Structure of Matter
3. Quarks
Learning outcomes
- I can describe quarks as fundamental particles.
- I can identify the six types (flavours) of quarks.
- I can explain how quarks combine to form hadrons.
- I can determine the quark composition of protons and neutrons.
- I can distinguish between quarks and leptons.
Looking Deeper Inside Matter
When scientists first discovered atoms, atoms were thought to be the smallest possible pieces of matter. Later, experiments showed that atoms contain protons, neutrons, and electrons.
Scientists then discovered something even more surprising:
Protons and neutrons are not fundamental particles.
They contain smaller particles called quarks.
A quark is a fundamental particle, meaning that according to current experimental evidence, it does not have a smaller internal structure.
This gives us another level in the organization of matter:
Matter → atoms → protons and neutrons → quarks
Electrons are different. Electrons are also fundamental particles, but they belong to another family called leptons.
The Six Flavours of Quarks
There are six types of quarks. Physicists call the different types flavours.
They are:
- Up
- Down
- Charm
- Strange
- Top
- Bottom
These six quarks are commonly organized into three generations.
| Generation | Quark | Symbol | Electric Charge |
|---|---|---|---|
| First | Up | u | +⅔ |
| First | Down | d | −⅓ |
| Second | Charm | c | +⅔ |
| Second | Strange | s | −⅓ |
| Third | Top | t | +⅔ |
| Third | Bottom | b | −⅓ |
Notice an unusual feature: quarks have fractional electric charges.
Up, charm, and top quarks have charge:
+⅔e
Down, strange, and bottom quarks have charge:
−⅓e
Here, e represents the magnitude of the elementary charge.
Quark Generations
The six quarks occur in three generations:
Generation 1:
Up and down
Generation 2:
Charm and strange
Generation 3:
Top and bottom
Ordinary stable matter is made almost entirely from first-generation particles.
That means the protons and neutrons in the atoms around us are built primarily from up and down quarks.
The heavier quarks can be produced in high-energy processes such as particle accelerator collisions and cosmic-ray interactions, but they are unstable and rapidly transform into lighter particles.
Quarks Combine to Form Hadrons
Quarks are unusual because we do not normally observe individual quarks by themselves.
Instead, quarks combine to produce composite particles called hadrons.
A hadron is a particle made from quarks held together by the strong interaction.
Two major families of hadrons are:
Baryons – traditionally described as three-quark combinations.
Mesons – quark-antiquark combinations.
Protons and neutrons are examples of baryons.
More precisely, modern particle physics also recognizes exotic hadrons such as tetraquarks and pentaquarks, but baryons and mesons provide the essential starting point for understanding hadronic matter.
Protons
A proton contains two up quarks and one down quark.
Proton = uud
We can use the charges of the quarks to check the proton's charge.
Up quark = +⅔
Up quark = +⅔
Down quark = −⅓
Therefore:
+⅔ + ⅔ − ⅓ = +1
So the proton has an overall electric charge of:
+1
Neutrons
A neutron contains one up quark and two down quarks.
Neutron = udd
Again, we can calculate the total charge.
Up quark = +⅔
Down quark = −⅓
Down quark = −⅓
Therefore:
+⅔ − ⅓ − ⅓ = 0
The neutron therefore has no overall electric charge.
Comparing Protons and Neutrons
The difference between a proton and neutron is therefore surprisingly small at the level of their valence quarks.
| Particle | Valence Quark Composition | Charge |
|---|---|---|
| Proton | uud | +1 |
| Neutron | udd | 0 |
Changing just one valence quark changes the particle.
This becomes particularly important when studying beta decay, where the weak interaction allows one type of quark to transform into another.
For example, during beta-minus decay, a down quark in a neutron changes into an up quark.
At the nucleon level:
neutron → proton + electron + antineutrino
At the valence-quark level:
udd → uud
What Holds Quarks Together?
Quarks interact through the strong interaction, the strongest of the four fundamental interactions.
The particles that mediate the strong interaction are called gluons.
Quarks possess a property called colour charge. Despite the name, colour charge has nothing to do with visible colour. The terms red, green, and blue are simply labels physicists use to describe this property.
Gluons interact with colour charge and help bind quarks together.
Quarks + strong interaction → hadrons
Quark Confinement
One of the most unusual properties of quarks is that they are not normally found alone.
This phenomenon is called quark confinement.
If enough energy is supplied while attempting to separate quarks, that energy can produce new quark-antiquark pairs rather than simply releasing an isolated quark.
As a result, experiments detect combinations of quarks rather than free individual quarks.
This is very different from electrons, which can exist as free particles.
Antiquarks
Every quark has a corresponding antiparticle called an antiquark.
Antiquarks have properties opposite to the corresponding quark, including opposite electric charge.
For example:
Up quark:
u = +⅔
Up antiquark:
ū = −⅔
Down quark:
d = −⅓
Down antiquark:
d̄ = +⅓
Antiquarks are especially important in forming another type of hadron: the meson.
Baryons and Mesons
Hadrons can be classified according to their quark structure.
Baryons
Ordinary baryons contain three valence quarks.
Examples include:
Proton = uud
Neutron = udd
Mesons
Mesons contain a quark and an antiquark.
For example, a positively charged pion can be represented as:
π⁺ = u d̄
The charges are:
+⅔ + ⅓ = +1
Therefore the pion has charge +1.
A Simple Particle Hierarchy
The organization of matter can now be expanded:
Matter
↓
Atoms
↓
Nucleus + electrons
↓
Protons + neutrons
↓
Quarks
Protons and neutrons are hadrons.
Electrons are leptons.
Quarks and leptons are both considered fundamental particles.
Quarks vs Leptons
Quarks and leptons are two major families of fundamental matter particles in the Standard Model.
| Property | Quarks | Leptons |
|---|---|---|
| Fundamental? | Yes | Yes |
| Six types? | Yes | Yes |
| Experience strong interaction? | Yes | No |
| Can normally exist freely? | No | Yes, for charged leptons and neutrinos |
| Can form hadrons? | Yes | No |
| Example | Up quark | Electron |
The most important difference is:
Quarks experience the strong interaction.
Leptons do not.
The Six Leptons
For comparison, there are also six leptons:
- Electron
- Electron neutrino
- Muon
- Muon neutrino
- Tau
- Tau neutrino
These are also organized into three generations.
| Generation | Quarks | Leptons |
|---|---|---|
| 1 | up, down | electron, electron neutrino |
| 2 | charm, strange | muon, muon neutrino |
| 3 | top, bottom | tau, tau neutrino |
This organization forms an important part of the Standard Model of particle physics.
Did You Know?
A proton is often drawn as if it simply contains three small balls representing two up quarks and one down quark.
The real situation is much more complex.
Inside a proton is a highly dynamic system involving:
- three valence quarks
- gluons
- temporary quark-antiquark pairs
Much of the proton's mass does not come directly from simply adding the masses of its three valence quarks. A large fraction arises from the energy associated with the strong interaction and the motion of particles inside the proton.
So the familiar uud description identifies the proton's valence-quark composition, but the proton's internal structure is considerably richer.
Key Terms
Quark – A fundamental particle that experiences the strong interaction.
Flavour – The type of quark, such as up, down, strange, charm, top, or bottom.
Hadron – A composite particle made from quarks and bound by the strong interaction.
Baryon – A type of hadron whose ordinary examples, such as protons and neutrons, contain three valence quarks.
Meson – A hadron consisting of a quark-antiquark pair.
Gluon – A force-carrying particle associated with the strong interaction.
Lepton – A fundamental particle that does not experience the strong interaction.
Antiquark – The antiparticle corresponding to a quark.
Quark confinement – The phenomenon that prevents quarks from normally being observed individually.
Valence quark – A quark contributing to the defining quantum properties of a hadron.
Key Takeaways
- Quarks are fundamental particles.
- There are six quark flavours: up, down, charm, strange, top, and bottom.
- Quarks have fractional electric charges of +⅔ or −⅓.
- Quarks combine to form composite particles called hadrons.
- A proton has valence-quark composition uud.
- A neutron has valence-quark composition udd.
- The charges of the individual quarks combine to produce the overall charge of a proton or neutron.
- Quarks are held together through the strong interaction, mediated by gluons.
- Quarks are not normally observed individually because of quark confinement.
- Quarks experience the strong interaction, while leptons do not.
- Quarks and leptons form the two major families of fundamental matter particles in the Standard Model.