Mystery inside the atom: Scientists try to solve the mystery of the EMC effect

Beijing time on February 25th, no one really knows what’s going on inside the atom, but two research teams have come up with the answer to that question. Now, both competing teams want to prove their theory is correct. What we can be sure of now is that within the atom, electrons rotate along the “orbit” of the outer layer, followed by “a large” empty space, and at the center of this space, there is a very small nucleus that gives the atom most of its mass.

The nucleus is a close combination of protons and neutrons, which come together and are bound by so-called strong nuclear forces, also known as strong interactions. The number of protons and neutrons determines whether an atom is iron, oxygen, or radon, and whether it is radioactive or stable.

However, no one knows how protons and neutrons (together called nuclei) behave inside atoms. Outside of atoms, protons and neutrons have a certain size and shape. Each nucleus is made up of three smaller particles, quarks. The interaction between quarks is so strong that no external force can deform them, not even the strong nuclear forces between the nucleus particles. But decades of research have found that this theory is somewhat wrong. Experiments have shown that protons and neutrons appear to be much larger than they should be in the nucleus. Physicists have developed two competing theories, trying to explain this strange mismatch, and the proponents of each theory are quite certain that the other theory is wrong. However, both camps agree that whatever the right answer is, it must come from outside their own realm.

Gerald Miller, a nuclear physicist at the University of Washington, said physicists at the University of Washington knew that the orbit of a nucleus was small and compact, at least in the 1940s. The movement of the nucleus is constrained and there is little energy. They are less likely to bounce around because of the limitations of strong nuclear forces.

In 1983, physicists at CERN noticed a strange phenomenon: electron beams bounce destitted from iron atoms in a very different way than they did from free protons. This is unusual. If the protons in a hydrogen atom are as big as the protons in the iron atom, the electrons should bounce back in the same way.

At first, the researchers didn’t know what they were observing. But over time, scientists began to think that it was a matter of size. For some reason, protons and neutrons in the heavy nuclei look much larger than they did outside the nucleus. The researchers call this phenomenon the EMC effect and are named after the European Small Partnership, which discovered it by chance. The effect can also be extended to the significant difference in the dynamic distribution of quarks within protons or neutrons in the nucleus than in free nuclei, in other words, the speed of movement of the quarks that make up protons and neutrons will decrease significantly once they are confined to the nucleus. In physicists’ view, the EMC effect violates existing nuclear physics theories.

Or Hen, a nuclear physicist at the Massachusetts Institute of Technology, makes a point that could explain what researchers observe. Quarks are subatomic particles that make up the nucleus and interact strongly with a given proton or neutron, while quarks of different protons and neutrons do not interact strongly. The strong interaction between the quarks inside the nucleus is so great that it dwarfs the strong nuclear force seq that binds the nucleus to other nuclei.

“Imagine you sitting in a room, talking to two friends, and the window is closed, ” hearn said Hearn. ” “The three people in the room can be thought of as three quarks inside a proton or neutron, and the breeze outside the window is the interaction between the proton or neutron and other nuclei outside. Even if a small amount of air flows through the closed window, it will hardly affect the people in the room.

In other words, as long as the nucleus is still in orbit, the force sitories between the nucleus will be difficult to affect the interaction between the quarks inside the nucleus. However, Hearn points out that recent experiments have shown that at any given time, about 20 percent of the nuclei in the nucleus are actually out of orbit. They are paired with other nuclei and interact with “short-range correlations.” In this case, the interaction between the nucleus has much higher energy than is usual. This is because quarks have broken through the barriers of their respective nuclei and begun to interact directly, and this quark-quark interaction is much more powerful than the nuclear-nuclear interaction.

These interactions break the quark’s wall inside a single proton or neutron, Hearn said. Quarks that make up one proton and quarks that make up another proton begin to occupy the same space. This causes protons (or neutrons, as the case may be) to stretch and blur. They grow fast, albeit for a short time. This causes the average size of all nuclei in the nucleus to be skewed, resulting in an EMC effect.

Mystery inside the atom: Scientists try to solve the mystery of the EMC effect

Most physicists now accept this explanation of the EMC effect. Gerald Miller, who worked with Hearn in some key studies, agrees. But not everyone thinks Hearn’s team solved the EMC effect. Ian Cloet, a nuclear physicist at The National Laboratory in Argonne, Illinois, said he doesn’t think Hearn’s findings are fully supported by the data.

Kreut argues that the EMC effect problem remains unresolved. This is because the basic model of nuclear physics already contains many of the short-range pairs described by Hearn. “If you use this model to study the EMC effect, you won’t be able to describe the EMC effect, ” says Kreuter. No one has yet successfully interpreted the EMC effect using the framework. So in my opinion, it’s still a mystery. “

Mr Kreuter said the experiments mr Hearn and his co-authors had done were “brave” and “very good science” but did not completely solve the problem of the nucleus. “It is clear that the traditional nuclear physics model … is not a very There is no explanation for this EMC effect,” he said. “

QCD is quantum chromodynamics, the rule system that controls quark behavior. The transition from nuclear physics to quantum chromodynamics is a bit like looking at the same image in a different way. Nuclear physics is like watching with a first-generation flip phone, while quantum chromodynamics is a picture on a high-resolution TELEVISION. High-resolution television provides more detail, but its construction is much more complex.

Both Kreukert and Hearn say the problem is that the complete QCD equation for describing all quarks in the nucleus is too difficult to understand. Kreuter estimates that it will take about 100 years for modern supercomputers to reach the speed of computing to solve this problem. Even if today’s supercomputers are fast enough, these equations have not yet developed to the point where they can be replaced.

However, Kreuter believes it is possible to use QCD to answer some questions. Now, these answers offer a new explanation for the EMC effect: nuclear average field theory.

Kreuter does not agree that 20 percent of nuclei in the nucleus are involved in short-range associations. He points out that experiments do not prove this, and that there are theoretical problems with this view. So he came up with a new model. “What I know is that there are very powerful nuclear forces inside the nucleus, ” he said. ” “

These fields have very small distances, negligible effects outside the nucleus, but very powerful in the nucleus. In Kreut’s model, these force fields, which he called the “average field,” actually deform the internal structure of protons, neutrons, and mesons, a particle that carries strong nuclear forces.

“It’s like you put an atom in a strong magnetic field and its internal structure changes, ” says Kreuter. In other words, the average field theoretical physicist sits that the sealed room that Hearn describes is not really strict, and that the holes in the walls will blow in, blowing quarks everywhere, stretching them.

Mr Kreuter acknowledges that short-range correlations may explain some of the EMC effect, while Mr Hearn says the average field may also play a role. The question is, which one is dominant?

Miller, who has also worked extensively with Kreuter, said the advantage of the average field was a more solid theoretical foundation, but Kreuter did not do all the necessary calculations. At the moment, there is plenty of experimental evidence that Hearn has the upper hand in the debate.

Both Hearn and Kreuter say the results of experiments in the next few years are likely to solve the problem. Hearn referred to an ongoing experiment at the Jefferson National Accelerator Center in Virginia. The researchers will observe changes in the nucleus as it approaches each other bit by bit. Mr Kreuter said he wanted to see a “polarization EMC experiment” that would use proton spin, a quantum feature, to break the effect. Perhaps this experiment could reveal unknown details of the EMC effect, which could help with the next calculation.

This is a friendly debate. “It’s great because it means we’re still making progress,” Miller said. In fact, there are two competing theories that mean the problem is exciting and dynamic. Now we finally have the experimental tools to solve these problems. “

As for what will happen in the future, let’s wait and see. (Any day)