What is a quark?

In the 1960s, American physicists Murray Gell-Mann and G. Zweig each independently proposed that hadrons such as neutrons and protons are composed of more basic units-quarks. (quark), many Chinese physicists call it "stratons". They have a fractional charge, 2/3 or -1/3 times the charge of an electron, and a spin of 1/2. The word "quark" was adapted by Murray Gell-Mann from a line in James Joyce's novel Finnegan's Wake. The original theory to explain strongly interacting particles required three kinds of quarks, called the three flavors of quarks, which are up quark (up, u), down quark (down, d) and strange quark (strange, s). The J/ψ particle was discovered in 1974, requiring the introduction of the fourth quark, charm quark (charm, c). The Y particle was discovered in 1977, requiring the introduction of the fifth quark, bottom quark (bottom, b). The sixth quark, the top quark (top, t), was discovered in 1994 and is believed to be the last quark.

Quark theory holds that all baryons are composed of three quarks, and antibaryons are composed of three corresponding antiquarks. For example, proton (uud), neutron (udd). Quark theory also predicts the existence of a particle (sss) composed of three strange quarks. This particle was observed in a hydrogen bubble chamber in 1964 and is called a negative ω particle.

Quarks are divided into three generations according to their characteristics, as shown in the following table:

Symbol Chinese name English name Charge (e) Mass (GeV/c^2)

u Up quark up +2/3 0.004

d Down quark down -1/3 0.008

c Charm quark charm +2/3 1.5

t Top quark top +2/3 176

b Bottom quark bottom -1/3 4.7

In quantum chromodynamics, in addition to the "flavor" properties, quarks also have three "color" properties, namely red, green and blue. "Color" here does not mean that quarks actually have color, but uses the word "color" to vividly describe a physical property of quarks themselves. Quantum chromodynamics believes that general matter has no "color". The "colors" of the three quarks that make up baryons are red, green and blue, so when superimposed together they become colorless. Therefore, including the attributes of 6 flavors and 3 colors, there are 18 kinds of quarks, and their corresponding 18 kinds of antiquarks.

Quark theory also believes that mesons are bound states composed of a quark and an anti-quark of the same color. For example, the [[π+ meson]] predicted by Japanese physicist Hideki Yukawa is composed of an up quark and an anti-down quark, and the π-meson is composed of an anti-up quark and a down quark. They are both Colorless.

Five kinds of quarks except the top quark have been discovered through experiments. The Chinese scientist Ting Zhaozhong won the Nobel Prize in Physics for the discovery of the charm quark. One of the main research directions of high-energy particle physicists in the past decade has been the top quark (t).

As for the sixth "top quark" newly discovered in 1994, it is believed to be the last one. Its discovery allows scientists to obtain a complete picture of quarks, which will help study the origin of the universe at the beginning of the Big Bang. How the universe evolved in less than a second, because the intense heat initially generated by the Big Bang created top excitation particles.

Research shows that some stars may become "quark stars" at the end of their evolution. When a star cannot withstand its own gravitational pull and continues to shrink, the increase in density will squeeze out the quarks. Eventually, a star the size of the sun may shrink to only seven or eight kilometers in size, but it will still shine.

Quark theory believes that quarks are trapped inside particles, and there are no individual quarks. Some people have objected, arguing that quarks are not real. However, almost all predictions made by quark theory agree well with experimental measurements, so most researchers believe that quark theory is correct.

In 1997, Russian physicist Diakonov and others predicted the existence of a particle composed of five quarks with a mass 50% greater than that of a hydrogen atom. In 2001, Japanese physicists discovered evidence of the existence of pentaquark particles when they bombarded a piece of plastic with gamma rays at the SP Ring-8 accelerator. It was subsequently confirmed by physicists at the Thomas Jefferson National Accelerator Laboratory in the United States and the Institute of Theoretical and Experimental Physics in Moscow. This pentaquark particle is composed of 2 up quarks, 2 down quarks and an antistrange quark. It does not violate the standard model of particle physics. This is the first time a particle composed of more than three quarks has been discovered. Researchers believe that this particle may be just the first member of the "pentaquark" family of particles to be discovered, and that there may also be particles composed of four or six quarks.