore, this possibility also includes new physical factors.

The significance of these three possibilities is considerable, and it is most consistent with the solar elbow model in terms of signal energy spectrum and other information. HOWEVER, XENON INTERNATIONAL COOPERATION STRESSED THAT THE AVAILABLE EVIDENCE WAS INSUFFICIENT TO THE FOLLOWING CONCLUSIONS.

With the future “one ton of xenon” upgraded to “XENONnT”, the scale of the liquid molybdenum increases by two times, and the background noise is further depressed, the experiment can only give the final answer: is the data fluctuations, impurity pollution, or exciting new physics?