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What is a Quark? Spin, Colors, and Flavors

Joseph Comunale, Sergey Segal
  • Author
    Joseph Comunale

    Joseph Comunale obtained a Bachelor's in Philosophy from UCF before becoming a high school science teacher for five years. He has taught Earth-Space Science and Integrated Science at a Title 1 School in Florida and has Professional Teacher's Certification for Earth-Space Science.

  • Instructor
    Sergey Segal

    Sergey has a Masters in Biomedical Engineering and has taught science and mathematics courses at the University of Wisconsin-Madison.

Learn the definition of quark in the Standard Model, and understand the concept of quark flavor and color. Explore the idea of quark matter in modern physics. Updated: 12/24/2021

What is a Quark?

When the Ancient Greek philosopher and atomist, Democritus first hypothesized the existence of the atom, he did so by performing a thought experiment. That is, by imagining a set of hypothetical circumstances and following a line of reasoning to see what conclusions follow. Democritus' thought experiment was imagining taking a thing, such as a rock, or a piece of paper, and cutting it in half. Next, one of those halves is subsequently cut in half also. Then a further halving. Imagine this continues and no matter how small the pieces of the thing get, imagine the knife used to cut is infinitely sharp can continue cutting. Democritus concluded that there must be a point at which the thing cannot be cut into smaller pieces; he imagined that there must be a fundamental piece of matter that itself could not be reduced to anything smaller. Democritus called this fundamental piece or particle atomos which means indivisible. This is the origin of the word atom.

However, when modern science began to take closer looks at the atom, it was discovered that it was not indivisible and that there were particles that make it up, i.e., subatomic particles. First, the electron was discovered, then the nucleus and proton, and then the neutron.

At the advent of using large hadron colliders which accelerate hadron particles such as protons and neutrons to speeds close to the speed of light and collide them, they too are broken up into smaller particles which are now called quarks. But exactly what is a quark? And is the quark the fundamental unit of matter like the atom was supposed to be?

Hadron colliders are used to break hadrons up into their component quarks.

This illustration shows a hadron collider which is used to collide hadrons.

The simple quark definition or quark meaning is a tiny particle that is among the most fundamental units of matter. Quarks have not been found to be composed of any further smaller particles or units of matter, which is what makes them fundamental. It is possible that quarks do not have geometry or shape and are as small as things get; infinitely small. What defines quarks as particles or at least particle-like is that they can follow paths through space. There are six types of quarks which are referred to as different flavors:

  • Up quarks
  • Down quarks
  • Strange quarks
  • Charmed quarks
  • Bottom quarks
  • Top quarks

Quark Model

This diagram outlines all the elementary particles in the standard model of physics.

This standard model or quark model shows what is a quark when compared to other elementary particles.

Quarks are a part of the Standard Model of particle physics which is a theory that describes and explains three out of four of the fundamental forces of the universe: electromagnetic, and strong and weak interactions or nuclear force. Gravity is a fourth fundamental force of the universe, but the Standard Model has not been able to explain gravity through its terms. Quarks interact with one another through the strong interaction or strong nuclear force. This is the force that binds quarks together to form hadron particles baryons and mesons; baryons being composed of at least three quarks such as protons and neutrons, and mesons being composed of usually one quark and one antiquark.

This diagram shows the quark flavors that make up a proton. Notice the gluons that bind them together.

This diagram shows the quark flavors that make up a proton.

Another portion of the standard model is quantum chromodynamics (QCD) which explains the strong interaction between quarks. A force particle that transfers the strong interaction between the quarks is called a gluon. Gluons act as a wave function between quarks which binds the quarks together and is analogous to an attraction between differing charges.

Quarks Defined

Imagine taking a block of butter and cutting it in half. After doing that, you take one of the resulting pieces and cut that one in half, and then do it again for the smaller pieces that follow, over and over. For the purposes of this thought experiment, your knife is infinitely thin, and you can keep going for an indefinitely long time. Initially, you will see smaller and smaller chunks of butter, until they become invisible to the naked eye. As you keep going, you will get down to the level of molecules, individual atoms, and finally subatomic particles.

You may be familiar with protons, neutrons, and electrons, which are the basic constituents of atoms. The protons and neutrons make up the nucleus of an atom, while the electrons orbit around this nucleus. However, even protons and neutrons are made up of more elementary particles, called quarks, which you can think of as tiny entities that are among the most basic constituents of all matter.

Let's proceed to take a closer look at these particles.

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Flavors of Quarks

The following table outlines each of the different flavors of quarks and their properties, including electric charge, mass, and spin. Mass is measured in electron volts (eV), megaelectron volts (MeV), or Gigaelectron volts (GeV).

Generation Quarks Mass in Electron Volts (eV/c^2) Spin Charge
First Up 1.7-3.3 MeV/c^2 1/2 +2/3
First Down 4.1-5.8 MeV/c^2 1/2 -1/3
Second Strange 101 MeV/c^2 1/2 -1/3
Second Charm 1270 MeV/c^2 1/2 +2/3
Third Bottom 4.19-4.67 GeV/c^2 1/2 -1/3
Third Top (most massive) 172 GeV/c^2 1/2 +2/3

Looking at the table, spin describes a property of subatomic particles that is analogous to having angular momentum or spinning about an axis. However, particles like quarks do not have a well-defined axis because of a lack of a defined volume. What spin really tells quantum physicists is how the particle may appear from different directions, and is therefore analogous to shape. Because there is no well-defined axis of these shapes, the particle's spin can be thought of as an oscillation.

  • Particles with 0 integer spin are like a dot, which looks the same from every direction.
  • Particles with 1 integer spin are like a spade, which looks different from different sides and which has to be rotated 360 degrees in order to be looking at the same side.
  • Particles with 2 integer spin are like a queen of diamonds, which looks the same when rotated 180 degrees.
  • Particles with half-integer spin-like quarks have to be rotated 720 degrees in order to be looking at the same side.

Obviously, this doesn't make a lot of sense. But, the universe at the scale of the quantum world is under no obligation to make sense in regards to using a language that has evolved only describing things in the macro universe, or at the scale that humans live. An analogy and demonstration of a complex three-dimensional shape might help understand how something can be spun around 360 degrees and not be back at its starting position. A person holding a glass of water with the base of the cup sitting in the palm of their outstretched hand, with their elbow bent and pointing down can be thought of as a shape. Specifically, the arm and hand holding the cup is the shape or particle for this example. If the person was required to spin the cup around without spilling the water (therefore keeping it vertical), they would have to spin the cup around 720 degrees, by rotating their hand and wrist, raising their elbow up, and rotating their hand, wrist, and the cup under their raised elbow before bringing everything back to the start. It's easy to try this at home. Drawing a dot on the cup helps keep track and recognize that the cup must rotate 720 degrees in order for it and the person's arm to return back to the starting position.

Quarks: A Closer Look

There are six types, also known as flavors, of quarks: up, down, strange, charm, bottom, and top. These flavors differ based on mass and electric charge. By the way, the electric charge property I just mentioned is the same one that exists in electric circuits or that causes a hair brush to attract small strips of paper after you brush your hair with it. The 'up', 'charm', and 'top' quarks have a positive charge of +2/3, while the 'down', 'strange', and 'bottom' quarks have a negative charge of -1/3. Don't worry about the numbers, but remember that opposite charges (positive-negative) attract, while like charges (positive-positive or negative-negative) repel each other.

The masses of quarks, from heaviest to lightest, are as follows: top (heaviest), bottom, charm, strange, down, and up (lightest). Keep in mind that since we are talking about elementary particles, you can think of their masses as just another property that characterizes them. After all, you can't really weigh quarks on your scale at home.

Another property of quarks is color, also referred to as color charge. In order to satisfy the Pauli Exclusion Principle, which asserts that no two quarks can exist in the same quantum state at the same time, the property of color was ascribed to quarks. Three colors could be assigned to individual quarks, and a common color scheme is red, green, and blue.

For the purposes of our discussion, it's important to mention that every quark has an antimatter particle associated with it, called an antiquark. You can think of antiquarks as quarks with opposite electric and color charges.

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Video Transcript

Quarks Defined

Imagine taking a block of butter and cutting it in half. After doing that, you take one of the resulting pieces and cut that one in half, and then do it again for the smaller pieces that follow, over and over. For the purposes of this thought experiment, your knife is infinitely thin, and you can keep going for an indefinitely long time. Initially, you will see smaller and smaller chunks of butter, until they become invisible to the naked eye. As you keep going, you will get down to the level of molecules, individual atoms, and finally subatomic particles.

You may be familiar with protons, neutrons, and electrons, which are the basic constituents of atoms. The protons and neutrons make up the nucleus of an atom, while the electrons orbit around this nucleus. However, even protons and neutrons are made up of more elementary particles, called quarks, which you can think of as tiny entities that are among the most basic constituents of all matter.

Let's proceed to take a closer look at these particles.

Quarks: A Closer Look

There are six types, also known as flavors, of quarks: up, down, strange, charm, bottom, and top. These flavors differ based on mass and electric charge. By the way, the electric charge property I just mentioned is the same one that exists in electric circuits or that causes a hair brush to attract small strips of paper after you brush your hair with it. The 'up', 'charm', and 'top' quarks have a positive charge of +2/3, while the 'down', 'strange', and 'bottom' quarks have a negative charge of -1/3. Don't worry about the numbers, but remember that opposite charges (positive-negative) attract, while like charges (positive-positive or negative-negative) repel each other.

The masses of quarks, from heaviest to lightest, are as follows: top (heaviest), bottom, charm, strange, down, and up (lightest). Keep in mind that since we are talking about elementary particles, you can think of their masses as just another property that characterizes them. After all, you can't really weigh quarks on your scale at home.

Another property of quarks is color, also referred to as color charge. In order to satisfy the Pauli Exclusion Principle, which asserts that no two quarks can exist in the same quantum state at the same time, the property of color was ascribed to quarks. Three colors could be assigned to individual quarks, and a common color scheme is red, green, and blue.

For the purposes of our discussion, it's important to mention that every quark has an antimatter particle associated with it, called an antiquark. You can think of antiquarks as quarks with opposite electric and color charges.

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Frequently Asked Questions

What is meant by quark flavor and color?

Quark flavor describes a certain type of positive or negative partial charge. For example, an up quark has two-thirds a positive charge. Color is another type of charge or attraction that has to do with strong nuclear force, which binds quarks together to make hadron particles. Quarks of the same flavor can come in different colors. This allows quarks of the same flavor to be attracted to each other because they are different colors.

Is a quark matter?

A quark might be the smallest unit of matter. Quarks are matter. They have mass. Quarks come together in threes to make protons and neutrons which make up the nuclei of atoms, the most basic unit of elements.

What are the 6 types of quarks?

There are six different flavors of quarks: up quarks, down quarks, strange quarks, charmed quarks, top quarks, and bottom quarks. Each of these quarks varies in its mass, and its fractional charges.

How many flavors of quark are there?

There are six different flavors of quarks: up, down, strange, charm, top, and bottom. Top quarks are the most massive quarks. Quarks come together to make hadrons. 2 up quarks and 1 down quark make a proton. 1 up quark and 2 down quarks make a neutron.

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