Will axions be the best choice for us to find dark matter?

A6021 composite image of bullet cluster. Bullet galaxy clusters are composed of a pair of galaxy clusters that collide head-on. They have been widely studied and are considered to be strong evidence of the existence of dark matter. One galaxy cluster passing through the other, like a bullet passing through an apple, may show clear signs of separation of dark matter (blue) from hot gas (pink).

According to foreign media reports, scientists have been exploring the true “identity” and composition of dark matter. Now, researchers have found new evidence to support a hypothetical particle called Axion, which is likely to be one of the main components of dark matter.

In 1933, astronomers first inferred that there was some invisible matter in the universe. Later, these “lost masses” were called dark matter. Since then, dark matter has been the mysterious object pursued by astronomers and physicists all over the world. Although some theories believe that dark matter accounts for 85% of all matter in the universe, it is still a mystery what these invisible substances are made of.

What is an Axion?

Some researchers believe that dark matter may actually be a strange particle called “Axion”. This is an imaginary elementary particle with extremely small mass and energy. Axions were first proposed in 1977 to solve the so-called “strong CP problem” in particle physics – why is CP not destroyed in quantum chromodynamics?

This is a difficult problem that has remained unresolved for a long time. CP is the product of two symmetrical operations in particle physics: C symmetry is charge symmetry, and quantum operation is charge conjugate operation, which can convert a charged particle into its antiparticle; P is parity, and parity operation will cause the mirror image of a physical system. In strong interaction and electromagnetic interaction, CP conversion operation has no effect on the whole physical system, that is, CP symmetry; However, in a certain weak interaction, this symmetry will be slightly broken (CP destruction). The study of CP destruction is still a very active field in theoretical physics and experimental physics.

In CP symmetry, if a particle exchanges with its antiparticle, or when it is inverted or becomes a mirror image, the laws of physics involved should be the same. In quantum chromodynamics, strong interaction may lead to CP destruction, but scientists have never observed this destruction.

“We already know that this CP symmetry distinguishes between particles and antiparticles. We also know that CP destruction occurs in weak interactions. This is a mystery. Why is this symmetry not violated in strong interactions?” said John Ellis, a particle physicist at CERN, which runs the Large Hadron Collider Ellis has been engaged in this research since the Axion was proposed.

In 1977, researchers proposed an extension of the standard model. In this extension, strong interaction will not destroy CP symmetry. “This theory predicts the existence of axions”. In 2020, a physics team found the first direct evidence of the existence of axions, making this hypothetical particle more reasonable. It also aroused the interest of the scientific community in axions, and further supported the view that axions may be the best candidate particles of dark matter.

“Dark matter accounts for the vast majority of matter in the universe, and we don’t know what it is. One of the most prominent questions in all science is’ what is dark matter? ‘” Benjamin safdi, the lead author of the new study and an assistant professor of physics at the University of California, Berkeley, said in a statement, “We speculate that this is an unknown new particle, and the Axion may be this kind of particle. It may be produced in large quantities in the big bang and float in the universe, which can explain the observations already made in astrophysics.”

Give up wimp?

A660202 dark matter “hair”. Some researchers believe that dark matter is distributed throughout the universe in this “network” pattern.

In two reviews published on February 23 in the journal science advances, researchers described how axions become the main candidate particles of dark matter, and how physicists will study such particles more deeply, perhaps explaining the mystery of dark matter.

So far, among the many candidate components that may form dark matter, the “weakly interacting massive particles” (wimp) has been in the leading position. Wimp is a very general term. It describes a hypothetical particle that still stays in the theoretical stage. It interacts very weakly with ordinary matter only through weak nuclear force and gravity. It is predicted that the mass of wimp is 1 to 1000 times that of proton.

However, although wimp is the most promising candidate component of dark matter, in the past few years, wimp has not appeared in the Large Hadron Collider or in the direct search for dark matter. Therefore, as wimp gradually loses its halo as a candidate particle of dark matter.

Can we capture axions?

As the research on wimp reached an impasse, researchers began to explore what steps can be taken to confirm the existence of Axion and further determine whether it may be dark matter. The researchers “have the possibility to capture” the Axion in their review and finally confirm its existence.

In these two reviews, the researchers proposed several different methods that physicists can use to predict the mass of axions and study them as candidates for dark matter. These methods include the use of haloscope, which can “observe the microwave photon signal from the silver halo”. (a galactic halo is a large spherical region of space around a galaxy that extends beyond the range of visible matter.)

The researchers expect that in such experiments, axions will be converted into electromagnetic waves in microwave cavities, although this will be very rare. Theoretically, this electromagnetic wave can be detected.

The researchers also proposed other methods that physicists are currently using and may be used to find axions, including using ground-based telescopes, such as the Axion Solar Telescope (CAST) of the European Center for nuclear research, to detect axions produced by the sun’s core, and even discover axions in the magnetosphere of neutron stars, where they may be converted into photons, And leave obvious spectral characteristics.

Looking for quality

In these two reviews, researchers have explored cutting-edge technologies that allow scientists to detect axions, and one of the more popular Axion detection technologies is to try to detect electromagnetic waves in microwave cavities.

However, in another new study published in the journal Nature communications on February 25, researchers used the supercomputer of the National Center for energy research and scientific computing (NERSC) – one of the largest supercomputers in the world – to simulate the generation of axions.

In the team’s simulation, the Axion was produced almost immediately after the big bang; They also take into account the total mass of dark matter in the universe and the total number of axions produced, so as to estimate the possible mass of axions. The simulation results show that the mass of the Axion is more than twice that expected by theoretical physicists, reaching 40 to 180 μ Ev (roughly equivalent to one tenth of an electron mass).

The research team also found that axions can produce electromagnetic waves with higher frequency than expected, which is beyond the range usually used to detect Axion electromagnetic waves in experiments. In these Axion experiments, the simulation program does not know which frequency to tune to, so many different possibilities must be scanned.

For the simulation results, the researchers described that the Axion in the early universe might behave like a “Rider falling from a Mustang”.

You can imagine that these ropes are composed of axions. In the process of cosmic expansion, the ropes are thrown around to form rings, connected with each other, and go through many violent dynamic processes. The shafts on the edge are trying to hold on to the ropes, but when some too violent processes occur, they will throw out of the ropes. The axions that break away from the rope will eventually become dark matter in a long time.

Of course, the research team did not solve the dark matter problem, nor did they give an exact description of the Axion. But as more and more researchers continue to promote such experiments, in the near future, we may be able to better understand what these hypothetical particles are and finally solve the mystery of dark matter. (Ren Tian)