Particle refers to the smallest component of matter that can exist in a free state. The first particles discovered were atoms, electrons and protons. In 1932, neutrons were discovered, confirming that atoms are composed of electrons, protons and neutrons. They are more basic material components than atoms, so they are called elementary particles. In the future, more and more such particles have been discovered, with the total number exceeding hundreds, and the number continues to increase. In addition, some of these particles have not been found to have an internal structure in experiments so far, and some experiments have shown that they have an obvious internal structure. . It seems that these particles do not belong to the same level, so the term elementary particles has become a thing of the past and is now collectively referred to as particles. Particles are not specific substances that actually exist like neutrons, protons, etc., but their collective name and a model concept.

For example:

Mesons

Physicists don’t know how to incorporate these particles into a complete physical theory. They try to explain the fundamental forces between these particles. Mode of action. In doing so, they imitated the electromagnetic force between photons carrying charged particles and wanted to use another type of force-carrying particles - mesons. But what are mesons made of?

Quark

For a period of time, the situation was extremely chaotic. But the development of quark theory in the 1960s and 1970s brought clarity to the situation. Quark theory holds that all known particles can be divided into two families. A family composed of quarks that can 'perceive' the strong force that only acts between quarks is called hadrons. Another group is called leptons. They cannot sense the strong force, but participate in interactions mediated by the so-called weak force (or weak interactions). For example, the process of radioactive decay (including β-decay) is caused by weak interactions. Hadrons can participate in strong interactions and sense weak forces.

Leptons

are truly elementary particles, they are not made of anything else. The typical lepton is the electron, and the electron is accompanied by another kind of lepton called a neutrino (strictly speaking, it should be an electron neutrino). When electrons participate in processes such as radioactive decay, neutrinos are always involved.

For reasons unknown to anyone, this basic image has been replicated twice, producing three 'generations' of leptons. In addition to the electron itself, there are also heavier particles called muons, which are exactly like electrons except that they are 207 times heavier than electrons; there is also an even heavier particle called tau particle, which has a mass close to twice that of a proton. Each of these two types of heavy electrons has its own neutrino, so there are six types (three pairs) of particles in the lepton family. Although both muons and tau particles can be created with energy in particle accelerators or from cosmic rays, they quickly decay and are converted into electrons or neutrinos.

The Hadron Tribe

The Hadron Tribe itself is further divided into two categories. Particles composed of three quarks are called baryons, which are what we often call 'matter' particles, including protons and neutrons (both baryons and leptons are members of the fermion family, and fermions are actually another name for ordinary matter particles) . Particles made of pairs of quarks are called mesons, and they are the particles that carry the fundamental force, although there are other mesons (the carriers of these forces and other mesons are also called bosons).

There are interactions between particles, including strong interaction, electromagnetic interaction, weak interaction and gravitational interaction. The gravitational interaction is very weak and can be ignored. Through these interactions, new particles are produced or particle transformation phenomena such as particle decay occur. Particles are divided into the following categories according to the properties involved in the interaction: ① Gauge particles. That is, the media particles that transmit interactions have been discovered to include photons that transmit electromagnetic effects and W and Z particles that transmit weak interactions. ②? Lepton. Particles that do not directly participate in the strong interaction but can directly participate in the electromagnetic interaction and the weak interaction. The ones that have been discovered include electrons, ? There are 12 kinds of particles and their antiparticles. ③ Hadron. Particles that directly participate in the strong interaction, as well as electromagnetic and weak interactions. Among them, hadrons with a spin of an integer are called mesons, and hadrons with a spin of a half-integer are called baryons.

There are a large number of hadrons, most of which are particles that decay through strong interactions. Their lifespan is extremely short and they are unstable particles, also called?*** vibration states.

Various particles have their own intrinsic properties, including the particle’s mass m (rest mass, expressed in energy), life span τ (average life span, referring to the average life span of the stationary system), and charge Q (expressed in terms of energy). The charge of the proton is in units), spin J (in units), parity P, isospin I, isospin third component I3, baryon number B, lepton number Le,, Lr, singular number S, ? Charming number C, base number d and so on.

With the accuracy of existing experiments, leptons behave like point particles and do not show an internal structure, while hadrons show that they are composite particles with a certain structure. According to the perspective of modern particle physics, mesons are composed of a pair of positive and negative quarks, baryons are composed of three quarks, and leptons and quarks belong to the same level.