What Is the Standard Model?

The Standard Model of particle physics is the scientific theory used to describe the known fundamental particles of matter and three of the four fundamental interactions.

It provides physicists with a framework for answering two major questions:

What is matter made of?

and

How do fundamental particles interact?

The Standard Model includes:

• quarks
• leptons
• force-carrying bosons
• the Higgs boson

It successfully describes the strong, electromagnetic, and weak interactions.

However, it does not provide a quantum description of gravity.

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The Main Groups of Particles

A useful starting point is to divide the particles we encounter into two broad categories:

Fundamental particles

and

Composite particles

Fundamental particles have no known smaller internal structure.

Composite particles are made from combinations of fundamental particles.

Within the Standard Model, the fundamental particles include:

Quarks + leptons + gauge bosons + Higgs boson

Particles such as protons and neutrons are not fundamental. They are composite particles called hadrons.

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Fermions: The Matter Particles

Quarks and leptons belong to a larger group called fermions.

Fermions are the fundamental particles that make up matter.

There are:

6 quarks + 6 leptons = 12 fundamental matter particles

These are organized into three generations.

Most ordinary matter is made from first-generation particles.

That means the matter around us depends primarily on:

• up quarks
• down quarks
• electrons

Electron neutrinos also participate in many nuclear processes.

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Quarks

There are six flavours of quarks:

• Up (u)
• Down (d)
• Charm (c)
• Strange (s)
• Top (t)
• Bottom (b)

Quarks experience:

• the strong interaction
• the electromagnetic interaction if electrically charged
• the weak interaction
• gravity, although gravity is not included in the Standard Model

Quarks have fractional electric charges.

Up-type quarks:

u, c, t = +⅔

Down-type quarks:

d, s, b = −⅓

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Leptons

There are also six leptons:

• Electron
• Electron neutrino
• Muon
• Muon neutrino
• Tau
• Tau neutrino

Unlike quarks, leptons do not experience the strong interaction.

The electron, muon, and tau have electric charge −1.

The neutrinos have no electric charge.

The electron is especially important because it is a major component of ordinary atoms.

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Bosons

Another major category is the bosons.

Some bosons act as force-carrying particles, allowing fundamental particles to interact.

The Standard Model includes the following gauge bosons:

There are eight types of gluons, although Standard Model diagrams usually represent them with a single gluon entry.

The Higgs boson is also a boson, but it has a different role.

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The Photon

The photon is associated with the electromagnetic interaction.

It has:

• no electric charge
• zero rest mass
• spin 1

Photons are responsible for electromagnetic interactions between charged particles.

Light itself consists of photons.

Therefore, photons appear in phenomena involving:

• visible light
• radio waves
• X-rays
• gamma rays
• electricity
• magnetism

All electromagnetic radiation consists of photons.

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Gluons

Gluons are the force carriers of the strong interaction.

They act between particles carrying colour charge, particularly quarks.

Gluons bind quarks together inside hadrons such as protons and neutrons.

For example:

Proton = uud

Neutron = udd

Without the strong interaction mediated by gluons, these quark combinations would not form protons and neutrons.

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W and Z Bosons

The W⁺, W⁻, and Z⁰ bosons carry the weak interaction.

The weak interaction is responsible for several types of particle transformation and radioactive decay.

For example, beta-minus decay can be represented at the nucleon level as:

n → p + e⁻ + ν̄ₑ

At the quark level, a down quark transforms into an up quark through a process involving a W⁻ boson.

The W and Z bosons are very massive, which contributes to the extremely short range of the weak interaction.

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Where Are the Hadrons?

A common source of confusion is that hadrons are not fundamental particles in the Standard Model.

A hadron is a composite particle made from quarks.

Two important categories are:

Baryons

Ordinary baryons contain three valence quarks.

Examples:

Proton = uud

Neutron = udd

Mesons

Mesons contain a quark and an antiquark.

For example:

π⁺ = u d̄

So when classifying particles:

Quark → fundamental fermion

Electron → fundamental fermion and lepton

Photon → boson

Proton → hadron and baryon

Neutron → hadron and baryon

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Particle Classification

This classification is useful because it distinguishes fundamental particles from particles made from other particles.

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The Higgs Field

One of the most important parts of the Standard Model is the Higgs field.

Unlike an ordinary object located in one particular place, the Higgs field exists throughout space.

Certain fundamental particles interact with this field.

The strength of this interaction contributes to the particle's rest mass.

Particles that interact more strongly with the Higgs field generally acquire greater mass through the Higgs mechanism.

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The Higgs Boson

The Higgs boson is an excitation of the Higgs field.

A useful comparison is to think about waves on the surface of water.

The water exists throughout the region.

A wave is an excitation of that water.

Similarly:

Higgs field → exists throughout space

Higgs boson → excitation of the Higgs field

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

Its discovery provided strong evidence for the Higgs mechanism predicted by the Standard Model.

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Does the Higgs Explain All Mass?

A common misconception is:

"The Higgs boson gives everything its mass."

This is too simple.

The Higgs mechanism explains the masses of fundamental particles such as electrons, quarks, and the W and Z bosons.

However, most of the mass of ordinary matter comes from protons and neutrons.

Much of the mass of protons and neutrons arises from the energy of the strong interaction and the motion of quarks and gluons inside them.

So the Higgs mechanism is essential, but it is not the whole story behind the mass of everyday objects.

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A Map of the Standard Model

The Standard Model can be summarized as:

FERMIONS

Matter particles

→ Quarks

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

→ Leptons

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

BOSONS

→ Gauge bosons

• photon
• gluons
• W⁺
• W⁻
• Z⁰

→ Scalar boson

• Higgs boson

Composite particles such as protons, neutrons, and mesons are built from the quarks described by the model.

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What Does the Standard Model Explain?

The Standard Model has been extraordinarily successful.

It helps explain:

• the structure of matter at the particle level
• electromagnetic interactions
• the strong interaction
• the weak interaction
• radioactive beta decay
• interactions between quarks
• interactions involving neutrinos
• particle production and decay
• many results from particle accelerator experiments

Many predictions of the Standard Model have been confirmed experimentally with extremely high precision.

But the model is not complete.

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Limitation 1: Gravity

The most obvious limitation is that the Standard Model does not include a quantum theory of gravity.

The other three fundamental interactions are incorporated into the Standard Model.

Gravity is described extremely successfully on large scales by Einstein's general theory of relativity.

However, physicists have not yet developed and experimentally confirmed a complete theory that successfully combines gravity with quantum physics.

A hypothetical quantum particle called the graviton is sometimes proposed, but no graviton has been detected.

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Limitation 2: Dark Matter

Astronomical observations provide strong evidence for dark matter.

Dark matter appears to produce gravitational effects on:

• galaxies
• galaxy clusters
• gravitational lensing
• the large-scale structure of the Universe

However, none of the known Standard Model particles provides a complete explanation for the dark matter inferred from these observations.

This suggests that there may be particles or physics beyond the Standard Model.

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Limitation 3: Dark Energy

Observations indicate that the expansion of the Universe is accelerating.

The name dark energy is given to whatever is responsible for this large-scale phenomenon.

The Standard Model of particle physics does not provide a satisfactory explanation for the observed nature and magnitude of dark energy.

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Limitation 4: Neutrino Mass

In the simplest original formulation of the Standard Model, neutrinos were treated as massless.

However, the discovery of neutrino oscillations demonstrated that neutrinos have non-zero masses.

The Standard Model can be extended to accommodate neutrino masses, but their origin remains an important area of research.

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Limitation 5: Matter and Antimatter

The Standard Model includes antimatter and allows some differences between the behaviour of matter and antimatter.

However, these known effects do not appear sufficient to explain why the observable Universe contains far more matter than antimatter.

Understanding this matter-antimatter asymmetry remains one of the major unanswered questions in physics.

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Did You Know?

The Standard Model is both incredibly successful and obviously incomplete.

It describes experimental results involving fundamental particles with remarkable accuracy, yet some of the biggest questions in physics remain unanswered:

What is dark matter?

What is dark energy?

Why is there more matter than antimatter?

How does gravity fit with quantum physics?

Why do the fundamental particles have the particular masses and properties that we observe?

Answering these questions could require physics that goes beyond the Standard Model.

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Key Terms

Standard Model – The theory describing known fundamental particles and the strong, electromagnetic, and weak interactions.

Fermion – A particle with half-integer spin; fundamental fermions include quarks and leptons.

Quark – A fundamental matter particle that experiences the strong interaction.

Lepton – A fundamental matter particle that does not experience the strong interaction.

Boson – A particle with integer spin; several bosons act as force carriers.

Gauge boson – A boson associated with a fundamental interaction.

Hadron – A composite particle made from quarks.

Baryon – A hadron whose ordinary examples contain three valence quarks.

Meson – A hadron containing a quark and antiquark.

Higgs field – A field that exists throughout space and participates in the mechanism through which certain fundamental particles acquire mass.

Higgs boson – The particle associated with excitations of the Higgs field.

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Key Takeaways

• The Standard Model describes the known fundamental particles and three fundamental interactions.
• Fundamental matter particles are divided into quarks and leptons.
• Quarks and leptons are fermions.
• Quarks combine to produce composite particles called hadrons.
• Protons and neutrons are hadrons, not fundamental particles.
• The photon carries the electromagnetic interaction.
• Gluons carry the strong interaction.
• W and Z bosons carry the weak interaction.
• The Higgs field plays an important role in giving fundamental particles their rest masses.
• The Higgs boson is an excitation of the Higgs field.
• The Standard Model does not provide a quantum description of gravity.
• It does not fully explain phenomena including dark matter, dark energy, neutrino masses, and the matter-antimatter asymmetry.
• The search for physics beyond the Standard Model is one of the major areas of modern physics.