14.2 Quarks | Particle Physics | Physics 11

Introduction

In the early 20th century, protons, neutrons, and electrons were considered the fundamental, indivisible building blocks of matter. This view was challenged in 1964 when physicists Murray Gell-Mann and George Zweig independently proposed that protons and neutrons were not elementary after all, but were instead composite particles made of even smaller constituents called quarks. This revolutionary idea was experimentally confirmed in 1968 through deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC), which essentially "saw" inside the proton.

Quarks are a fundamental part of the Standard Model of Particle Physics. They are a type of elementary particle, meaning they have no discernible internal structure. Specifically, they are fermions (matter particles) with a spin of 1/2. Along with leptons, quarks form the fundamental constituents of all visible matter in the universe.

Properties of Quarks

Quarks are defined by a unique set of properties that distinguish them from all other particles.

1. Flavors

Quarks come in six different types, or flavors, which are organized into three generations of increasing mass. For every quark flavor, there is a corresponding anti-quark with the same mass but an opposite electric charge.

Generation	Flavor	Symbol	Charge	Mass Range	Key Characteristics
First	Up	$u$	$+ 2/3 e$	~2.2 MeV/c²	The lightest quark. A proton contains two up quarks.
First	Down	$d$	$- 1/3 e$	~4.7 MeV/c²	Slightly heavier than the up quark. A neutron has two.
Second	Charm	$c$	$+ 2/3 e$	~1.27 GeV/c²	Much heavier; found in exotic particles.
Second	Strange	$s$	$- 1/3 e$	~95 MeV/c²	Found in particles that have unusually long lifetimes.
Third	Top	$t$	$+ 2/3 e$	~173 GeV/c²	The most massive elementary particle discovered; decays almost instantly.
Third	Bottom	$b$	$- 1/3 e$	~4.18 GeV/c²	Decays quickly, often into a charm quark.

Only the up and down quarks are stable and make up the ordinary matter of protons and neutrons.

2. Color Charge

Quarks possess a type of "charge" called color charge. This is the charge associated with the strong nuclear force, in the same way that electric charge is associated with the electromagnetic force.

There are three types of color charge: red, green, and blue (and their corresponding anti-colors for anti-quarks).
This property has no connection to the visible colors of light; it is simply a naming convention for a physical property.
The theory describing the interactions of color charge is called Quantum Chromodynamics (QCD).

3. Confinement

A bizarre and fundamental rule of the strong force is that quarks are never found in isolation; they are always bound together in groups. This phenomenon is called confinement.

The strong force between quarks, mediated by gluons, increases with distance.
If you try to pull two quarks apart, the energy in the force field between them grows until it is energetically cheaper to create a new quark-antiquark pair from that energy. The result is not two free quarks, but two new composite particles.

Hadrons: Composite Particles from Quarks

Quarks combine to form a class of composite particles called hadrons. Hadrons are always "color-neutral," meaning the combination of their quarks' color charges results in a net "white" or neutral state. Hadrons are divided into two categories:

1. Baryons

Composition: Made of three quarks (or three anti-quarks for anti-baryons).
Properties: Baryons are fermions (with half-integer spin) and include the most familiar matter particles. The combination of three colors (red, green, and blue) makes them color-neutral.
Baryon Number: A conserved quantum number, with baryons having a value of +1.
Examples:
- Proton: Composed of two up quarks and one down quark ( $uu d$ ). Charge: $(+ 2/3) + (+ 2/3) + (- 1/3) = + 1$ .
- Neutron: Composed of one up quark and two down quarks ( $u dd$ ). Charge: $(+ 2/3) + (- 1/3) + (- 1/3) = 0$ .

2. Mesons

Composition: Made of one quark and one anti-quark.
Properties: Mesons are bosons (with integer spin) and are all unstable. They are color-neutral because their constituent quark has a color and the anti-quark has the corresponding anti-color (e.g., red and anti-red).
Baryon Number: 0.
Examples:
- Pion ( $π^{+}$ ): Composed of an up quark and a down anti-quark ( $u \overset{ˉ}{d}$ ).
- Kaon ( $K^{+}$ ): Composed of an up quark and a strange anti-quark ( $u \overset{s}{ˉ}$ ).

Possible Questions/Answer

Q: Why can't we observe a single, free quark? A: This is due to the principle of confinement. The strong force that binds quarks together increases with distance, so an infinite amount of energy would be required to separate them completely. Instead, the energy put into separating them creates new quark-antiquark pairs.
Q: What holds quarks together inside a hadron? A: Quarks are held together by the strong nuclear force, which is mediated by force-carrying particles called gluons.

Introduction

Properties of Quarks

Quarks are defined by a unique set of properties that distinguish them from all other particles.

1. Flavors

Generation	Flavor	Symbol	Charge	Mass Range	Key Characteristics
First	Up	$u$	$+ 2/3 e$	~2.2 MeV/c²	The lightest quark. A proton contains two up quarks.
First	Down	$d$	$- 1/3 e$	~4.7 MeV/c²	Slightly heavier than the up quark. A neutron has two.
Second	Charm	$c$	$+ 2/3 e$	~1.27 GeV/c²	Much heavier; found in exotic particles.
Second	Strange	$s$	$- 1/3 e$	~95 MeV/c²	Found in particles that have unusually long lifetimes.
Third	Top	$t$	$+ 2/3 e$	~173 GeV/c²	The most massive elementary particle discovered; decays almost instantly.
Third	Bottom	$b$	$- 1/3 e$	~4.18 GeV/c²	Decays quickly, often into a charm quark.

Only the up and down quarks are stable and make up the ordinary matter of protons and neutrons.

2. Color Charge

There are three types of color charge: red, green, and blue (and their corresponding anti-colors for anti-quarks).
This property has no connection to the visible colors of light; it is simply a naming convention for a physical property.
The theory describing the interactions of color charge is called Quantum Chromodynamics (QCD).

3. Confinement

A bizarre and fundamental rule of the strong force is that quarks are never found in isolation; they are always bound together in groups. This phenomenon is called confinement.

The strong force between quarks, mediated by gluons, increases with distance.
If you try to pull two quarks apart, the energy in the force field between them grows until it is energetically cheaper to create a new quark-antiquark pair from that energy. The result is not two free quarks, but two new composite particles.

Hadrons: Composite Particles from Quarks

1. Baryons

Composition: Made of three quarks (or three anti-quarks for anti-baryons).
Properties: Baryons are fermions (with half-integer spin) and include the most familiar matter particles. The combination of three colors (red, green, and blue) makes them color-neutral.
Baryon Number: A conserved quantum number, with baryons having a value of +1.
Examples:
- Proton: Composed of two up quarks and one down quark ( $uu d$ ). Charge: $(+ 2/3) + (+ 2/3) + (- 1/3) = + 1$ .
- Neutron: Composed of one up quark and two down quarks ( $u dd$ ). Charge: $(+ 2/3) + (- 1/3) + (- 1/3) = 0$ .

2. Mesons

Composition: Made of one quark and one anti-quark.
Properties: Mesons are bosons (with integer spin) and are all unstable. They are color-neutral because their constituent quark has a color and the anti-quark has the corresponding anti-color (e.g., red and anti-red).
Baryon Number: 0.
Examples:
- Pion ( $π^{+}$ ): Composed of an up quark and a down anti-quark ( $u \overset{ˉ}{d}$ ).
- Kaon ( $K^{+}$ ): Composed of an up quark and a strange anti-quark ( $u \overset{s}{ˉ}$ ).

Possible Questions/Answer

Q: Why can't we observe a single, free quark? A: This is due to the principle of confinement. The strong force that binds quarks together increases with distance, so an infinite amount of energy would be required to separate them completely. Instead, the energy put into separating them creates new quark-antiquark pairs.
Q: What holds quarks together inside a hadron? A: Quarks are held together by the strong nuclear force, which is mediated by force-carrying particles called gluons.