Classification of Particles

Over time, more and more particles were discovered, with electrons, protons, neutrinos, and neutrons, the basic blocks that make up the universe, becoming part of a larger family of particles.

How many particles are there?

At the moment, the standard model features seventeen elemental particles. It includes particles like quarks, which make up matter, and photons, which are responsible for the energy exchanges in the matter.

Figure 1. Installation of the Large Hadron Collider, which is used for research on elementary particles. Source: Kars Alfrink, Flickr (CC BY 2.0).

What is the classification of elemental particles?

The classification of elemental particles divides them by their properties or interactions. As the particle number increased, a more general classification was needed, and so the particles were divided into three large families, leptons, hadrons, and field carriers (bosons). The three families have distinct characteristics and are subject to different forces, having different roles in the universe.

Particle classification chart

The particles can be classified as follows:

Figure 2. Classification of elemental particles with examples of the members of each family, some of which overlap. Source: Manuel R. Camacho, StudySmarter.

Quarks

Quarks are the particles that make up the hadrons. Quarks are responsible for giving hadrons their charge. Examples include the bottom quark and the down quark, which make up protons and neutrons. Each quark has its fundamental charge. Quarks, electrons, and neutrinos also form another sub-family known as fermions.

Quarks and hadrons

If you add up the charge of all quarks, you get the total fundamental charge of the particle. If you then add the baryon number, you obtain the sign number that tells you if the particle is matter or antimatter. See the following table showing the quark charges.

We can see that if we have two up quarks and one down quark, we have a proton with a charge of 1. If we have one up quark and two down quarks, we have a neutron with a charge of 0.

An antiproton combines two anti-up and one anti-down quark. An antiproton has the same mass as a proton, but its charge is negative.

Strange particles are additional particles that are formed when high-energy beams collide with atoms. Strange particles disintegrate more slowly than other quarks. They possess a quality that has been named the strange number.

Hadrons

Hadrons are particles that compose most of the matter’s mass. The neutron is an example of a hadron. Hadrons have the following key characteristics:

• Hadrons are held together by a force known as the strong nuclear force, which glues together all particles that make up the nucleus.
• Hadrons are composed of particles called quarks.
• Quarks carry the electric charge of the hadrons.

The proton is a hadron that consists of three quarks, each of which has a charge. Two of the quarks have a charge of ⅔ e, while the third one has a charge of -⅓. The sum of the charges is 2 (⅔) +1 (-⅓), with the total or elementary charge of the proton being +1.

Hadrons can be classified into two categories:

• Mesons, which consist of an even number of quarks such as protons and neutrons.
• Baryons, which consist of an odd number of quarks like the muon.

Antiquarks and antimatter, such as positrons (positive electrons), antiprotons, and antineutrons, belong to the same families as their counterparts (quarks, electrons, protons, and neutrons). The key difference between them and normal matter is that some of their characteristics, such as the electrical charge, are the opposite.

Leptons

Leptons are elemental particles that cannot split themselves into smaller particles. They are not affected by the strong nuclear force that keeps protons and neutrons together in the nucleus. Leptons have a charge like hadrons, they are affected by the weak nuclear force, and they can be classified into charged particles and neutral particles.

• Charged: they possess and carry electrical charges like the electron, muon, and tau.
• Neutral: their charge is zero like that of neutrons.

Field particles or force carriers

The model of the universe consists of particles that make up matter and four elemental forces. These forces are gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. They are also known as bosons.

• Gravity: the graviton, a proposed particle, has not yet been found.
• Electromagnetic force: the photon is the particle responsible for carrying the electromagnetic energy in waves and for interacting with the atoms exchanging energy.
• Strong nuclear force: the force responsible for keeping the centre of the atoms together. The particle carrying this force is the gluon.
• Weak nuclear force: the force responsible for the disintegration of atoms and radiation. The particles carrying this force are the W and Z bosons.

The Higgs boson

The Higgs boson has been one of the last particles to have been discovered. Proposed by Peter Higgs and discovered in the LHC (Large Hadron Collider), it provides information about how particles obtain their mass.