What Is a Quark?

Quark is a basic particle that participates in strong interactions and is the basic unit of matter. The quarks combine with each other to form a composite particle called a hadron. The most stable of hadrons are protons and neutrons, which are the units that make up the nucleus. Due to a phenomenon called "quark confinement", quarks cannot be directly observed or separated, and can only be found in hadrons. For this reason, most of what we know about quarks comes from indirect observations of hadrons.

There are currently six known quarks. The type of quark is called "taste". They are up (u), down (d), odd (s), (c), bottom (b) and top (t). The mass of the upper and lower quarks is the lowest of all. The heavier quark will pass a call

Quark has many different inherent characteristics, including

More than 200 hadrons were observed experimentally in the 1950s and 1960s. Therefore, exploring the possible internal structure of such a large number of hadrons and establishing their "periodic table of elements" became a problem for particle physicists at the time. It is these explorations that led to the concept of quark. As early as 1949, E. Fermi and CN Yang first tried to explain the meson with protons and antiprotons, neutrons and antineutrons, etc. ^[1]

The word quark is

See also: Particle physics

Currently, it is believed that the sixth "top quark" discovered in 1995 is the last quark. Its discovery has led scientists to complete images of quarks, which can help study the universe

Quark opportunity

Towards the end of the 19th century, Marie Curie opened the door to atoms, proving that atoms are not the smallest particles of matter ^[38-39] . Soon scientists discovered two subatomic particles: electrons and protons. In 1932, James Chadwick discovered neutrons, and this time scientists thought that the smallest particles were found ^[40-41] .

The particle accelerator was invented in the mid 1930s to have the ability to accelerate charged particles to high energy collisions. In the 1950s, Donald Glaser invented the "bubble chamber", which accelerated subatomic particles to near the speed of light, and then ejected this low-pressure bubble chamber filled with hydrogen. When these particles collide with the hydrogen nucleus, a new group of strange particles will be produced. When these particles diffuse from the collision point, they will leave an extremely tiny bubble, exposing their tracks. Scientists can't see the particles themselves, but they can trace the bubbles. Using a bubble cell image, scientists can estimate the size, charge, direction of motion, and speed of each particle, but they cannot determine their identity. By 1958, nearly 100 names were used to identify and describe these new particles detected.

Quark presenter

One of the quark presenters, Murray Gell-Mann, was born on September 15, 1929, in a Jewish family in New York, USA. He had a strong interest in science when he was a child. He entered Yale University at the age of 14 and transferred to the Massachusetts Institute of Technology after receiving a bachelor's degree in 1948. In 1951 Gellman went to work at the Institute for Advanced Studies at Princeton University. Lecturer at the University of Chicago in 1953. He participated in the research group with Fermi as the core. In 1955, Gelman went to California Institute of Technology as an associate professor of theoretical physics, and later became a professor, becoming the youngest tenured professor of California Institute of Technology.

Quark's other author was George Zweig, a Jewish family born in Moscow, Russia. He originally wanted to become a particle physicist under the guidance of Richard Feynman, but later turned to neurobiological research. learn. He worked as a scientific researcher at Los Alamos National Laboratory and at the Massachusetts Institute of Technology, but switched to the financial industry in 2004.

Zweig

Gailman

Quark inference hypothesis

Gelman believes that if several basic concepts of nature are applied, it may be possible to clarify the hundreds of particles found at the time. He first assumed that nature was simple and symmetrical. He also assumes that like all other matter and forces in nature, these subatomic particles are conserved (that is, mass, energy, and charge are not lost during the collision, but are preserved). Using these theories as a guide, Gellman began to classify and simplify the reactions during nuclear division. He created a new measurement method called "strangeness." The term was introduced by him from quantum physics. Singularity can measure the quantum state of each particle. He also assumed that the singularity was preserved in each reaction.

What we know about the structure of matter

Quark model

Gelman found that he could establish a simple mode of reaction for nuclear division or synthesis. But several models do not seem to follow the law of conservation. He then realized that if protons and neutrons were not basic matter, but consisted of three smaller particles, then he could make all collision reactions follow a simple conservation law.

Quark proof

After two years of hard work, Gellman proved that these smaller particles must exist in protons and neutrons. He named it "k-works" and later abbreviated "kworks". Soon after, he read "three quarks" in James Joyce's work, and renamed the new particles quark.

Jerome Friedman of the Massachusetts Institute of Technology (MIT), Henry Kendall, and Richard Taylor of the Stanford Linear Accelerator Center (SLAC), because Won the 1990 Nobel Physics series at Stanford from 1967 to 1973 using Stanford's most advanced two-kilometer electron linear accelerator for a series of ground-breaking experimental work on the deep inelastic scattering of protons and neutrons by electrons prize. This shows that people finally recognized the existence of quarks scientifically.

The experiments performed at the Stanford Linear Accelerator Center are similar to those performed by E. Rutherford to verify the nuclear model. Just as Rutherford's observation of the large-angle scattering phenomenon of a large number of alpha particles predicted the existence of nucleus in the atom, the large-angle scattering phenomenon of a large number of electrons at the center of the Stanford linear accelerator was unexpected, confirming the point-like group in the nucleus structure Minute.

Although Gellman and Zweig theoretically predicted the existence of quarks, no one could come up with convincing kinetic experiments to confirm the experiments performed by the Stanford Linear Accelerator Center-MIT Collaboration Group. this point. In fact, the role of quarks in hadron theory was unclear to theorists during that period. As C. Jarlskog said at the Nobel Prize Presentation Ceremony to the King of Sweden, "The quark hypothesis was not the only hypothesis at the time. For example, there is a model called 'nuclear democracy' that No particle can be called a basic unit. All particles are equally basic and constitute each other. "

In 1962 Stanford began to build a large linear accelerator with an energy of 10-20 GeV. After a series of improvements, the energy can reach 50 GeV. Two years later, W. Panofsky, director of the Stanford Linear Accelerator Center, received support from several young physicists who worked with him while he was director of the Stanford High Energy Physics Laboratory. One member and served as the leader of an experimental group. Soon Friedman and Kendall joined in. They were MIT teachers at the time. They have been doing electron scattering experiments on the 5GeV Cambridge electron accelerator. This accelerator is a cyclotron and its capacity is limited. But there will be a 20GeV accelerator in Stanford, which can produce "absolutely strong" beams, high current density, and external beams. A team from the California Institute of Technology has also joined the collaboration, and their main job is to compare electron-proton scattering and positron-proton scattering. In this way, scientists from the Stanford Linear Accelerator Center, the Massachusetts Institute of Technology, and the California Institute of Technology formed a large research team (this team is called Group A). They decided to build two spectrometers, one with a large acceptance spectrometer of 8GeV and the other with a small acceptance spectrometer of 20GeV. The difference between the newly designed energy spectrometer and earlier energy spectrometers is that they use a straight line to focus a little in the horizontal direction, instead of the point-by-point focus of the old equipment. This new design allows the scattering angle to spread out horizontally and the momentum to spread out vertically. The measurement of momentum can reach 0.1%, and the accuracy of the scattering angle can reach 0.3 milliradians.

At that time, the mainstream of physics believed that protons had no point structure, so they expected that the scattering cross section would decrease rapidly with increasing q (q is the four-dimensional momentum transferred to the nucleus). In other words, they expected that large-angle scattering would be small, and the experimental results were unexpectedly large. In their experiments, they used various theoretical assumptions to estimate the count rate, none of which included component particles. One hypothesis uses the structure function observed in elastic scattering, but the experimental results differ from the theoretical calculations by 1 to 2 orders of magnitude. This is an amazing discovery, and people don't know what it means. No one in the world (including the quark inventor and the entire theoretical community) can specifically and accurately say: "Go and find quarks, I believe they are in the nucleus." In this case, the theorist at the center of the Stanford linear accelerator is Bjorken proposed the idea of calibration independence. When he was a graduate student at Stanford, he completed the study of inelastic scattering kinematics with L. Hand. When Bjorken returned to Stanford in February 1965, due to the influence of the environment, he naturally started again. Questions about electronics. He remembered listening to L. Schiff at the Stanford Academic Conference in 1961. Inelastic scattering is a method for studying the distribution of instantaneous charges in protons. This theory explains how electron inelastic scattering gives protons in the nucleus. And neutron momentum distribution. At that time, Gellman introduced stream algebra into field theory, abandoning some errors in field theory and maintaining the reciprocal relationship of stream algebra. S. Adler derived the neutrino reaction sum rule using localized flow algebra. Björken spent two years studying the scattering of high-energy electrons and neutrinos by flow algebra in order to calculate the integral of the structure function over the entire summation rule and find out the shape and size of the structure function. Structure functions W ₁ and W _{2 are} generally functions of two variables. These two variables are the square q of the four-dimensional momentum transfer and energy transfer v. Bjorken argues that the structure function W ₂ depends only on the dimensionless ratio of these variables = 2Mv / q (M represents the mass of the proton), that is, vW ₂ = F (), which is Bijoken's scale independence. In arriving at scale independence, he used many parallel methods, the most controversial of which is the point structure. The rule of summation in stream algebra implies a point structure, but it is not required. However, based on this suggestion, Bjorken combined with some other strong interaction concepts that converge the summation rule, such as Reggie's pole, naturally derived the irrelevance of the structure function calibration ^[41] .

After the irrelevance of calibration was proposed, many people did not believe it. As Friedman puts it: "These opinions are raised, and we are not completely sure. He is a young man, and we feel his ideas are amazing. We do not expect to see point structures, what he said A lot of nonsense. "In late 1967 and early 1968, experimental data on deep inelastic scattering began to accumulate. When Kendall showed Bjorken the new analysis of the data, Bjorken suggested analyzing the data with a scale-independent variable . According to the diagram drawn by the old method, Kendall said: "The data are scattered, filled with graph paper like chicken paw prints. When processing the data according to Bjorken's method (W ₂ vs. ), they use a strong A powerful way to bring it together ^[25] . I remember how Balmor felt when he discovered his empirical relationship-the wavelength of the hydrogen spectrum was fitted with absolute precision. "In August 1968, at the Fourteenth International High Energy Physics At the meeting, Friedman reported the first result, and Panovsky, as the leader of the conference, hesitantly raised the possibility of a nuclear point structure.

After collecting 6 ° and 10 ° scattering data from a 20GeV spectrometer, Group A set out to use an 8GeV spectrometer for 18 °, 26 °, and 34 ° scattering. According to these data, it is found that the second structure function W ₁ is also a function of a single variable , that is, it follows the Bijoken scale independence ^[25] . All these analysis results are still correct today, and even after more accurate radiation correction, the difference of the results is not more than 1%. Since 1970, experimenters have conducted similar scattering experiments with neutrons. In these experiments, they have alternately measured hydrogen (proton) and deuterium (neutron) for one hour to reduce system errors.

As early as 1968, Feynman of Caltech had thought that hadrons were made up of smaller "portions". When he visited the center of the Stanford Linear Accelerator in August of the same year, he saw that the data of inelastic scattering had nothing to do with the Bjorken scale. Feynman believes that partons are approximately freely distributed in high-energy relativity nuclei, that is, the structure function is related to the momentum distribution of partons. This is a simple model of dynamics, and another term for Bjorken's point of view. Feynman's work greatly stimulated theoretical work, and several new theories emerged. After C. Gllan and D. Gross found that the ratios R ₁ and W ₂ of R and partial spins are closely related, experimental data from the Massachusetts Institute of Technology at Stanford Linear Accelerator Center and Feynman The requirement for quarks eliminated other assumptions. The analysis of neutron data clearly shows that the neutron yield is different from the proton yield, which further denies other theoretical assumptions.

One year later, the neutrino inelastic scattering performed in the heavy bubble chamber of the European Nuclear Research Center extended the experimental results of the Stanford Linear Accelerator Center. Subsequent muton-depth inelastic scattering, electron-positron collision, proton-antiproton collision, and hadron injection all showed quark-quark interactions. All of these strongly prove the quark structure of hadrons.

It took the physics community several years to accept quarks, which was mainly due to the contradiction between the point structure of quarks and their strong constraints in hadrons. Just as Josskog said at the Nobel prize ceremony, the quark theory cannot completely and exclusively explain the experimental results. The Nobel Prize-winning experiments show that protons also contain electrically neutral structures. Gluon. " Gluons glue together quarks in protons and other particles. In 1973, Gross, F. Wilczek and HD Politzer independently discovered the asymptotic freedom theory of non-Abelian gauge fields [25,26]. This theory holds that if the interaction between quarks is caused by color gauge gluons, the coupling between quarks decreases logarithmically over a short distance. This theory (later called Quantum Chromodynamics) easily explains all experimental results at the center of a Stanford linear accelerator. In addition, the opposite of asymptotic freedom, the increase in the strength of long-distance coupling (called infrared slavery) illustrates the mechanism of quark confinement. The father of quarks, Gelman, said at the 16th International Conference on High Energy Physics in 1972: "Theoretically, quarks are not required to be truly measurable in the laboratory. It can exist in imagination. "In short, the electron inelastic scattering experiments at the center of the Stanford linear accelerator showed the quark-like behavior, which is the experimental basis of quantum chromodynamics.

In 1967, Steven Weinberg and Abdus Salam independently obtained the unified norm theory of weak electricity ^{[9] [10]} . In order to introduce the weak interaction of quarks in this model in 1970, Grachau et al. Improved the method used in the classical four Fermi weak effects introduced by Kabib and introduced quark, ^[15] and proved it in 1974. Need to introduce ^[17] . In 1973, Japanese physicists Makoto Kobayashi and Toshi Masakawa introduced the third-generation quark to explain the destruction of time inversion in weak interactions ^[23] , and it was experimentally confirmed that they won the Nobel Prize in Physics in 2008. ^[42]