Particle Classification

At the smallest known scales, matter and interactions are described using particle physics.

Scientists have discovered many different particles. To understand how these particles are related, they classify them according to properties such as:

• composition
• mass
• electric charge
• spin
• interactions
• stability

Three important categories encountered in particle physics are:

hadrons, leptons, and bosons

These classifications help scientists organize particles and understand the fundamental structure of matter.

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The Standard Model

The Standard Model of particle physics is the modern theory describing known elementary particles and three of the four fundamental interactions.

It describes:

• electromagnetic interaction
• strong interaction
• weak interaction

Gravity is not currently incorporated into the Standard Model in the same way.

The Standard Model contains two broad types of elementary particles:

fermions

and:

bosons

Fermions make up matter.

Bosons are associated with interactions and, in the case of the Higgs boson, the Higgs field.

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Elementary vs Composite Particles

An important distinction is between:

elementary particles

and:

composite particles

Elementary particle

As far as current experiments show, it has no smaller internal structure.

Examples:

• electron
• quark
• neutrino
• photon

Composite particle

Made from smaller particles.

Examples:

• proton
• neutron

Protons and neutrons are made from:

quarks

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Quarks

Quarks are elementary particles.

There are six types, called:

flavours

They are:

• up
• down
• charm
• strange
• top
• bottom

Quarks combine to form particles called:

hadrons

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Quark Electric Charges

Unlike electrons and protons, quarks have fractional electric charges.

The up-type quarks:

• up
• charm
• top

have charge:

+2/3 e

The down-type quarks:

• down
• strange
• bottom

have charge:

−1/3 e

where e is the magnitude of the elementary charge.

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Hadrons

A hadron is a composite particle made from quarks.

Hadrons experience the:

strong interaction

Two important groups of hadrons are:

baryons

and:

mesons

Therefore:

Hadrons → baryons + mesons

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Baryons

A baryon is a hadron made from three quarks.

Important examples include:

• proton
• neutron

These particles form atomic nuclei and therefore make up much of the ordinary matter around us.

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The Proton

A proton contains:

two up quarks + one down quark

Written:

uud

Calculate its charge:

up = +2/3 e

up = +2/3 e

down = −1/3 e

Total:

+2/3 + 2/3 − 1/3 = +1

Therefore:

proton charge = +e

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The Neutron

A neutron contains:

one up quark + two down quarks

Written:

udd

Calculate its charge:

+2/3 − 1/3 − 1/3 = 0

Therefore:

neutron charge = 0

This explains how a neutron can be electrically neutral even though its constituent quarks carry electric charge.

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Comparing Protons and Neutrons

Both are:

• baryons
• hadrons
• composite particles
• made from three quarks

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Mesons

A meson is another type of hadron.

Mesons consist primarily of:

one quark + one antiquark

Examples include:

• pions
• kaons

Mesons are generally unstable and often appear in:

• particle collisions
• cosmic-ray interactions
• nuclear processes

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Baryons vs Mesons

Both are hadrons because both are built from quarks and participate in the strong interaction.

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The Strong Interaction

Quarks inside hadrons are bound together by the:

strong interaction

The particles associated with transmitting the strong interaction between quarks are:

gluons

Gluons themselves are elementary bosons.

The underlying theory describing quarks and gluons is called:

quantum chromodynamics (QCD)

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Colour Charge

The strong interaction involves a property called:

colour charge

Quarks carry colour charge.

The names commonly used are:

• red
• green
• blue

These are simply labels.

They have nothing to do with ordinary visible colours.

Antiquarks carry corresponding:

anticolours

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Quark Confinement

Individual quarks are not normally observed travelling freely under ordinary conditions.

Instead, they are confined inside hadrons.

This phenomenon is called:

quark confinement

Trying to separate quarks requires increasing energy, and the system can eventually produce additional quark-antiquark pairs rather than releasing a single isolated quark.

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Leptons

A second major particle family is:

leptons

Unlike hadrons, leptons are not made from quarks.

They are considered:

elementary particles

There are six leptons:

• electron
• electron neutrino
• muon
• muon neutrino
• tau
• tau neutrino

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The Electron

The electron is probably the most familiar lepton.

Properties include:

• elementary particle
• electric charge −1e
• relatively low mass
• stable

Electrons are responsible for much of:

• atomic structure
• electricity
• chemical bonding
• electronics

The electron is:

a lepton, not a hadron

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Muons

The muon is similar to an electron but much more massive.

It has:

• charge −1e
• spin 1/2
• no known internal structure

Muons are unstable and decay into lighter particles.

They are commonly produced by:

cosmic rays

and in:

particle accelerators

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Tau Particles

The tau is another negatively charged lepton.

It is considerably more massive than both:

• electron
• muon

It is also highly unstable.

Thus the charged leptons form a pattern:

electron → muon → tau

with increasing mass.

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Neutrinos

Each charged lepton has an associated neutrino:

electron → electron neutrino

muon → muon neutrino

tau → tau neutrino

Neutrinos:

• have no electric charge
• have extremely small masses
• interact through the weak interaction
• interact very weakly with ordinary matter

Huge numbers of neutrinos can pass through matter without interacting.

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Lepton Generations

Leptons are organized into three generations.

Ordinary stable matter is dominated by particles from the:

first generation

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Quark Generations

Quarks are also arranged into three generations.

Therefore, the matter particles of the Standard Model have a repeating generational structure.

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First-Generation Matter

Most ordinary matter is built primarily from:

• up quarks
• down quarks
• electrons

Protons:

uud

Neutrons:

udd

Atoms contain:

protons + neutrons + electrons

Therefore, nearly all familiar matter can be constructed using just:

up quarks + down quarks + electrons

Electron neutrinos also belong to the first generation, though they are not structural components of ordinary atoms.

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Fermions

Quarks and leptons belong to a broader category called:

fermions

Fermions have half-integer spin.

The elementary matter fermions in the Standard Model have:

spin 1/2

Therefore:

matter fermions → quarks + leptons

Hadrons require slightly more care because they are composite particles.

Baryons are composite fermions, while mesons are bosons.

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Bosons

Bosons are particles with integer spin.

In the Standard Model, several elementary bosons are associated with fundamental interactions.

Important examples include:

• photon
• gluon
• W⁺ boson
• W⁻ boson
• Z boson
• Higgs boson

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The Photon

The photon is the quantum particle associated with the electromagnetic field.

It is involved in electromagnetic interactions between electrically charged particles.

Photons:

• have zero electric charge
• have zero rest mass
• travel at the speed of light in vacuum

Light itself consists of:

photons

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Gluons

Gluons are associated with the strong interaction.

They bind quarks together inside hadrons such as:

• protons
• neutrons

Unlike photons, gluons themselves carry the type of charge associated with the strong interaction.

This contributes to the complex behaviour of the strong force.

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W and Z Bosons

The:

W⁺, W⁻ and Z bosons

are associated with the:

weak interaction

The weak interaction is involved in processes such as:

• beta decay
• neutrino interactions
• particle transformations

For example, beta-minus decay ultimately involves a down quark changing into an up quark through the weak interaction.

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Beta Decay at the Quark Level

A neutron contains:

udd

A proton contains:

uud

During beta-minus decay, one down quark effectively changes into an up quark.

Therefore:

udd → uud

which means:

neutron → proton

At the fundamental level, this process involves the weak interaction and a W boson.

The overall decay is:

n → p + e⁻ + ν̄ₑ

where ν̄ₑ is an electron antineutrino.

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Higgs Boson

The Higgs boson is associated with the:

Higgs field

Interactions with the Higgs field contribute to the masses of fundamental particles such as:

• quarks
• charged leptons
• W bosons
• Z bosons

The Higgs boson was discovered experimentally in 2012 at the Large Hadron Collider.

Its discovery provided major confirmation of the Standard Model.

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Force-Carrying Bosons

A simplified comparison is:

Gravity is not successfully incorporated into the Standard Model.

A hypothetical quantum particle called the:

graviton

is sometimes proposed for gravity, but it has not been experimentally confirmed.

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Important Classification Detail

The categories:

hadron, lepton, boson

are useful, but they are not three completely parallel, mutually exclusive categories.

For example:

• leptons are elementary fermions
• baryons are hadrons and are fermions
• mesons are hadrons and are bosons
• photons are elementary bosons

Therefore, particle classification can involve several properties at once.

A particle may belong to more than one classification scheme.

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A Better Classification Tree

A useful way to organize particles is:

Particles

→ Elementary particles

→ quarks
→ leptons
→ elementary bosons

and:

Composite particles

→ hadrons

→ baryons
→ mesons

This avoids the mistaken idea that quarks themselves are hadrons.

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Antiparticles

Most particles have corresponding:

antiparticles

An antiparticle has:

• the same mass as its corresponding particle
• opposite values for certain quantum properties
• opposite electric charge when the particle is charged

Examples include:

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The Positron

The antiparticle of the electron is the:

positron

The electron has charge:

−e

The positron has charge:

+e

Both have the same mass.

The positron is commonly written:

e⁺

while the electron is:

e⁻

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Antiprotons

A proton consists of:

uud

An antiproton consists of the corresponding antiquarks:

ūūd̄

Its electric charge is:

−e

The antiproton has the same mass as the proton but opposite electric charge and opposite baryon number.

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Antineutrons

The neutron is electrically neutral.

Its antiparticle, the:

antineutron

is also electrically neutral.

However, neutron and antineutron are not identical.

Their internal quark structures differ:

neutron:

udd

antineutron:

ū d̄ d̄

They also have opposite values of quantities such as baryon number.

This demonstrates that:

an antiparticle does not necessarily have the opposite electric charge if the original particle is neutral.

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Particle-Antiparticle Annihilation

When a particle encounters its antiparticle, they can undergo:

annihilation

Their mass-energy is converted into other particles.

A familiar example is:

electron + positron → photons

Often:

e⁻ + e⁺ → γ + γ

Energy and momentum must be conserved.

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Why Two Photons?

If an electron and positron annihilate approximately at rest, producing a single photon would generally fail to conserve both energy and momentum.

Two photons can travel in opposite directions.

This allows:

total momentum = 0

while conserving energy.

Each photon commonly has energy approximately:

511 keV

when the electron and positron annihilate essentially at rest.

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Pair Production

The reverse process can also occur.

Sufficiently energetic electromagnetic radiation can produce:

particle + antiparticle

For example:

photon energy → electron + positron

This is called:

pair production

Because each electron or positron has rest energy:

0.511 MeV

the minimum combined rest energy is:

1.022 MeV

Additional conditions, such as interaction with a nearby nucleus, are normally needed to conserve momentum.

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Matter and Antimatter

Ordinary matter contains overwhelmingly more matter than antimatter in our observable surroundings.

Why the universe contains much more matter than antimatter is an important unanswered question in modern physics.

Particle physicists investigate this through phenomena involving:

• particle decays
• CP violation
• neutrinos
• high-energy collisions

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Conservation Laws

Particle interactions must obey conservation laws.

Important conserved quantities include:

• energy
• momentum
• electric charge

Other quantum numbers are also important in particle physics.

For example:

baryon number

and:

lepton number

are useful for analyzing many reactions.

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Baryon Number

Baryons are assigned:

B = +1

Antibaryons:

B = −1

Particles that are not baryons:

B = 0

For example:

proton:

B = +1

electron:

B = 0

antiproton:

B = −1

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Lepton Number

Leptons are assigned lepton numbers.

In a simplified treatment:

lepton:

L = +1

antilepton:

L = −1

non-lepton:

L = 0

For beta-minus decay:

n → p + e⁻ + ν̄ₑ

the electron and antineutrino contributions allow lepton number to balance.

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Comparing Major Particle Families

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Comparing Quarks and Leptons

Both quarks and leptons are elementary:

fermions

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Comparing Hadrons and Bosons

This comparison needs care.

Some hadrons are themselves bosons.

For example:

mesons are bosons

while:

baryons are fermions

Therefore:

hadron describes composition and strong-interaction behaviour.

boson describes quantum statistics and integer spin.

The categories answer different questions.

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Why Classification Matters

Imagine trying to study hundreds of particles without organizing them.

Patterns would be difficult to recognize.

Classification allows scientists to identify relationships such as:

proton and neutron → both baryons

electron and muon → both charged leptons

photon and gluon → both elementary bosons

These patterns helped scientists develop the Standard Model.

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Classification Reveals Structure

Before quarks were understood, protons, neutrons, and many other hadrons appeared to be unrelated particles.

Scientists eventually discovered patterns among them.

These patterns suggested that hadrons might be made from smaller constituents.

The quark model successfully explained many of these relationships.

Thus classification did more than organize particles.

It helped reveal:

deeper physical structure

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Particle Generations

The Standard Model contains three generations of matter particles.

First generation

• up
• down
• electron
• electron neutrino

Second generation

• charm
• strange
• muon
• muon neutrino

Third generation

• top
• bottom
• tau
• tau neutrino

The heavier generations are generally unstable and decay into lighter particles.

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Why Ordinary Matter Uses the First Generation

Stable ordinary matter is built mainly from:

up quarks + down quarks + electrons

The heavier particles usually decay rapidly.

For example:

• muons decay
• tau particles decay
• top quarks decay extremely rapidly

Therefore, the first generation dominates the stable matter around us.

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Worked Example: Classifying a Proton

Consider a proton.

It contains:

uud

Therefore it is made from three quarks.

Three-quark hadron:

baryon

Therefore the proton is:

• a hadron
• a baryon
• a composite particle
• a fermion

It is not:

• a lepton
• an elementary particle

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Worked Example: Classifying an Electron

An electron has no known smaller internal structure.

It is:

• elementary
• a lepton
• a fermion

It is not:

• a hadron
• a baryon
• a meson

Its antiparticle is the:

positron

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Worked Example: Classifying a Pion

A pion consists primarily of:

quark + antiquark

Therefore it is:

• composite
• a hadron
• a meson
• a boson

This illustrates why "hadron" and "boson" are not necessarily mutually exclusive classifications.

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Worked Example: Classifying a Photon

A photon:

• is elementary
• is a boson
• is associated with electromagnetism
• has zero electric charge
• has zero rest mass

It is not:

• a hadron
• a lepton
• a quark

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Worked Example: Proton Charge

Proton:

uud

Charges:

u = +2/3

u = +2/3

d = −1/3

Total:

+2/3 + 2/3 − 1/3 = +1

Therefore:

Q = +e

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Worked Example: Neutron Charge

Neutron:

udd

Charges:

u = +2/3

d = −1/3

d = −1/3

Total:

+2/3 − 1/3 − 1/3 = 0

Therefore the neutron is electrically neutral.

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Worked Example: Identify the Unknown Particle

Suppose a particle:

• is elementary
• has charge −1e
• does not experience the strong interaction
• is stable
• is found in atoms

The particle is the:

electron

Classification:

lepton

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Worked Example: Identify the Family

Suppose a particle consists of:

one quark + one antiquark

It belongs to the:

meson family

Mesons belong to the larger category:

hadrons

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Particle Classification Summary

A useful structure is:

Elementary particles

Matter particles:

quarks + leptons

Elementary bosons:

photon + gluons + W/Z + Higgs

Quarks combine to produce:

hadrons

Hadrons divide mainly into:

baryons + mesons

Baryons include:

protons + neutrons

Mesons include:

pions + kaons

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Common Mistakes

Mistake: Saying protons are elementary particles.
Protons are made from quarks.

Mistake: Saying neutrons contain no charged particles because they are neutral.
Neutrons contain electrically charged quarks whose charges add to zero.

Mistake: Saying quarks are hadrons.
Quarks combine to form hadrons.

Mistake: Saying all hadrons contain three quarks.
Baryons contain three quarks, while mesons consist primarily of a quark and antiquark.

Mistake: Saying electrons are hadrons.
Electrons are leptons.

Mistake: Saying neutrinos are bosons.
Neutrinos are leptons and fermions.

Mistake: Saying every boson is a force carrier.
The Higgs boson has a different role, and composite bosons such as mesons also exist.

Mistake: Treating hadrons, leptons, and bosons as three completely mutually exclusive categories.
For example, mesons are both hadrons and bosons.

Mistake: Saying antiparticles always have opposite electric charge.
Neutral particles can have distinct antiparticles without changing electric charge.

Mistake: Saying antimatter has negative mass.
Antiparticles have the same mass as their corresponding particles.

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Key Terms

Standard Model — theory describing elementary particles and the electromagnetic, weak, and strong interactions.

Elementary particle — particle with no known smaller internal structure.

Composite particle — particle made from smaller constituents.

Quark — elementary fermion that participates in the strong interaction.

Hadron — composite particle made from quarks and subject to the strong interaction.

Baryon — hadron composed primarily of three quarks.

Meson — hadron composed primarily of a quark and antiquark.

Lepton — elementary fermion that does not participate in the strong interaction.

Neutrino — electrically neutral lepton with very small mass and weak interactions.

Fermion — particle with half-integer spin; elementary matter particles are fermions.

Boson — particle with integer spin.

Photon — elementary boson associated with electromagnetism.

Gluon — elementary boson associated with the strong interaction between quarks.

W and Z bosons — elementary bosons associated with the weak interaction.

Higgs boson — particle associated with the Higgs field.

Antiparticle — particle corresponding to another particle with the same mass but opposite values of certain quantum properties.

Positron — antiparticle of the electron.

Annihilation — process in which a particle and antiparticle are converted into other particles.

Pair production — production of a particle-antiparticle pair from sufficient energy.

Quark confinement — phenomenon preventing isolated quarks from normally being observed.

Particle generation — one of three repeating families of quarks and leptons in the Standard Model.

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Key Takeaways

• Particle classification helps scientists organize matter and identify fundamental relationships.
• The Standard Model describes quarks, leptons, and elementary bosons.
• Quarks and leptons are elementary matter particles.
• There are six quark flavours: up, down, charm, strange, top, and bottom.
• Hadrons are composite particles made from quarks.
• Hadrons are divided mainly into baryons and mesons.
• Baryons consist primarily of three quarks.
• Protons are uud and neutrons are udd.
• Mesons consist primarily of a quark and an antiquark.
• Leptons are elementary particles and do not participate in the strong interaction.
• The six leptons are the electron, muon, tau, and their three associated neutrinos.
• Quarks and leptons are organized into three generations.
• Ordinary matter is built mainly from first-generation up quarks, down quarks, and electrons.
• Elementary bosons include photons, gluons, W and Z bosons, and the Higgs boson.
• Photons are associated with electromagnetism.
• Gluons are associated with the strong interaction.
• W and Z bosons are associated with the weak interaction.
• Antiparticles have the same mass as their corresponding particles but opposite values of certain quantum properties.
• The positron is the antiparticle of the electron.
• Particle-antiparticle annihilation can convert mass-energy into other particles.
• Classification terms can overlap: a meson is both a hadron and a boson.
• The organization of particles into families helped reveal the underlying quark structure of matter and led to the development of the Standard Model.

Check Your Understanding

1. What is an elementary particle?
2. What is a composite particle?
3. Name the six quark flavours.
4. What is a hadron?
5. What are the two main families of hadrons?
6. What is the quark composition of a proton?
7. Use quark charges to explain why a proton has charge +1e.
8. What is the quark composition of a neutron?
9. Use quark charges to explain why a neutron is electrically neutral.
10. Explain the difference between a baryon and a meson.
11. Name the six leptons.
12. Why is an electron classified as a lepton rather than a hadron?
13. What are neutrinos, and why are they difficult to detect?
14. Name the main elementary bosons in the Standard Model.
15. Which boson is associated with electromagnetism?
16. Which bosons are associated with the weak interaction?
17. What role do gluons play?
18. What is an antiparticle?
19. Name the antiparticle of the electron.
20. Explain why classifying particles into families helps scientists understand the fundamental structure of matter.