# 20.1: Fundamental Particles

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Who says scientists don’t have a sense of humor? Look at the odd names and images in this chart. Do you have any idea what they represent? Would it surprise you to learn that they represent the most fundamental particles of matter? In fact, particles with funny names like charm quarks and strange quarks make up all the matter in the universe.

## The Search for Fundamental Particles

Scientists have long wanted to find the most basic building blocks of the universe. They asked, “what are the fundamental particles of matter that cannot be subdivided into smaller, simpler particles,” and “what holds these particles together?” The quest for fundamental particles began thousands of years ago. Scientists thought they had finally found them when John Dalton discovered the atom in 1803 (see the timeline in Table below). The word atom means “indivisible,” and Dalton thought that the atom could not be divided into smaller, simpler particles.

Timeline of Discovery of Fundamental Particles
Year Discovery
1803

John Dalton discovers the atom. 1897

J.J. Thomson discovers the electron, the first lepton to be discovered. 1905

Albert Einstein discovers the photon, the first boson to be discovered. 1911

Ernest Rutherford discovers the proton, the first particle to be discovered in the nucleus of the atom. 1932

James Chadwick discovers the neutron, another particle in the nucleus. 1964

Murray Gell-Mann proposes the existence of quarks, the fundamental particles that make up protons and neutrons. 1964-present Through the research of many scientists, many other fundamental particles (except gravitons) are shown to exist.

For almost 100 years after Dalton discovered atoms, they were accepted as the fundamental particles of matter. But starting in the late 1890s with the discovery of electrons, particles smaller and simpler than atoms were identified. Within a few decades, protons and neutrons were also discovered. Ultimately, hundreds of subatomic particles were found.

## Leptons and Quarks and Bosons, Oh My!

Today, scientists think that electrons truly are fundamental particles that cannot be broken down into smaller, simpler particles. They are a type of fundamental particles called leptons. Protons and neutrons, on the other hand, are no longer thought to be fundamental particles. Instead, they are now thought to consist of smaller, simpler particles of matter called quarks. Scientists theorize that leptons and quarks are held together by yet another type of fundamental particles called bosons. All three types of fundamental particles—leptons, quarks, and bosons—are described below. The following Figure below shows the variety of particles of each type.

• There are six types of quarks. In ordinary matter, virtually all quarks are of the types called up and down quarks. All quarks have mass, and they have an electric charge of either +2/3 or -1/3. For example, up quarks have a charge of +2/3, and down quarks have a charge of -1/3. Quarks also have a different type of charge, called color charge, although it has nothing to do with the colors that we see. Quarks are never found alone but instead always occur in groups of two or three quarks.
• There are also six types of leptons, including electrons. Leptons have an electric charge of either -1 or 0. Electrons, for example, have a charge of -1. Leptons have mass, although the mass of electrons is extremely small.
• There are four known types of bosons, which are force-carrying particles. Each of these bosons carries a different fundamental force between interacting particles. In addition, there is a particle which may exist, called the "Higgs Boson", which gives objects the masses they have. Some types of bosons have mass; others are massless. Bosons have an electric charge of +1, -1, or 0.

Q: Protons consist of three quarks: two up quarks and one down quark. Neutrons also consist of three quarks: two down quarks and one up quark. Based on this information, what is the total electric charge of a proton? Of a ne utron?

A: These combinations of quarks give protons a total electric charge of +1 (2/3 + 2/3 – 1/3 = 1) and neutrons a total electric charge of 0 (2/3 – 1/3 – 1/3 = 0).

## Force-Carrying Particles

The interactions of matter particles are subject to four fundamental forces: gravity, electromagnetic force, weak nuclear force, and strong nuclear force. All of these forces are thought to be transmitted by bosons, the force-carrying fundamental particles. The different types of bosons and the forces they carry are shown in Table below. Consider the examples of gluons, the bosons that carry the strong nuclear force. A continuous exchange of gluons between quarks binds them together in both protons and neutrons. Note that force-carrying particles for gravity (gravitons) have not yet been found.

Bosons and Fundamental Forces
Type of Bosons Fundamental Force They Carry Particles They Affect Distance over Which They Carry Force
Gluons strong nuclear force quarks only within the nucleus

W bosons

Z bosons

weak nuclear force leptons and quarks only within the nucleus
Photons electromagnetic force leptons and quarks all distances
Gravitons (hypothetical) force of gravity leptons and quarks all distances

Q: Which type of boson carries force between the negative electrons and positive protons of an atom?

A: Photons carry electromagnetic force. They are responsible for the force of attraction or repulsion between all electrically charged matter, including the force of attraction between negative electrons and positive protons in an atom.

Q: Gravitons have not yet been discovered so they have only been hypothesized to exist. What evidence do you think leads scientists to think that these hypothetical particles affect both leptons and quarks and that they carry force over all distances?

A: Gravity is known to affect all matter that has mass, and both quarks and leptons have mass. Gravity is also known to work over long as well as short distances. For example, Earth’s gravity keeps you firmly planted on the ground and also keeps the moon orbiting around the planet.

## The Standard Model

Based on their knowledge of subatomic particles, scientists have developed a theory called the standard model to explain all the matter in the universe and how it is held together. The model includes only the fundamental particles in the Table above. No other particles are needed to explain all kinds of matter. According to the model, all known matter consists of quarks and leptons that interact by exchanging bosons, which transmit fundamental forces. The standard model is a good theory because all of its predictions have been verified by experimental data. However, the model doesn’t explain everything, including the force of gravity and why matter has mass. Scientists continue to search for evidence that will allow them to explain these aspects of force and matter as well.

Launch the simulation below to learn more about the variety of fundamental particles discovered around the world over the last century. The simulation is presented like a zoo map with three major areas: the lepton safari, the boson pond, and the hadron forest. Have fun exploring the Particle Zoo:

Interactive Element

## Summary

• For centuries, scientists searched for the fundamental particles of matter and the “glue” that holds them together. At first, scientists thought that atoms were the fundamental particles. Now they know that there are smaller, simpler particles than atoms that make up matter and carry the forces that hold matter together.
• Protons and neutrons are made up of fundamental particles of matter called quarks. Electrons are another type of fundamental particles of matter called leptons. Bosons are fundamental particles that carry forces between fundamental particles of matter.
• Scientists think that a different type of boson carries each of the four fundamental forces in the universe (strong and weak nuclear forces, electromagnetic force, and gravity).
• The standard model is a simple theory that explains all the matter in the universe and its interactions (except for the mass of matter and the force of gravity). According to the standard model, all known matter consists of quarks and leptons, which interact by exchanging force-carrying particles called bosons.

## Review

1. Outline the order in which fundamental particles were discovered.
2. Make a table comparing and contrasting the three types of fundamental particles. Include an example of each type in your table.
3. Make a two- or three-dimensional model of a hydrogen atom (1 proton and 1 electron) that represents all of its fundamental particles, including force-carrying particles.
4. What is the standard model? In what ways is it incomplete?