1, all neutrons are composed of three quarks, and antineutrons are composed of three corresponding antiquarks, such as protons and neutrons. Proton consists of two upper quarks and one lower quark, and neutron consists of two lower quarks and one upper quark.
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They have a fractional charge, which is 2/3 times or-1/3 times the electron charge, and their spins are 1/2 or-1/2. First of all, three kinds of quarks are needed to explain the strong interaction particle theory, which is called quark three flavors. They are upper quark (up, u), lower quark (down, d) and odd quark [1](strange, s). 1974 The discovery of the J/ψ particle requires the introduction of the fourth quark charm quark (Charm, C). The particle υ was found in 1977, which requires the introduction of the fifth quark bottom (b). The sixth quark top, T quark (T), was found in 1994 and is considered as the last quark. Quark theory holds that all baryons are composed of proton (uud) and neutron (UDD). Antibaryons consist of three corresponding antiquarks. Quark theory also predicted the existence of a particle (sss) composed of three strange quarks, which was observed in the hydrogen bubble chamber of 1964 and was called negative ω particle. Top quarks, bottom quarks, odd quarks and charm quarks will decay into upper quarks or lower quarks in a short time due to their huge mass (see the table below). Quarks are divided into three generations according to their characteristics, as shown in the following table:
Charge quantity of Chinese and English name symbols in substitution function /e mass /MeV.c-2
Quarks+1/2iz =+1/2u+2/31.5 to 4.0 on1
1 ? 1/2 Iz=？ 1/2 quark) d? 1/3 4 to 8
2 ? 1/2 S=？ 1 strange quark) s? 1/3 80 to 130
2+ 1/2 C= 1 charm quark C+2/3 1 150 to 1350.
3 ? 1/2 B′=？ 1 bottom quark) b? 1/3 4 100 to 4400
3+ 1/2 T= 1 top quark t+2/3171400 2100.
Some physicists in our country call quarks straton, because they think that even straton is not the initial element of matter, but only one layer in the infinite layer of matter structure.
In quantum chromodynamics, quarks not only have the characteristics of "taste", but also have three characteristics of "color", namely, red, green and blue. Here, "color" does not mean that quarks really have color, but uses the word "color" to vividly compare a physical property of quarks themselves. Quantum chromodynamics holds that general matter has no "color", and the "colors" of the three quarks that make up baryons are red, green and blue respectively, so they are colorless when superimposed together. So it includes six flavors and three colors, * * * has 18 quarks and their corresponding 18 antiquarks.
Quark theory also holds that mesons are bound states composed of a quark and an antiquark of the same color. For example, Japanese physicist Hideki Yukawa predicted that [[π+meson]] consists of an upper quark and an anti-lower quark, while π-meson consists of an anti-upper quark and a lower quark, which are colorless.
Five kinds of quarks except the top quark were discovered through experiments. Ding Zhaozhong, a scientist from China, won the Nobel Prize in physics for discovering charm quarks (also known as J particles). Top quark (T) is a main direction of high-energy particle physicists in recent ten years.
As for the sixth "top quark" newly discovered by 1994, I believe it is the last one. Its discovery enables scientists to obtain a complete picture of quarks, which helps to study how the universe evolved in less than a second at the beginning of the Big Bang, because the high heat generated at the beginning of the Big Bang will produce top quark particles.
Studies have shown that some stars may become "quarks" at the end of evolution. When a star cannot resist its gravitational contraction, quarks will be squeezed out by a large increase in density. Eventually, a star the size of the sun may shrink to only seven or eight kilometers, but it will still shine.
Quark theory holds that quarks are all trapped inside particles and there is no single quark. Some people object that quarks do not really exist. However, almost all the predictions made by quark theory are in good agreement with the experimental measurements, so most researchers think that quark theory is correct.
1997, Russian physicist Diane Konov and others predicted that there was a particle composed of five quarks, the mass of which was 50% greater than that of hydrogen atom. In 200 1 year, Japanese physicists discovered the evidence of the existence of pentaquark particles when they bombarded a piece of plastic with gamma rays on the SP Ring-8 accelerator. It was later confirmed by physicists at Thomas Jepperson National Accelerator Laboratory and Moscow Institute of Theoretical and Experimental Physics. This pentaquark particle consists of two upper quarks, two lower quarks and an anti-singular quark, which does not violate the standard model of particle physics. This is the first time that a particle composed of more than three quarks has been found. Researchers believe that this particle may be only the first member of the "five quarks" particle family to be discovered, and there may be particles composed of four or six quarks.
One by one, nine experimental groups claimed to have found evidence of five quarks. But in other high-energy experimental groups and their data, including the use of lepton colliders, such as Zeus experiment in DESY, Germany, Belle in KEK, Japan and BaBar in SLAC, USA, as well as CDF and D? In the experiment, no evidence that should exist was observed. Therefore, the existence of the so-called five quark particles is still a controversial topic. At the same time, Chun8 also plans to further improve the efficiency, and the radiation is 10 times stronger than the current one, so as to obtain more experimental data for statistical confirmation.
At present, human beings are only making bold assumptions and scientific verification. Quark is a possible hypothesis to explain some phenomena that cannot be explained by human beings at present, but human beings have never found direct evidence of quark.
199665438+On February 2nd, Science and Technology Daily published Professor Cui Junda's article "Composite Space-time Theory is not Pathological Science". Cui further pointed out in the article: "The existence of quarks is not universally recognized in physics. The differences can be traced back to the 1970s. Zhu Hongyuan, a physicist in China, and Heidelberg, the founder of quantum mechanics, all think that many physicists all over the world have spent so much effort to find quarks. If quarks really existed, they should have been discovered long ago.
It is certainly wrong for this scientist to deny quarks, just like the sentence "If quarks really existed, they should have been discovered long ago" is obviously a fallacy, which is equivalent to saying "If cancer really existed, it should have been cured long ago".
In short, science cannot be anything false and emotional. Quarks cannot directly prove its existence, nor can they prove (even indirectly) that it does not exist. At present, it is only a hypothesis.
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The discovery of quarks
/kloc-at the end of 0/9th century, Marie Curie opened the door of atoms and proved that atoms are not the smallest particles of matter. Soon, scientists discovered two kinds of subatomic particles: electrons and protons. 1932, james chadwick discovered neutrons. This time, scientists thought that the smallest particle was found.
In the mid-1930s, the particle accelerator was invented. Scientists can break down neutrons into protons, break down protons into heavier nuclei, and observe what happens when they collide. In 1950s, Donald Glaser invented the "bubble chamber", accelerating subatomic particles to near the speed of light, and then throwing out this low-pressure bubble chamber filled with hydrogen. After these particles collide with protons (hydrogen nuclei), the protons split into a group of strange new particles. When these particles diffuse from the collision point, they will leave a tiny bubble and expose their traces. Scientists can't see the particles themselves, but they can see the traces of these bubbles.
These tiny trajectories on bubble chamber images (each of which indicates the transient existence of a previously unknown particle) are diverse and numerous, which makes scientists both surprised and puzzled. They can't even guess what these subatomic particles are.
Murray gherman, 1929 was born in Manhattan, and he is a veritable child prodigy. At the age of 3, he could mentally calculate the multiplication of large numbers; The 7-year-old won the spelling bee 12-year-old child; At the age of 8, he is as intelligent as most college students. However, at school, he felt bored, agitated and suffered from serious writing obstacles. Although it is easy for him to finish papers and research project reports, he seldom finishes them.
Nevertheless, he successfully graduated from Yale University and worked in MIT, University of Chicago (for Fermi) and Princeton University (for Oppenheimer). At the age of 24, he decided to concentrate on studying the strange particles in bubble chamber's images. Through the bubble chamber image, scientists can estimate the size, charge, direction and speed of each particle, but they cannot determine their identity. Up to 1958, nearly 100 names are used to identify and describe these detected new particles.
Murray gherman believes that if several basic concepts about nature are applied, it is possible to understand these particles. He first assumed that nature is simple and symmetrical. He also assumed that, like all other substances and forces in nature, these subatomic particles are conserved (that is, mass, energy and charge are not lost in collisions, but preserved).
Under the guidance of these theories,
[Our understanding of material structure so far]
So far, our understanding of material structure
Gherman began to classify and simplify the reactions in proton splitting. He created a new measurement method called "singularity". This word was introduced by him from quantum physics. Singularity can measure the quantum state of each particle. He also assumed that singularities existed in every reaction.
Gherman found that he could set up a simple reaction model of proton splitting or synthesis. But there are several patterns that don't seem to follow the law of conservation. Then he realized that if protons and neutrons are not solid substances, but are composed of three smaller particles, then he can make all collision reactions follow simple conservation laws.
After two years of hard work, gherman proved that these smaller particles must exist in protons and neutrons. He named it "k- works" and later abbreviated it as "KWOKS". Soon after, he read a sentence "three quarks" in James Joyce's works, so he renamed this new particle quark.
Jerome Friedman of Massachusetts Institute of Technology (MIT), Henry kendall and RichardTaylor of Stanford Linear Accelerator Center (SLAC), during the period from 1967 to 1973, conducted a series of pioneering experiments on deep inelastic scattering of protons and neutrons in Stanford by using the most advanced two-kilometer electron linac at that time, and won the Nobel Prize in physics. This shows that people finally realize the existence of quarks in science.
Canadian Taylor 1950 got a bachelor of science degree, 1952 got a master's degree, 1962 got a doctorate from Stanford, 1968 got an associate professor from Stanford linear accelerator center, and 1970 got a professor. American Friedman 1950 at the University of Chicago. 1953 obtained the master's degree, and 1956 obtained the doctor's degree. 1960 came to MIT as an associate professor, 1967 was promoted to professor, 1983- 1988 was the head of the physics department of the institute. American Kendall was born in 65438. 1954 received his Ph.D. in physics from MIT, and two years later he became an associate professor at Stanford University and a professor at MIT from 1967.
The experiment done by Stanford Linear Accelerator Center is similar to that of E. Rutherford to verify the nuclear model. Just as Rutherford predicted the existence of atomic nuclei by observing the large-angle scattering of a large number of alpha particles, the Stanford Linear Accelerator Center confirmed the point-like components in the nuclear structure through the large-angle scattering of a large number of electrons, which was unexpected before and is now understood as quarks.
Gherman predicted the existence of quarks in 1964, and G Zweig of California Institute of Technology also made this prediction independently. No one can come up with a convincing dynamic experiment to confirm the existence of quarks in protons and neutrons before the MIT experiment at Stanford Linear Accelerator Center. Theorists at that time were not clear about the role of quarks in hadron theory. As Jowers C Jarlskog said when introducing the winners to the King of Sweden at the Nobel Prize ceremony, "Quark hypothesis was not the only hypothesis at that time. For example, there is a model called' nuclear democracy', which holds that no particle can be called a basic unit, and all particles are equally basic and constitute each other. "
1962, Stanford began to build a large linear accelerator with energy of 10-20 GeV. After a series of improvements, the energy can reach 50GeV ... Two years later, W panofsky, director of the Stanford Linear Accelerator Center, was supported by several young physicists who worked with him when he was the director of the Stanford High Energy Physics Laboratory, Taylor. And served as the leader of an experimental group. Soon, Friedman and Kendall joined in. At that time, they were teachers at MIT. They have been doing electron scattering experiments on the 5GeV Cambridge Electron Accelerator, which is a cyclotron with limited capacity. But Stanford will have a 20GeV accelerator. It can produce "absolutely strong" radiation beam, high current density and external radiation beam. A team from California Institute of Technology has also joined the cooperation, and their main work is to compare electron-proton scattering and positron-proton scattering. This means that scientists from Stanford Linear Accelerator Center, Massachusetts Institute of Technology and California Institute of Technology have formed a huge research team (this team is called Group A). They decided to build two energy spectrometers. One is a large acceptance spectrometer with 8GeV, and the other is a small acceptance spectrometer with 20GeV. The difference between the newly designed spectrometer and the early spectrometer is that they focus point by point in the horizontal direction, instead of the point by point focus in the old equipment. This new design can make the scattering angle spread out in the horizontal direction and the momentum spread out in the vertical direction. The measurement of momentum can reach 0.65438 0%, and the accuracy of scattering angle can reach 0.3 milliradians.
At that time, the mainstream of physics thought that protons had no point structure, so they expected that the scattering cross section would decrease rapidly with the increase of q2 (Q is the four-dimensional momentum transferred to the nucleus). In other words, they expected little large-angle scattering and the experimental results were unexpectedly large. In the experiment, they used various theoretical assumptions to estimate the counting rate. None of these assumptions includes constituent particles. One hypothesis uses the structural function observed in elastic scattering, but the experimental results are one or two orders of magnitude different from the theoretical calculation. This is an amazing discovery, and people don't know what it means. No one in the world (including the inventor of quarks and the whole theoretical circle) specifically and accurately said, "Look for quarks, I believe they are in the nucleus." In this case, Bjorcken, a theorist at Stanford Linear Accelerator Center, put forward the idea of calibration independence. When he was a graduate student at Stanford, he completed the study of inelastic scattering kinematics with his L-hand. When Bjorcken returned to Stanford in February 1965, due to the influence of the environment, naturally, he started the subject of electronics again. He remembers that in 196 1 year, he heard L. Schiff say that inelastic scattering is a method to study the instantaneous charge distribution in protons. This theory shows how inelastic scattering of electrons gives the momentum distribution of neutrons and protons in the nucleus. At that time, gherman introduced stream algebra into field theory. Abandoning some mistakes in field theory and maintaining the reciprocity of flow algebra, S. Adler derived the summation rule of neutrino reaction by using localized flow algebra. Bjorcken used stream algebra to study the scattering of high-energy electrons and neutrinos in order to calculate the integral of structural function to the global summation rule, which took two years. And find out the shape and size of the structure function. Generally speaking, the structural functions W 1 and W2 are functions of two variables. These two variables are the square q2 of four-dimensional momentum transfer and energy transfer V. Bjorcken thinks that the structure function W2 only depends on the dimensionless ratio ω = 2mV/Q2 of these variables (m stands for proton mass), that is, vW2=F(ω), which is Bjorcken scale independence. He used many parallel methods, the most speculative of which was the point structure. The summation rule of stream algebra implies the point structure, but it does not necessarily require the point structure. However, according to this suggestion, Bjorcken combined with other strong interaction concepts, such as Reggie pole, makes the summation rule converge, and naturally obtains the independence of structural function scaling.
After calibration irrelevance was put forward, many people did not believe it. As Friedman said, "These views have been put forward, and we are not completely sure. He is a young man, and we think his ideas are great. We didn't expect to see some structure, but what he said was just a lot of nonsense. " 25438+0967 and 1968, the experimental data of deep inelastic scattering have begun to accumulate. When Kendall showed Bjorcken a brand-new data analysis, Bjorcken suggested using the scale-independent variable ω to analyze the data. According to the chart drawn by the old method, Kendall said, "The data is scattered, just like the chicken paw prints are covered with chart paper. When data are processed according to Bjorcken's method (vW2 vs), they are concentrated in a powerful way. I remember balmer's feeling when he discovered his empirical relationship-the wavelength of hydrogen spectrum is absolutely accurate. " 1968 In August, Friedman reported the first result. As the leader of the conference, panofsky hesitated to put forward the possibility of the nuclear point structure.
After collecting the data of 6 and 10 scattering from 20GeV spectrometer, Group A began to use 8GeV spectrometer to do 18, 26 and 34 scattering. According to these data, it is found that the second structural function W 1 is also a function of univariate ω, that is, it obeys Bjorcken scale independence. All these analysis results are still independent today. Even after more accurate radiation correction, the difference of the results is not more than 65438 0%. Starting from 1970, the experimenter made a similar scattering experiment with neutrons. In these experiments, they alternately measured hydrogen (protons) and deuterium (neutrons) for one hour to reduce the system error.
As early as 1968, R. Feynman of California Institute of Technology thought that hadrons were made up of smaller "partons". When he visited Stanford Linear Accelerator Center in August of the same year, he saw that inelastic scattering data had nothing to do with Bjorcken scale. Feynman thinks that some protons are in high-energy relativistic nuclei.
That is to say, the structure function is related to the momentum distribution of partons. This is a simple dynamic model and another expression of Bjorcken's viewpoint. Feynman's work greatly stimulated the theoretical work, and several new theories appeared. After C. Gllan and D. Gross concluded that the ratio R of W 1 and W2 is closely related to the spin of partons, Stanford Linear Accelerator Center-Massachusetts.
Herman's demand for quarks excludes other hypotheses. Neutron data analysis clearly shows that neutron yield is different from proton yield, which further denies other theoretical hypotheses.
A year later, inelastic scattering of neutrinos in CERN heavy bubble chamber strongly expanded the experimental results of Stanford Linear Accelerator Center. In order to consider the difference between the electromagnetic interaction between quarks and the weak-current interaction between neutrinos, the Stanford Linear Accelerator Center was calibrated.
It is completely consistent with the data of Stanford Linear Accelerator Center. Quark-quark interaction is manifested in later muon deep inelastic scattering, electron-positron collision, proton-antiproton collision and hadron jet. All these strongly prove the quark structure of hadron.
It took physics several years to accept quarks, which is mainly due to the contradiction between the point structure of quarks and their strong constraints in hadrons. As Jowers Kaug said at the Nobel Prize ceremony, quark theory cannot completely and uniquely explain the experimental results. Nobel Prize-winning experiments show that protons also contain electrically neutral structures. People soon discovered that this is the "gluon". In protons and other particles, gluons bind quarks together. 1973, Gro, F. Wilcek and H. D. Rizel independently discovered the asymptotic freedom theory of non-Abelian gauge fields. This theory holds that if the interaction between quarks is caused by color gauge gluons, then the coupling between quarks is logarithmically weakened in a short distance. This theory (later called quantum chromodynamics) can easily explain all the experimental results of Stanford Linear Accelerator Center. In addition, the opposite of asymptotic freedom and the increase of long-distance coupling intensity (called infrared slavery) explain the mechanism of quark confinement. Gherman, the father of quarks, said at the 16th International Conference on High Energy Physics 1972: "Quarks are not required to be truly measurable in the laboratory in theory, but like magnetic monopoles, they can exist in imagination." In a word, the inelastic electron scattering experiment at the center of Stanford linear accelerator shows the point-like behavior of quarks, which is the experimental basis of quantum chromodynamics.