< h1 class="pgc-h-arrow-right" > introduction</h1>
In the continuous exploration of the world by human beings, people's cognition of the world from macroscopic to microscopic, slowly understand that matter is composed of atoms, atoms are composed of atomic nuclei and electrons moving around the nucleus, and the nucleus is a complex structure of the whole composed of protons and neutrons, step by step to peel away the cocoon, scientists have found that neutrons, protons and other baryons still have internal structure, they are composed of a variety of different quarks. This constitutes the face of particle physics today. What exactly are elementary particles? What are their classifications and compositions?
< h1 class="pgc-h-arrow-right" > what elementary particles are</h1>
Elementary particles are the smallest or/and most basic units of matter that people perceive, and are the basis for the composition of various objects. That is, the smallest volume of matter without changing the properties of the substance. It is the basis for the composition of all kinds of objects, and it is not concluded that it is not a substance because it is small.
As early as the ancient Greeks and the Spring and Autumn Warring States period, some people have proposed the concept of the atom, Keiko once said: "The smallest is inside, called the small one", meaning that the smallest matter is inseparable. This smallest unit, democritus called it an "atom," proposed atomic materialism, which holds that everything in the world is made of atoms. But for more than 2,000 years, no one explored in depth what atoms or "smallest units" were, so the concept of "atoms" initially belonged to the philosophical category.
The Atom of Democritus
In 1789, the French scientist Lavoisier defined the term atom, and since then, atoms have been used to denote the smallest units in chemical variation. Initially scientists thought that atoms were already the smallest units of matter and could no longer be separated. But in 1897, Joseph John Thomson determined that the particles in the cathode rays were negatively charged according to the trajectory of the cathode rays in the discharge tube under the action of electromagnetic and magnetic fields, measured their charge-to-mass ratio, discovered the electron and its subatomic properties, and shattered the idea that atoms were indivisible.
Later Thomson also proposed the famous raisin cake model to depict the internal structure of atoms:
An atom is a small sphere filled with a uniformly distributed, positively charged fluid. There are also several electrons in the sphere, all of which are in this positively charged liquid, just as many corks are immersed in a basin of water, and these electrons are arranged in equal intervals on the circumference concentric with the positive sphere, and at a certain speed they make a circular motion to emit electromagnetic radiation, and the atomic spectrum reflects the radiation frequency of these electrons. Since the sum of the negative charges carried by the electrons is equal to the sum of the positive charges carried by the electric liquid, but the signs are opposite, the atoms appear neutral from the outside. In this model of atoms proposed by Thomson, electrons are embedded in a positively charged liquid, like raisins dotted in a piece of cake.
In 1914, Rutherford bombarded hydrogen with cathode rays, resulting in the electrons of the hydrogen atom being knocked out and turned into a positively charged cation, which is actually the nucleus of hydrogen. Rutherford speculated that it was the anode ray that people had previously discovered as opposed to cathode rays, and its charge was one unit and its mass was also a unit, which Rutherford named protons. In 1932, Chadwick discovered neutrons in an experiment bombarding nuclei with α particles. It was then recognized that the nucleus of an atom was made of protons and neutrons, resulting in a unified image of a world in which all matter is made up of basic structural units—protons, neutrons, and electrons.
With the discovery of few "particles" such as protons, neutrons, electrons, μ, etc., these particles are considered to be the smallest units that make up matter, calling them "elementary particles" (meaning indivisible).
Particles have dual properties, which is what we call wave-particle duality, and in the long-term exploration process, scientists classify particles by properties such as mass, size, lifetime, symmetry, spin, conservation and so on.
What is symmetry? In the 1920s, Dirac proposed the famous Dirac theory, and predicted that every fundamental Fermi particle in the universe must have its corresponding antiparticle, both with the same mass but carrying the opposite charge. In 1932, Anderson discovered that the positron (that is, the antiparticle of the electron), he filled the cloud chamber with oversaturated ether gas, when the matter emitted positrons, the positrons through the cloud chamber, in the positron orbit appeared droplet line, through the applied magnetic field to measure the deflection direction and radius of the positron can know its charged sign, and charge-mass ratio (ratio of charge to mass), thus confirming Dirac's prediction.
The antiparticle is the same as the corresponding particle in mass, spin, average lifetime, and magnetic moment, if charged. The two carry equal amounts of electricity and sign the opposite. The orientation relationship between magnetic moment and spin is also opposite. When an antiparticle meets the corresponding particle, it annihilates and transforms into another particle.
What is spin? In 1925, G.E. Ulenbeck and S.A. Guzmitt were inspired by the Pauli incompatibility principle to analyze some experimental results of atomic spectra and proposed that electrons have intrinsic motion- spin, and have spin magnetic moments associated with electron spins. The spin of a particle is caused by the intrinsic angular momentum (the displacement of the object to the origin and the physical quantity associated with the momentum), the particle spin is its intrinsic property, a kind of angular momentum inherent in the particle, and its magnitude is quantized and cannot be changed (but the direction of the spin angle momentum can be changed by operation).
What is conservation? Matter is constantly moving and changing, and there are some things that do not change in change, which is conservation. Particles are created and decayed according to the law of conservation of energy. In addition, there are other conservation laws, such as conservation of mass, conservation of momentum, conservation of angular momentum, and conservation of discontinuities in microscopic phenomena, conservation of electric charge, as well as conservation of baryon numbers, conservation of lepton numbers, conservation of singular numbers, conservation of isotopic rotation, etc.
<h1 class="pgc-h-arrow-right" > classification of elementary particles</h1>
There are currently 61 species of elementary particles, divided into two categories, namely fermions and bosons, with semi-odd spins (1/2, 3/2...). Particles with a spin of 0 or positive integers are called fermions, which obey the Fermi-Dirac statistic; particles with a spin of 0 or positive integers are called bosons, which obey boson-Einstein statistics.
The basic fermion fee is the particles that make up matter, divided into 2 classes: quarks and leptons. And these 2 types of basic fermions are divided into a total of 24 flavors (taste refers to the type of elementary particles):
12 quarks: Includes upper quark (u), lower quark (d), odd quark (s), cannica quark (c), bottom quark (b), top quark (t), and their corresponding 6 antiparticles. It should be pointed out that there are 3 colors of quarks, such as red upper quark, blue upper quark, and green upper quark, so the subdivision of quarks is 36 kinds.
12 leptons: including electrons (e), muons (μ), potters (τ), neutrinos νe, neutrinos νμ, neutrinos ντ, and corresponding 6 antiparticles, including 3 antineutrines.
Bosons, on the other hand, are divided into elementary particles and composite particles. The basic boson can be divided into four major interactions according to physics:
Gravitational interactions: gravitons
Electromagnetic interaction: photons
Weak interaction (the interaction that causes atoms to decay): W and Z bosons, W bosons have two types, +1 (W+) and −1 (W−) unit charges, respectively. W+ is the antiparticle of W−. The Z boson (Z0), on the other hand, is electrically neutral and is an antiparticle of itself. All three particles are very short-lived, with a half-life of about 3 seconds.
Strong interaction (interaction between quarks): gluons
Their function is to transfer fundamental interactions, and in addition to these four, they also include Higgs particles that provide mass to other elementary particles, and the interaction of Higgs particles with other particles gives other particles mass. The stronger the interaction, the greater the mass.
Antiparticles are not marked on the figure
The composite boson is composed of an even number of fermions, and the common ones are mesons, deuterium nuclei, helium-4 and so on.
PS：
1, hadrons do not belong to elementary particles, hadrons are a kind of subatomic particles, including baryons and mesons, protons, neutrons, superons belong to the baryon class, baryons are composed of three quarks or three antiquarks, their spin is always half.
2. What does it mean that fermion constituent substances and boson transport function? For example, a gluon is a boson responsible for transmitting a strong nuclear force. They bind quarks together to form protons, neutrons, and other hadrons.
Photons are media particles that transmit electromagnetic interactions. Charged particles interact by emitting or absorbing photons, and pairs of positive and negative charged particles can be annihilated into photons, and they can also be produced in the electromagnetic field.
< h1 class="pgc-h-arrow-right" > the research progress and significance of elementary particles</h1>
Currently, the graviton is currently a hypothetical particle in physics that transmits gravity. The presence of free quarks and gluons has also not been found in experiments. In both quarks and gluons have "color" quantum numbers, one guess is that colored quarks and gluons are like being imprisoned in a "cage" of overall colorlessness, a phenomenon called "color confinement" (color confinement). The explanation of "color confinement" has various theoretical evidence, but it is still at the forefront of research.
The study of elementary particles has led to the birth of many disciplines, such as quantum chromodynamics, and in 1973, American scientists Gross, Pollitz, and Wieldsk explained through a perfect mathematical model that the closer the quarks are, the weaker the strong force. When the quarks are very close to each other, the strong force is so weak that they can fully operate as free particles. This phenomenon is called "asymptotic freedom." Conversely, the greater the distance between the quarks, the stronger the force. The discovery of "asymptotic freedom" led to the birth of a completely new theory, quantum chromodynamics (QCD).
Many scientists believe that it is likely that all four interactions were unified at higher energies (planck scales) in the earlier universe, a theory known as the "Grand Unification Theory." At present, scientists have constructed a Standard Model on the unified theory of weak electricity proposed by Glashow to describe the three basic forces of strong, weak and electromagnetic forces and the elementary particles that make up all matter, and scientists have paired fermions with bosons to describe the forces between fermions.
The Standard Model was considered the most likely to achieve a great unification of physics, but after the quark theory was proposed, it was recognized that elementary particles also have complex structures, and now elementary particles have become historical terms, and people have replaced them with "particles".
As scientists deepen their research on particles, human beings will slowly unveil the mysteries of the universe and understand the ultimate mystery of the universe.