Describe quarks, hadrons, and field particles in the standard model

Ontario Grade 12 Physics

Revolutions in Modern Physics: Quantum Mechanics and Special Relativity

A Grade 12 introduction to the particle groups in the Standard Model

In earlier physics courses, matter is often described as being made of atoms. Atoms contain smaller parts, including protons and neutrons in the nucleus. The Standard Model is a model that organizes the known fundamental particles and describes how some of them interact. A fundamental particle is one not known to be made of smaller particles. This lesson focuses on three ideas: quarks, hadrons, and field particles. It uses particle symbols and charge bookkeeping, not a motion or force diagram, because the goal is to describe particle types and how they fit together.

What you will learn

Describe quarks and identify their basic properties.
Explain how quarks form hadrons, including baryons and mesons.
Describe the roles of the Standard Model's field particles.
Use particle symbols, charges, and quark content to classify familiar particles.

1. Quarks: one family of fundamental particles

Quarks are fundamental particles. They are not found as isolated particles under ordinary conditions. Instead, they are found inside composite particles called hadrons. Composite means made from smaller parts.

There are six quark flavours: up, down, charm, strange, top, and bottom. Here, flavour is the name of a quark type; it does not refer to taste. Up and down quarks make up the familiar protons and neutrons. The other four flavours are also part of the Standard Model, but they are not needed to describe ordinary atomic nuclei.

Quarks have electric charge, measured in coulombs in SI. At this level, their charges are usually stated as fractions of the elementary charge, represented by

e

. An up-type quark—up, charm, or top—has charge

+\frac{2}{3}e

. A down-type quark—down, strange, or bottom—has charge

-\frac{1}{3}e

. The elementary charge is the magnitude of the charge of a proton. An electron has charge

-e

.

A quark symbol names both its type and, when needed, its antiparticle. An antiquark is the antimatter partner of a quark. It has the same mass as its matching quark and the opposite electric charge. For example, an up antiquark has charge

-\frac{2}{3}e

. The symbol for an up quark is

u

; the symbol for an up antiquark is

\bar{u}

.

q_u=+\frac{2}{3}e, q_d=-\frac{1}{3}e

The six flavours are up, down, charm, strange, top, and bottom.
Up-type quarks have charge $+\frac{2}{3}e$ ; down-type quarks have charge $-\frac{1}{3}e$ .
An antiquark has the opposite electric charge to its matching quark.

2. Hadrons: particles made of quarks

A hadron is a composite particle made of quarks held together by the strong interaction. The strong interaction is one of the interactions described by the Standard Model. Quarks also have a property called colour charge. Despite the name, colour charge is not visible colour. It is a label used to describe how quarks take part in the strong interaction.

There are two main hadron groups. A baryon contains three quarks, or three antiquarks for an antibaryon. A meson contains one quark and one antiquark. These combinations have no net colour charge. The model does not describe quarks as normally appearing alone; they occur in hadrons.

Protons and neutrons are baryons. A proton contains two up quarks and one down quark, written

uud

. A neutron contains one up quark and two down quarks, written

udd

. Add the quark charges to find the hadron's total electric charge. The proton has charge

+e

, while the neutron has zero net charge.

A meson contains a quark and an antiquark. For example, a positively charged pion has quark content

u\bar{d}

. Its charge is found by adding the up-quark charge and the down-antiquark charge. The result is

+e

. Mesons and baryons are both hadrons, even though their quark combinations differ.

q_{\mathrm{hadron}}=\sum_i q_i

Baryons contain three quarks; mesons contain a quark and an antiquark.
Proton: $uud$ , with net charge $+e$ . Neutron: $udd$ , with net charge $0$ .
Electric charge is conserved in the quark bookkeeping: add the charges of the constituents.

3. Field particles: describing interactions

The Standard Model also includes field particles, often called force-carrying particles. In this Grade 12 description, they are the particles associated with interactions between matter particles. They are not extra quarks and are not hadrons. The model links each of the three interactions below to particular field particles.

The gluon is associated with the strong interaction between quarks. Gluons act on quarks' colour charge and help bind quarks into hadrons. The photon is associated with the electromagnetic interaction, which acts on electrically charged particles. A familiar example is the electromagnetic interaction between electrons and protons.

The weak interaction is associated with the

W^+

,

W^-

, and

Z^0

particles. These are called the weak field particles. The weak interaction can change one type of quark into another. For example, a down quark can change into an up quark in a process associated with the

W^-

particle. This description identifies the particle and interaction; it does not require a detailed account of the process.

The Higgs boson is associated with the Higgs field in the Standard Model. It is different from the gluon, photon, and weak field particles in the roles described here. The Standard Model includes it as a particle associated with the Higgs field. Do not treat it as a quark or a hadron.

Gluons are associated with the strong interaction; photons with the electromagnetic interaction.
The $W^+$ , $W^-$ , and $Z^0$ are associated with the weak interaction.
The Higgs boson is associated with the Higgs field; it is neither a quark nor a hadron.

4. Putting the particle categories together

A useful way to read a particle description is to ask two questions. First, is the particle fundamental in the Standard Model, or is it composite? Second, if it is composite, what quarks make it up? A quark is fundamental; a proton is composite and is a baryon; a pion is composite and is a meson.

Field particles belong to a different part of the description. They are associated with interactions, rather than being quark combinations. A gluon is not a hadron. A photon is not a quark. This separation prevents a common mix-up: a particle can be important in holding or linking matter together without being one of the matter particles inside a hadron.

Charge addition is a simple check on a proposed quark combination. The calculation gives the net electric charge, not the particle's mass or its complete behaviour. Use the quark content and charge rules to classify the particle, then name the interaction or field particle only when the description calls for it.

Quarks are fundamental; hadrons are composite.
Field particles are associated with interactions and are not hadron constituents in the same classification.
Adding constituent electric charges checks a hadron's net charge.

Particle categories and roles

Category	Description	Example
Quark	Fundamental particle; one of six flavours	Up quark, $u$
Baryon	Hadron made of three quarks	Proton, $uud$
Meson	Hadron made of one quark and one antiquark	Pion, $u\bar{d}$
Field particle	Particle associated with an interaction	Photon, $\gamma$

Worked example

Check a proton's charge

A proton has quark content

uud

. Find its net electric charge in terms of

e

and identify its hadron group.

Identify the constituents
The proton contains two up quarks and one down quark. It is a baryon because it contains three quarks.
Add their charges
Each up quark has charge $+\frac{2}{3}e$ , and the down quark has charge $-\frac{1}{3}e$ . Adding these charges gives the proton's net charge.
$(+\frac{2}{3}e)+(+\frac{2}{3}e)+(-\frac{1}{3}e)=+e$

Answer: The proton is a baryon with net charge

+e

, or approximately

+1.60\times10^{-19}\ \mathrm{C}

.

Check: The charge is positive, as expected for a proton. The units are coulombs when the elementary charge is written in SI units.

Worked example

Classify a neutron

A particle has quark content

udd

. Classify it as a baryon or meson and determine its net electric charge.

Use the quark count
The combination has three quarks and no antiquark. A hadron made of three quarks is a baryon.
Calculate the net charge
The up quark contributes $+\frac{2}{3}e$ . Each down quark contributes $-\frac{1}{3}e$ . Their total charge is zero.
$+\frac{2}{3}e-\frac{1}{3}e-\frac{1}{3}e=0$

Answer: The particle is a neutral baryon. This quark combination is a neutron.

Check: Three quarks indicate a baryon, and the charge sum is zero, matching a neutron.

Worked example

Classify a positively charged pion

A meson has quark content

u\bar{d}

. Determine its net electric charge and explain why it is a meson.

Identify the particle types
The combination has one up quark and one down antiquark. A hadron made of one quark and one antiquark is a meson.
Add the charges
The up quark has charge $+\frac{2}{3}e$ . The down antiquark has the opposite charge to a down quark, so its charge is $+\frac{1}{3}e$ .
$+\frac{2}{3}e+\frac{1}{3}e=+e$

Answer: The particle is a meson with net charge

+e

. It is commonly called a positively charged pion.

Check: The antiquark's charge is positive, not negative. Adding the two charges gives a positive result with units of charge.

Common mistakes and how to avoid them

Calling a proton a fundamental particle because it is found in an atomic nucleus.

Correction: A proton is a composite hadron. It contains three quarks:

uud

.

Calling a meson a three-quark particle.

Correction: A meson contains one quark and one antiquark. A three-quark hadron is a baryon.

Giving an antiquark the same electric charge as its matching quark.

Correction: An antiquark has the opposite electric charge. For example, a down antiquark has charge

+\frac{1}{3}e

.

Treating a photon or gluon as a type of hadron.

Correction: Photons and gluons are field particles associated with interactions. A hadron is made from quarks.

Assuming the names colour charge or flavour describe visible colour or taste.

Correction: They are particle-physics labels. Colour charge relates to the strong interaction; flavour names the type of quark.

Lesson summary

Quarks are fundamental particles with six flavours and fractional electric charges.
Hadrons are composite particles made of quarks: baryons contain three quarks, while mesons contain a quark and an antiquark.
Protons are $uud$ baryons with charge $+e$ ; neutrons are $udd$ baryons with charge $0$ .
Gluons, photons, and the $W^+$ , $W^-$ , and $Z^0$ are associated with the strong, electromagnetic, and weak interactions, respectively.
The Higgs boson is associated with the Higgs field and is not a quark or hadron.

Check your understanding

Question 1

Which quark combination describes a neutron?

$uud$
$udd$
$u\bar{d}$
$ddd$

Show answer and explanation

udd

A neutron is a baryon made of one up quark and two down quarks. The other choices are a proton, a meson, and a different three-quark combination.

Question 2

What is the net electric charge of

u\bar{d}

?

$0$
$-e$
$+e$
$+\frac{1}{3}e$

Show answer and explanation

+e

The up quark has charge

+\frac{2}{3}e

, and the down antiquark has charge

+\frac{1}{3}e

. Their sum is

+e

.

Question 3

Which particle is associated with the electromagnetic interaction?

Gluon
Photon
Neutron
Up quark

Show answer and explanation

Photon

The photon is associated with the electromagnetic interaction. A gluon is associated with the strong interaction, while a neutron and an up quark are matter particles.

Key terms

Antiquark: The antimatter partner of a quark, with opposite electric charge.
Baryon: A hadron made of three quarks, or three antiquarks for an antibaryon.
Composite particle: A particle made from smaller particles.
Elementary charge: The magnitude of the electric charge of a proton, represented by $e$ .
Field particle: A particle associated with one of the interactions described by the Standard Model.
Flavour: The name of a quark type, such as up or down.
H is the Higgs boson: A particle associated with the Higgs field in the Standard Model.
Hadron: A composite particle made of quarks and held together by the strong interaction.

Continue through SPH4U

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Physics (SPH4U), expectation F3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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