Elementary particle, a physical term, refers to peoplecognitionComposition ofmaterialThe smallest or/and most basic unit of is the basis of various objects.That is, the minimum under the premise of not changing the material propertiesvolumeSubstance.It is composed of various objectsBasics, and it will not be judged that it is not some kind of substance because it is small.But inQuark theorySince it was proposed, people have realized that elementary particles also have complex structures, so the term "elementary particles" is generally not mentioned.
According to the different forces, the basic particles are divided intoquark、LeptonandPropagatorThree categories.In the theoretical framework of quantum field theory, these basic particles are treated as point particles.
Fundamental particles have multidisciplinary scientific applications.Scientists useParticle acceleratorAccelerate some particles, and sometimes use particle collisions to study the properties and composition of basic particles.
Elementary particle, a physical term, refers to peoplecognitionComposition ofmaterialThe smallest or/and most basic unit of is the basis of various objects.That is, the minimum under the premise of not changing the material propertiesvolumeSubstance.
features
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size
The elementary particles are the basis of various objects. They are much smaller than atoms and molecules, and cannot be observed by the current electron microscope with the highest power.protonNeutrons are only one hundred thousandth of the size of atoms.Leptons andquarkIs smaller thanproton、neutronOne ten thousandth of.
quality
The mass of particles is another main characteristic quantity of particles.According to the gauge theory of particle physics, the mass of all gauge particles is zero.The norm invariance is broken in some wayquark, leptons and intermediate bosons.The existing particle mass range is large.Photons and gluons are massless, and the electronic mass is very small. The mass of π mesons is 280 times that of electrons;proton、neutronBoth are very heavy, close to 2000 times the electronic mass.
The heaviest particle known is the top quark.Six quarks have been found, ranging from lower quark to top quark, with mass ranging from light to heavy.The mass of neutrinos is very small. The measured mass of neutrinos is one seventh of the electronic mass.
life
The lifetime of particles is the third main characteristic quantity of particles.Electrons, protons and neutrinos are stable and called "long-lived" particles;Most other particles are unstable, that is, they can decay.A free neutron will decay into a proton, an electron and a neutrino;A pion decays into aMuonAnd a neutrino.The lifetime of particles is defined by the time when the intensity decays to half.Proton is the most stable particle. The measured proton lifetime is more than 10 to the 33rd power years.
Symmetry
There is symmetry between particles.There is a particle, there must be an antiparticle.In 1932, scientists found a particle with the same mass as the electron but with a positive charge, called a positron;Later, a negatively charged particle with the same mass as the proton was found, which was called antiproton;Subsequently, various anti quarks and anti leptons were also discovered.A pair of positive and anti particles can collideAnnihilation, become a photon carrying energy, that is, the particle mass is transformed into energy;On the contrary, when two high-energy particles collide, it is possible to produce a pair of new positive and anti particles, namelyenergyalsoCan be transformed into particles with mass。
spin
Particles also have another property - spin.A particle whose spin is a half integer is called a fermion, and a particle whose spin is an integer is called a boson.
It was Ralph Kronig, George Uhlenbeck and Samuel Goudsmit who first proposed the concept of rotation and corresponding angular momentum for elementary particles in 1925.However, later in quantum mechanics, through theoretical and experimental verification, it was found that basic particles can be regarded as indivisible point particles, so the rotation of objects cannot be directly applied to the spin angular momentum, so only spin can be regarded as an internal property, which is a kind of angular momentum inherent in particles, and its magnitude is quantized,It cannot be changed (but the direction of spin angular momentum can be changed by operation).
conservation
Matter is constantly moving and changing, and there are some things that remain unchanged in the process of change, namely conservation.The production and decay of particles must follow the law of conservation of energy.In addition, there are other conservation laws, such as mass conservation, momentum conservation, angular momentum conservation, and discontinuous parity conservation and charge conservation in microscopic phenomena, as well as baryon number conservation, lepton number conservation, singular number conservation, isospin conservation, etc.
Dual attribute
The particles in the micro world have dual attributes: particle property and wave property.Quantum field theory is the basic theory that describes the dual properties of particle and wave, as well as the generation and elimination process of particles.Quantum field theory
family
charge
Mass (MeV)
Average life (s)
Common decay products
Antiparticle
Lepton
μ
-e
one hundred and six
2.2*10^-6
evμv-e
μ+
e
-e
zero point five one one
stable
——
e+
ve
zero
zero
stable
——
v-e
vμ
zero
zero
stable
——
v-μ
τ
vτ
baryon
p
+e
nine hundred and thirty-eight point two six
stable
——
p-
n
zero
nine hundred and thirty-nine point five five
nine hundred and thirty
pev-e
n-
Λ
zero
one thousand one hundred and fifteen point six
2.5*10^-10
pπ-,nπ0
λ-
Σ+
+e
one thousand one hundred and eighty-nine point four
8*10^-10
pπ0,nπ+
Σ-
Σ0
zero
one thousand one hundred and ninety-two point five
Less than 10 ^ - 14
λ，radiation
Σ+
Σ-
-e
one thousand one hundred and ninety-seven point three
1.5*10^-10
nπ-
Σ+
Ξ-
-e
one thousand three hundred and twenty-one point two
1.7*10^-10
λπ-
Ξ+
Ξ0
zero
one thousand three hundred and fourteen point seven
3*10^-10
λπ0
Ξ0
meson
π+
+e
one hundred and thirty-nine point six
2.6*10^-8
μ+vμ
π-
π-
-e
one hundred and thirty-nine point six
2.6*10^-8
μ-vμ
π+
π0
zero
one hundred and thirty-five
10^-6
radiation
π0
K+
+e
four hundred and ninety-three point eight
1.2*10^-8
μ+vμ，π+π0
K-
K-
-e
four hundred and ninety-three point eight
1.2*10^-8
μ-vμ，π-π0
K+
K0
zero
four hundred and ninety-seven point eight
8.6*10^-11
π+π-，2π0
K0
(fast decay mode)
5.4*10^-8
3π0，π+π-π0
(slow decay mode)
π+μv-μ，π+ev-,π-μ+vμ-,π-e+v
K0 (antiparticle)
zero
four hundred and ninety-seven point eight
The decay mode is the same as K0
——
K0
η
zero
five hundred and forty-eight point eight
——
3 π 0, π 0 π+π -, π+π -, radiation
η
And gauge theory has successfully described particles and their interactions.
Family Properties
Note: The mass of particles in the table is given in the energy unit of 1MeV (megaelectron volts).If compared with the daily unit, 1MeV is equivalent to working at 1kW power 1.6 * 10 ^ - 16s
Standard Model of Interaction
Interaction standard model
structure
The secret of elementary particles
Paul Dirac
In 1932, Dirac's prediction about the existence of positrons was confirmed, and Anderson won the Nobel Prize in physics in 1936.In 1955, Segre and Chamberlin discovered antiprotons using high-energy accelerators, for which they won the 1959 Physics Prize.The next year, antiprotons were discovered.
In 1959, Wang Ganchang and others discovered anti sigma negative hyperons.These provide evidence for the existence of antimatter.After three years of efforts, Reines and others directly detected the anti neutrinos produced in the process of uranium fission in 1956 by using large reactors.He was awarded the 1995 Physics Prize.It was not until 1968 that neutrinos from the sun were detected.
In 1947, Powell used the photographic emulsion technology invented by himself to find the meson predicted in the meson field theory proposed by Hideki Tomokawa in 1934 in cosmic rays.Yukawa Hideki won the Physics Prize in 1949, and Powell won the Physics Prize in 1950.
By the end of the 1950s, the number of elementary particles had reached 30.Most of these particles are found in cosmic rays.Since Fermi first found resonance particles in 1951, more than 300 resonance particles have been found by the 1980s.
All elementary particles are resonance states. The discovery of resonance states has actually revealed the secret of elementary particles, that is, all elementary particles are resonance states.There are two kinds of resonance states, one is unstable, such as hadrons;The other is stable, such as electrons, neutrons, etc.They are not prone to spontaneous decay.There is no absolutely stable elementary particle, for example, the electron will be destroyed under certain conditions (when it meets the positron).The external cause of producing elementary particles is the intersection of matter waves, where wave packets are formed.The internal cause is the resonance at the intersection, which is objectively manifested as the resonance state - the generation of basic particles.
Quark model
Quark model
There are so many basic particles. Are they really the most basic and indivisible?In the past 40 years, a large number of experimental facts have shown that at least hadrons have internal structures.In 1964, Gelman put forward the quark model, thinking that mesons are composed of quarks and anti quarks, and baryons are composed of three quarks.For this reason, he won the 1969 physics prize.In 1990, Friedman, Kendall and Taylor won the physics prize for their pioneering work in the development of quark models in particle physics.In 1965, Feynman, Schwenger and Chaoyong Zhenyilang won the Physics Prize for their contributions to the renormalization of quantum electrodynamics and computational methods, which had a profound impact on elementary particle physics.Based on the quark model, Weinberg and Salam completed the weak electricity unified theory describing electromagnetic interaction and weak interaction.They won the physics prize in 1979.The development of unified field theory is moving forward to the grand unified theory that unifies strong interactions and the super unified theory that unifies gravity.And this combination of the theory of small universe and the study of large universe is promoting the progress of cosmology.
Elementary particle quark model
Today, in order to unify the four fundamental forces in the universe, Gabriele Veneziano has created string theory. One of the basic ideas of string theory is that the basic units of nature are not particles such as electrons, photons, neutrinos and quarks.These things that look like particles are actually very small closed loops of strings (called closed chords or closed chords). Different vibrations and motions of closed strings produce different kinds of basic particles.It has become a very important theory for human beings to explore the mysteries of the universe
Particle type
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hadron
Hadron is the general name of all particles involved in strong action.They are composed of quarks. Six kinds of quarks have been found: top quark, top quark, bottom quark, strange quark, charm quark and bottom quark.The theory predicted the existence of the top quark, which was found in Fermilab on January 30, 2007.
Most of the existing particles arehadronProtons, neutrons, pions, etc. are all hadrons.In addition, antimatter has also been found. There is a famous anti quark, which has been found and is studying its utilization method. It is speculated that there may even be anti earth and anti universe.Strangely, some quarks are even heavier than protons, which remains to be studied.
Lepton
Elementary particle
Leptons are particles that only participate in weak force, electromagnetic force and gravitational force, but not in strong interaction.It is different from bosons and quarks.All known charged leptons can carry a positive charge or a negative charge, depending on whether they are particles or antiparticles.All neutrinos and their antiparticles are electrically neutral.There are six kinds of leptons, including electron, electron neutrino, muon, muon neutrino, tau and tau neutrino.
Electronics, muzi (miaozi)Tau son(Tao Zi, Heavy Lepton) three particles with a unit negative charge, respectively expressed as e -, μ -, τ -, and their corresponding three uncharged neutrinos, namely, electron neutrino, muon neutrino, tau neutrino, respectively expressed as ve, ν μ, ν τ.Add the antiparticle of each of the above six particles to make a total of 12 leptons.(All neutrinos are not charged, and all neutrinos have antiparticles).Tau is an important particle discovered in 1975. It does not participate in strong interaction and belongs to lepton. However, its mass is very heavy, 3600 times that of the electron and 1.8 times that of the proton, so it is also called heavy lepton.
Propagator
Propagators are also elementary particles.There are 8 kinds of gluons that transfer strong effects. They were found indirectly in the three jet phenomenon in 1979. They can form gluon balls. Because of the color confinement phenomenon, they cannot be directly observed up to now.Photons transfer electromagnetic interactions, while W+, W - and Z0, gluons transfer strong interactions.The heavy vector boson was discovered in 1983. It is very heavy, 80 to 90 times as heavy as the proton.
Fermion
The basic fermions are divided into two categories: quarks and leptons
Experiments show that there are six kinds of quarks, and their respective antiparticles.These six quarks can be divided into three generations.They are
First generation: u (upper quark) d (lower quark)
Second generation: s (strange quark) c (charm quark)
Third generation: b (bottom quark) t (top quark)
In addition, it is worth pointing out that the reason why they could not be discovered by early scientists is that quarks never exist alone (except for top quarks, which are too heavy and decay too fast to be produced by early experiments).They always form mesons in pairs, or three together form baryons such as protons and neutrons.This phenomenon is called quark confinement theory.This is why early scientists mistook mesons and baryons for elementary particles.
There are six kinds of leptons and their antiparticles.Three of them are electronic and similarμSubandτChild.Each of the three has a neutrino associated with it.They can also be divided into three generations:
First generation: e (electron) (electric neutrino)
Second generation:（μμ neutrino
Third generation:（τTau neutrino
Boson
Boson is a particle with integral spin according to bose einstein statistics.
This is a kind of particles that play a mediating role and transfer interactions between particles.The reason why they are called "gauge bosons" is that they are closely related to the Young Mills gauge field theory of elementary particles.
There are four kinds of interactions in nature, so gauge bosons can also be divided into four categories.
Gravitational interaction: graviton
Electromagnetic interaction: photon
Weak interaction (the interaction that makes atoms decay): W and Z bosons, there are three kinds.
Strong interaction (interaction between quarks): gluon
Particle physics has proved that electromagnetic interaction and weak interaction originate from the same kind of interaction when the energy is extremely high in the early universe, which is called "weak current interaction".Many particle physicists suspect that when the early universe was at a higher energy (Planck scale), it was likely that these four interactions were all unified. This theory is called the "grand unified theory".But because the energy that the accelerator can reach is still very low relative to the Planck scale, it is difficult to verify. The main development direction of the grand unified theory is the superstring theory.
Gluons are strongly interacting mesons, with colored and anti colored bands, and have never been observed by detectors because of color tightness.However, like individual quarks, they produce hadron jets.In the high-energy environment, the annihilation of electrons and positrons produces three jets: a quark, an anti quark and a gluon are the first Higgs particles to prove the existence of gluons.
Higgs particle physicists believe that the interaction between Higgs particle and other particles makes other particles have mass.The stronger the interaction, the greater the mass.The Higgs particle itself has a great mass, but the accelerator energy is still unable to reach, and the theoretical calculation is also difficult.Physicists discovered the Higgs particle in July 2012.
The Standard Model predicts the existence of a neutral Higgs particle: H.But many scientists have suggested other possibilities.
61 elementary particles:
1、 Leptons (12 species) {leptons mainly participate in weak interactions, charged leptons also participate in electromagnetic interactions, not in strong interactions.}
Quantum field theory is the theoretical system of the structure, interaction and motion transformation laws of elementary particles.According to quantum field theory, each type of particle is described by the corresponding quantum field. The interaction between particles is the coupling between these quantum fields, and this interaction is quantum transferred by the gauge field.
Since the 1930s, fundamental particle theory has made great progress on the basis of experiments.In terms of particle structure, people have gone deep into a level through the study of symmetry, confirmed that hadrons are composed of stratons and anti stratons, and also had a new understanding of vacuum, especially the spontaneous breaking of vacuum.In terms of interaction, we have developed quantum electrodynamics that can describe electromagnetic interaction, and a unified theory of weak electricity that can uniformly describe weak interaction and electromagnetic interaction, which can be used to describe quantum chromodynamics of strong interaction.All of them are quantum gauge field theories without exception, and they are consistent with experiments to a large extent, so that people have a deeper understanding of the laws of various interactions.
The elementary particle theory is a developing theory in essence, which is not satisfactory in many aspects.Among them, there are two theoretical issues with philosophical significance that need to be clarified, namely, the issue of hierarchical structure (see the level of material structure) and the issue of interaction and unity (see the unified theory of interaction).At the atomic level of the material structure, the electron in the atom can be separated from the nucleus;At the nuclear level, the protons and neutrons that make up the nucleus can also be separated from the nucleus.However, after entering the "elementary particle" level, the situation has changed.This change is that although hadrons are composed of stratons and anti stratons with "color", they cannot be separated from hadrons.This phenomenon is called "color" confinement.Therefore, at the level of "elementary particles", the concept of matter separability has added new content.Separability does not mean separability. Hadrons are composed of stratons and anti stratons, but stratons and anti stratons cannot be separated from hadrons.Up to now, the reason for the phenomenon of "color" confinement has not been found a clear answer in theory.
In the 1980s, 36 kinds of stratons and anti stratons were known, and 12 kinds of leptons and anti leptons were known. In addition, the number of gauge field particles and Higgs particles, as the transmitter of force, was large, which made people imagine the structure of these particles.Physicists have given many theoretical models for this, but there are great differences among them. It is difficult to verify and judge which model is correct by experiments.
model theory
After the success of the weak electricity unified theory, people have explored the unity between strong action, weak action and electromagnetic action, and put forward various grand unified model theories.This theory predicts that protons will also decay, with a lifetime of about 1032 ± 2 years.But it has not been confirmed experimentally.
When exploring the unified theory of force, we must consider gravity.But there are important differences between gravity and weak force, electromagnetic force and strong force, because it is directly related to the measurement of space and time. Its transmitter, the spin of the graviton, is different from the transmitter of the other three forces. Its coupling constant has the dimension ～ (mass) - 2, so there will be infinite kinds of divergence, which cannot be normalized.If we consider the nonlinear nature of the gravitational equation proposed by Albert Einstein, it will increase the difficulty of quantization and renormalization of the gravitational theory.
According to the preliminary discussion,gravitational fieldIt is also a gauge field, which means that gravity and the other three basic forces will eventually be logically unified.However, from the depth of the problem, we can see that there are some key factors that people have not yet mastered.
Particle Discovery Table
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The concept of elementary particles is also changing with the development of physics, and people's understanding is also deepening to reveal the deeper level of the micro world.
Close up of the 10 fermi nucleus
Three generations of "ancestors and grandchildren" of "elementary particles" have followedThomson From the discovery of electrons to the discovery of neutrons in 1932, people realized that protons, neutrons, electrons and photons can be called elementary particles.At that time, it was thought that everything was clear: protons and neutrons constituted all atomic nuclei;Nucleus and electrons constitute all atoms and molecules in nature, while photons are only the smallest units that constitute light and electromagnetic waves.However, the good times did not last long. Such a "perfect" explanation of the material structure did not last long. People soon found that the basic particles discovered at that time could not satisfactorily explain the nuclear force.
first generation
In 1935, the famous Japanese physicist Hideki Tomokawa (1907-1981) boldly assumed that there might still be new particles that have not been discovered.Yukawa Hideki believes that just as electromagnetic interaction is realized by exchanging photons, nuclear force is realized by exchanging a meson between nucleons.He also estimated that the mass of this particle is about 200 times that of the electron.
In 1937, American physicist Carl David Anderson (1905~) found a charged particle in cosmic rays. Its mass is about 200 times that of the electron, and it is named "m (muon) meson".The success of theoretical prediction makes people feel gratified, but further investigation is very disappointing.Because this m-meson does not interact with the nucleus at all, it is obvious that it cannot be the particle predicted by Hideki Yukawa.
In 1947, the Brazilian physicists Sese, M. G. Latis and others found another meson - p meson in cosmic rays by using nuclear latex.The properties of the p meson completely conform to the prediction of Hideki Tomokawa, and can explain the nuclear force.In fact, "m meson" is not a meson but a lepton, so it is called "m meson".By 1947, people had known as many as 14 kinds of particles.They include photons (g), positive and negative electrons (e ±), positive and negative m-particles (m ±), three kinds of p-mesons (p ±, p0), protons (p) and neutrons (n) discovered at that time;The other four are the positron and antiproton neutrinos, antiprotons and antineutrons found in the laboratory in 1956.These 14 kinds of particles have their own applications, in which protons, neutrons and electrons constitute all stable substances;Photon is the transmitter of electromagnetic force, p meson transmits nuclear force, and neutrino plays an indispensable role in b decay (b decay is the transformation of atomic nucleus spontaneously emitting electrons or positrons, or capturing an electron on the electronic orbit in the atom);And m-particles appear in cosmic rays.These constitute the first generation particles.
Second generation
The stable order did not seem to last long, and the old theory of "perfection" was soon broken by a series of new questions.In 1947, when p-mesons were discovered, people took two pictures with V-shaped tracks in the cloud chamber using cosmic rays. The decay products were p ± mesons and protons (p).These two kinds of tracks cannot be explained by any first generation particles found at that time, so people naturally thought that they must be formed by the decay of two undiscovered particles.In the following years, people took more than 100000 cosmic ray pictures and finally discovered these two new uncharged particles.One of them is 1000 times the mass of the electron and is called "k0 meson";The other is about 2200 times of the electronic mass, called the l particle (read "Lamberta").We call them the second generation particles because they have two obvious characteristics: (1) fast production and slow decay;(2) Paired (synergetic) production, single decay.These characteristics cannot be explained by past theories, so they are also called "strange particles".
In order to quantitatively study these strange particles, it is not enough to rely on cosmic rays alone.In the early 1950s, some large accelerators were built one after another, making it possible for people to use the particles accelerated by the accelerator to bombard the atomic nucleus to study exotic particles.
By 1964, people had discovered a batch of strange particles, making the number of particle types discovered reach 33.These strange particles are collectively called "second generation particles".
Third generation
If more than 30 particles found are classified according to their stability, some of them are stable, such as protons, electrons, etc;Some particles will spontaneously decay into other particles, such as m ±, p ±, π 0, k0, λ 0, etc.Their decay time is generally 10-20~10-16 seconds or more than 10-10 seconds, belonging to electromagnetic decay and weak action decay respectively.
In the 1960s, due to the gradual increase of accelerator energy and the rapid development of high-energy detectors, fast decaying particles with decay time in the range of 10-24~10-23 seconds were also found experimentally, and their decay is a strong decay.These particles are called "resonance particles", also called "third generation particles".Because of their appearance, the number of particles has soared to hundreds.
development history
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The branch of physics that studies the structure and properties of substances in the micro world deeper than the atomic nucleus, and the mutual transformation of these substances under high energy, as well as their causes and laws.Also called high-energy physics.Its development has roughly gone through three stages.
Phase I
Quantum Mechanics
(1897~1937) can be traced back to the discovery of the first elementary particle electron in 1897.In 1932, J. Chadwick discovered neutrons in the experiment of bombarding nuclei with alpha particles, and then people realized that atomic nuclei are composed of protons and neutrons, thus forming a unified world image that all substances are composed of basic structural units - protons, neutrons, and electrons.Proton, neutron, electron and A. Einstein were proposed by RA. Millikan and AH. The photon confirmed by Compton's experiment, the neutrino in W. Pauli's hypothesis (finally confirmed by experiment in 1956), and PA. M. Dirac prophesied by CD. The positrons observed by Anderson in cosmic rays in 1932 are considered as elementary particles or subatomic particles.At this stage, quantum mechanics was established in theory, which is the basic law generally followed by the movement of microscopic particles.
On the basis of relativistic quantum mechanics, the quantum field theory is initially established by quantizing the field, which solves the problems of the particle nature of the field and the generation and annihilation of the description particles.With the development of nuclear physics, it is found that in addition to gravitational interaction and electromagnetic interaction, there are also strong interactions stronger than electromagnetic interaction and weak interactions between electromagnetic interaction and gravitational interaction in the range equivalent to the size of atomic nucleus. The former is the nuclear force of nucleon combination and nucleation, and the latter causes the β decay of atomic nucleus.The study of nuclear force realizes that nuclear force is generated by exchanging mesons, and the concept of isospin is established according to the charge independence of nuclear force.
Phase II
(1937~1964) successively found many particles.In 1937, muons were discovered from cosmic rays, and later it was confirmed that they did not participate in strong interactions. They can be grouped into the same category as the accompanying muneutrinos, electrons, and electron neutrinos, collectively called leptons.In 1947, π± meson was discovered, in 1950, π 0 meson was discovered, and in 1947, strange particles were also discovered.In the 1950s, particle accelerators and various particle detectors made great progress, thus starting a new era of using accelerators to study and discover a large number of basic particles, and the antiparticle of various particles was confirmed;A number of resonances with very short lifetime have been found.A large number of fundamental particles have been found, most of which are hadrons. People doubt the basic nature of these fundamental particles.People try to classify hadrons and put forward the successful "eight fold method" of hadrons classification.
Further explanation of parity nonconservation
The most important theoretical progress at this stage is the establishment of renormalization theory and the study of symmetry in interaction.With respect to quantum electrodynamics, which describes the quantization of electromagnetic fields, the divergence difficulty is eliminated through the renormalization method. The theoretical calculation of the anomalous magnetic moments of electrons and muons and Lamb shifts are in exact agreement with the experimental results.Quantum electrodynamics has been tested by many experiments and has become a successful basic theory to describe electromagnetic interaction.
Symmetry is linked with the law of conservation. The most important result of the research on symmetry in interaction is that Li Zhengdao and Yang Zhenning proposed parity nonconservation under weak action in 1956, which was confirmed by Wu Jianxiong et al.'s experiments and other experiments in 1957. These experiments also proved that the charge conjugate parity nonconservation under weak action.These studies promote the development of weak action theory.
Phase III
(1964 ～) marked by the quark model of hadron structure.In 1964, M. Gelman and G. Zweick proposed that hadrons are composed of quarks based on the eight fold method of hadron classification. There are three kinds of quarks: upper quark u, lower quark d and strange quark s. Their charge and baryon number are fractions.The quark model can explain various hadrons discovered at that time.The quark model is supported by the deep inelastic scattering of protons and neutrons by high-energy electrons and neutrinos later. The experiments show that there are point structures in protons and neutrons, which can be considered as evidence for the existence of quarks.In 1974, J/ψ particles were discovered, and their unique properties must introduce a new charm quark c; in 1979, another unique new particle γ was discovered, and the fifth quark, called bottom quark b, must be introduced.On the other hand, heavy leptons τ were discovered in 1975, and there are signs that there are tau neutrinos associated with τ, so there are 6 kinds of leptons.Up to now, no internal structure of leptons has been found in experiments.It is believed that leptons are particles at the same level as quarks.The symmetry between leptons and quarks means that there is a sixth top quark t.In 1995, the D0 and CDF experimental groups of Fermi National Laboratory found evidence of the existence of top quarks respectively.
At this stage, the most important theoretical progress is the establishment of the electroweak unified theory and the progress of strong interaction research.1961 S50. Grashaw proposed that the foundation was Yang Zhenning and R50. The non Abelian gauge theory proposed by Mills in 1954.According to this model, photons are particles that transmit electromagnetic effects, and the particles that transmit weak effects are W ± and Z0 particles. However, whether W ± and Z0 have static masses and how to normalize them in theory have not been solved.From 1967 to 1968, on the basis of the spontaneous breaking of symmetry, S. Weinberger and A. Salam developed Grasho's electric weak unified model, established a perfect theory of electric weak unified, clarified that gauge field particles W ±, Z0 can have static mass, predicted their mass at 80~100 GeV, and also predicted the existence of weak neutral current.
Weak neutral current was observed in 1973, and W ±, Z0 particles were found in 1983. Their mass (mW ≈ 80GeV, mZ ≈ 91GeV) and characteristics are completely consistent with the theoretical expectations.In 1973, G. Hoft, D.J. Gross and others developed the theory of quantum chromodynamics on the study of strong interaction.Like quantum electrodynamics, quantum chromodynamics is also a localized gauge theory.In this theory, the strong interaction between quarks is caused by quarks having color charge exchange chromogluons, which have no static mass but have color charges.The strong interaction is asymptotically free, that is, the strong interaction between quarks does not weaken with the increase of their distance, but on the contrary;When they are close to each other and in the interior of hadrons, the interaction is weak and can be approximately regarded as free, which can explain the confinement properties of quarks and gluons, the abnormal phenomena of deep inelastic scattering of leptons on hadrons, and jet phenomena.
In the deep exploration of particle physics, particle accelerators, detection means, data recording and processing, and the application of computing technology are constantly developing, which not only bring about the progress of particle physics itself, but also promote the development of the entire science and technology;The fruitful results of particle physics have played an important role in the study of the evolution of the universe.
