Classification of Elementary Particles. Quarks and Charm Revision Notes

In these revision notes for Classification of Elementary Particles. Quarks and Charm, we cover the following key points:

• How many types of interactions are there? Which are they?
• What are the main features of various types of interaction?
• How the elementary particles are classified?
• What are quarks? How many types of quarks are there?
• What is the elementary charge of each type of quarks? What about their spin, baryon number, strangeness, etc.?
• How many generations of quarks are there? Which quarks do belong to each generation?
• What are quark antiparticles? What are their corresponding charges?
• What are quark mediators? How do we express them?
• What is charm?
• What is the standard model? What are its main features?
• Why symmetry is so important in standard model?
• What scientists are trying to achieve in respect to various types of interactions? What are the missing steps in this regard?

Classification of Elementary Particles. Quarks and Charm Revision Notes

There are four types of interaction occurring in the universe:

1. Strong interaction. This is an interaction experienced by all elementary particles except leptons, photons and gravitons.
2. Electromagnetic interaction. This is an interaction experienced by all electrically charged elementary particles.
3. Weak interaction. It is present in all types of beta decay, in many elementary particles decay processes, as well as in all processes involving neutrino interaction with matter.
4. Gravitational interaction. This is an interaction experienced by all particles without exception, often neglected when dealing with elementary particles, as it is very weak at this level.

Based on the type of interaction they experience, elementary particles are classified in three major groups:

1. Photons (quants) γ, which are the carriers of EM interaction;
2. Leptons, a group composed by muons, electrons, the two types of neutrino and their corresponding antiparticles. All these elementary particles experience the weak interaction during their lifetime; and
3. Hadrons, responsible for the strong interaction. All particles that experience strong interaction such as protons, neutrons, hyperons and their corresponding antiparticles, mesons with or without strangeness as well as a number of resonances (elementary particles with a very short lifespan) are included in hadrons group. The two major subgroups of hadrons are mesons and baryons.

A quark is one of the fundamental particles in physics. Quarks join to form hadrons, such as protons and neutrons, which are components of the nuclei of atoms. They interact between them mainly through the strong force. Quarks exhibit the confinement property, i.e. a specific quark cannot exist separately but only in pair with another quark. A quark has an electric charge equal to 1/3 or 2/3 of the charge of electron, which was previously thought to be the smallest unit of electric charge. Likewise, quarks have a non-whole baryon number.

There are three generations of quarks, based on pairs of weak positive/negative, weak isospin. The first generation includes the up and down quarks, the second-generation includes strange, and charm quarks, and the third generation includes the top and bottom quarks.

Experiments show that quarks and antiquarks interact with each other by exchanging virtual particles called gluons. They are particles that have the spin equal to 1 and behave in a similar way to photons.

Since quarks have the spin 1/2, they belong to the fermions class that are subject to the exclusivity principle. To have a better understanding of quark types, scientists have classified them in "colours". They have proposed three colors to identify various types of quarks: red, green and blue. The exclusivity principle is applied for each color separately.

By analyzing the quarks behavior during various decay processes, made scientists think that there must exists another quark. They called it charm, C, which means "fascinating". It has the following values for each type of charge: q = 2/3, B = 1/3, S = 0 and C = +1.

The actual frame of elementary particles is temporary as there are many mysteries of micro-world still undisclosed. The actual framework of elementary particles is known as the "standard model" of structure of matter. This model includes three main families of elementary particles:

• Six leptons not related to the strong interaction;
• Six quarks from which all hadrons are made of;
• Particles that are mediators of various interactions.

Symmetry is a very important property of elementary particles that has helped a lot in the progress made in this field of science in the sense that particles not discovered yet have been predicted just by looking the symmetry of the corresponding schemes. The 8-vertices symmetry is often used to describe the structure of elementary particles.

Scientists have always sought to merge all types of interactions in a single theory known as the theory of universal interaction. Despite many attempts made in this direction, this is still a dream.