Quark Compositions | CIE AS Physics Revision Notes 2025

Protons & neutrons

Protons and neutrons are not fundamental particles
They are each made up of three quarks
Protons are made up of two up quarks and a down quark
Neutrons are made up of two down quarks and an up quark

Composition of protons & neutrons

Quarks in proton and neutrons, downloadable AS & A Level Physics revision notes

Protons and neutrons are made up of three quarks

You will be expected to remember these quark combinations for exam questions

Worked example

State how many 'up' quarks there are in the nucleus of iron, ${}_{26}^{56}{Fe}$ .

Answer:

Step 1: Calculate number of protons

The number of protons is from the proton number = 26 protons

Step 2: Calculate number of neutrons

The number of neutrons = nucleon number - proton number = 56 - 26 = 30 neutrons

Step 3: Up quarks in a proton

Protons are made up of uud quarks = 2 up quarks

Step 4: Up quarks in a neutron

Neutrons are made up of udd quarks = 1 up quark

Step 5: Total number of up quarks

26 protons x 2 up quarks = 52 up quarks
30 neutrons x 1 up quark = 30 up quarks
52 + 30 = 82 up quarks

Baryons & mesons

Hadrons are the group of subatomic particles that are made up of quarks
Hadrons can be either:
- Baryons
  - Made up of 3 quarks
- Mesons
  - Made up of a quark and anti-quark pair

Hadrons

Baryons & Mesons

Hadrons may be either a baryon or a meson

Anti-hadrons can be either:
- Anti-baryons
  - Made up of 3 anti-quarks
- Anti-mesons
- Made up of a quark and anti-quark pair

Anti-hadrons

Anti-hadrons may be either an anti-baryon or an anti-meson

Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
This means quarks in a baryon are either all quarks or all anti-quarks
- Combinations of quarks and anti-quarks do not exist in a baryon

$\bar{u} ud$ would not be a quark combination that exists

The anti-particle of a meson is still a quark-antiquark pair
However, the quark becomes the anti-quark and vice versa

Worked example

The baryon Δ⁺⁺ was discovered in a particle accelerator using accelerated positive pions on hydrogen targets.

Which of the following is the quark combination of this particle?

A: $uuu$ B: $cd \bar{s}$ C: $\bar{u} d$ D: $\bar{c} \bar{c} \bar{c}$

Answer: A

Since it is a baryon, its made up of three quarks
- This rules out option C
Baryons are made up of 3 quarks
Anti-baryons are made up of 3 anti-quarks
- This rules out option B
- There cannot be a combination of quarks and anti-quarks
The baryon Δ⁺⁺ has a charge of 2+
Adding up the charges of quarks
- A: $uuu = + \frac{2}{3} + \frac{2}{3} + \frac{2}{3} = + 2$
- D: $\bar{c} \bar{c} \bar{c} = - \frac{2}{3} - \frac{2}{3} - \frac{2}{3} = - 2$
Therefore, the correct combination is option A

Examiner Tip

Remembering quark combinations is useful for the exam. However, as long as you can remember the charges for each quark, it is possible to figure out the combination by making sure the combination of quarks add up to the charge of the particle (just like in the worked example)

Quark Compositions (CIE AS Physics)

Revision Note

Protons & neutrons

Composition of protons & neutrons

Worked example

Baryons & mesons

Hadrons

Anti-hadrons

Worked example

Examiner Tip

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Author: Leander