Quarks are "confined"? It turns out that humans can't make them "appear"...

In 1974, Ting Zhaozhong and Richter discovered the J/ψ particle, and in 1977, Lederman discovered the Y particle. Since then, charm quarks and bottom quarks have also been confirmed in experiments. This has made the quark model gradually popular, and more than 200 particles can be classified by different quarks. Unfortunately, quarks have not appeared so far due to quark confinement.

Why are quarks confined? As we have introduced before, the earliest conclusion of quark confinement was drawn from a series of experiments conducted at SLAC. At that time, people used electrons with energy close to 20GeV to bombard protons. As a result, it was found that there seemed to be some tiny charged bodies in the protons, suspected to be quarks. Later, with the increase of equipment energy, from tens of G to hundreds of G, and now to hundreds of T, even with such high energy, the quarks could not be knocked out by proton collision. So people were forced to give the theory of quark confinement. Later, with the emergence of quantum chromodynamics (QCD), quark confinement had another description. In fact, as early as 1965, the second year after Gell-Mann proposed the quark model, quantum chromodynamics (QCD) appeared. The theory of quantum chromodynamics believes that each quark has a unique flavor, that is, there are 6 flavors of quarks, and each flavor of quark can have three colors of red, green, and blue. Antiquarks have anti-colors. This property is called color charge. But obviously this flavor and color are not the flavor and color we understand. It simply represents two new quantum numbers. QCD is also a field theory, so it also needs a particle to transmit the interaction, that is, who transmits the strong force between quarks? This particle is called a gluon. There are 8 types of gluons, which are distinguished by color charge. So far, we finally know the internal structure of hadrons, which are composed of quarks and gluons. Why can't quarks be seen? Now we can put it another way, because each type of quark that makes up a hadron must have a different color, that is, one red, green and blue, so the superposition of the three colors becomes white. We see that all particles are white. Except for white, particles of other colors are invisible. This is also called color confinement.

But you may think that the color confinement is just a rehash of the old story, not the fundamental reason, because even the color properties of quarks are artificially determined, so is color confinement credible? It was not until 1973 that three people seemed to have given another explanation for quark confinement while solving other problems.

At that time, there was a little problem with quantum field theory. Some people doubted that quantum field theory was unreliable. The reason was simple: when two particles were infinitely close, the force would be infinite. In the history of physics, this infinity was too terrible and caused a lot of trouble. For example, gravity, the gravitational force between two objects is inversely proportional to the square of the distance. To put it bluntly, the closer the distance, the greater the gravity. Then you will find that when the distance approaches 0, the gravity will be infinite. Fortunately, this phenomenon does not exist in the macroscopic world, because we cannot get infinitely close, but particles are different. Some particles do not even have the concept of size. So at that time, physicist Landau suspected that QCD would likely have this phenomenon of infinite force, and he also named it "Landau singularity". So at that time, two people took the "Landau singularity" as the starting point and wanted to prove that QCD had problems. The two were a pair of teachers and students, the teacher was called Gross, and the student was called Wilczek. The theoretical basis of QCD is the Yang-Mills gauge field, so they began to study the Yang-Mills gauge field and tried to prove that it was wrong. As a result, through mathematical deduction, it was found that in mathematics, when two particles in the Yang-Mills gauge field are infinitely close, their interaction force is not infinite, but on the contrary, it tends to be infinitesimal. Within a certain range, the closer the particles are, the weaker the force is. At that time, people gave this phenomenon a name called asymptotic freedom. That is, within a certain range, the closer the particles are, the freer they are. On the contrary, the farther they are, the greater the interaction force is. So the two of them published a paper. Almost at the same time, another person named Politzer got the same answer and published a paper. This time, not only did they not prove that there was a problem with the QCD theory, but they made QCD famous, because once the asymptotic freedom theory appeared, the Landau singularity would naturally not appear, and people also found that asymptotic freedom seemed to explain quark confinement, that is, why can't quarks appear alone? Because the more you want to separate quarks, the stronger the strong force will be. The three people didn't expect that as more and more asymptotic freedom phenomena were discovered in experiments, 31 years later, in 2004, they also won the Nobel Prize in Physics.

Is asymptotic freedom the fundamental reason for quark confinement? Later, string theory also tried to explain the reason for quark confinement. String theory believes that quarks are connected by a string. When we try to cut the string, a pair of positive and negative quarks will quickly appear at the break. In this way, the string has a new endpoint and forms a new particle. However, string theory itself has not been confirmed yet, so this explanation naturally does not stand up to scrutiny. At present, people have found that quark confinement is likely caused by the properties of the vacuum, but whether this is true is still unknown.

So far, 60 of the 61 elementary particles in the standard model of particle physics have been discovered. Let's do the math. First, there are 6 kinds of quarks, up and down, strange charm, bottom and top, but these are only 6 flavors, and there is also color charge. There are three colors, 3x6=18, and then add the corresponding antiparticles, so there are 36 quarks in total. Then there are 6 kinds of leptons, electrons, muons, tau, as well as electric neutrinos, muon neutrinos, tau neutrinos, plus their antiparticles, there are 12 leptons in total. Quarks plus leptons are the particles that make up matter, a total of 36+12=48. In addition, there are gauge bosons that transmit mutual forces, including 8 kinds of gluons that transmit strong force, 1 kind of photon that transmits electromagnetic force, and three kinds of W± and Z0 that transmit weak force, a total of 12 kinds, 48+12 is exactly 60. So why is it said that the current standard model has 61 kinds of elementary particles? Could it be the graviton? No, gravity is not within the scope of the Standard Model, so the graviton does not count, and the graviton has not been confirmed. The last particle is the Higgs boson, known as the God particle. Strictly speaking, the Higgs boson must exist for the Standard Model to be valid, so people directly put the Higgs boson in when building the Standard Model, but what is this God particle? Can it be found?

Author: Mommy says popular science creator

Reviewer: Luo Huiqian, Associate Researcher, Institute of Physics, Chinese Academy of Sciences

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