AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Combinations of quarks and antiquarks required for baryons (proton and…
Full specification wording
Quarks and antiquarks: Combinations of quarks and antiquarks required for baryons (proton and neutron only), antibaryons (antiproton and antineutron only) and mesons (pion and kaon only).
Baryons are made from three quarks, antibaryons from three antiquarks, and mesons from one quark and one antiquark.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Baryons are made from three quarks, antibaryons from three antiquarks, and mesons from one quark and one antiquark. For the particles required here, the proton is uud and the neutron is udd. Their antiparticles reverse every quark to its antiquark: the antiproton is ūūd̄ and the antineutron is ūd̄d̄. Mesons contain a quark–antiquark pair: the π⁺ is ud̄, the π⁻ is ūd, the π⁰ is a superposition of uū and dd̄, the K⁺ is us̄, the K⁻ is ūs, the K⁰ is ds̄ and the anti-K⁰ is d̄s. Checking charge, baryon number and strangeness confirms each combination.
What earns the marks
- Recall that baryons contain three quarks and antibaryons contain three antiquarks, while mesons contain one quark and one antiquark.
- Give the quark composition of the proton as uud and the neutron as udd.
- Give the quark composition of the antiproton as ūūd̄ and the antineutron as ūd̄d̄.
- Give the quark composition of the pion as ud̄, ūd or a uū/dd̄ combination for the neutral pion.
- Give the quark composition of the kaon as us̄, ūs, ds̄ or d̄s according to its charge and strangeness.
- Check a proposed combination by adding charges, baryon numbers and strangeness values to confirm it matches the required particle.
Common mistakes and how to put them right
Writing a meson as three quarks or a baryon as a quark–antiquark pair.
Put it right: Baryons have three quarks, mesons have one quark and one antiquark.
Giving the antiproton as ūūd instead of ūūd̄, which leaves the wrong charge and baryon number.
Confusing the neutral kaon with the neutral pion.
Put it right: K⁰ contains a strange quark or antiquark, so it has non-zero strangeness, whereas π⁰ does not.
Exam tips
- Learn the quark compositions of the proton, neutron, antiproton, antineutron, pion and kaon as required, and practise writing them with clear antiquark symbols.
- For each particle, verify charge and baryon number from the quark content before committing to an answer.
- When a question asks for a meson, remember that the quark and antiquark need not be of the same flavour, as in K⁺ = us̄.
Try the lesson
The start of the Quarks and antiquarks: Combinations of quarks and antiquarks required for baryons (proton and neutron only), antibaryons (antiproton and antineutron only) and mesons (pion and kaon only). lesson, made from checked teaching notes for this specification point.
Have a go first
Which combination of quarks makes up a proton?
Listen
Baryons are made from three quarks, antibaryons from three antiquarks, and mesons from one quark and one antiquark. For the particles required here, the proton is uud and the neutron is udd. Their antiparticles reverse every quark to its antiquark: the antiproton is ūūd̄ and the antineutron is ūd̄d̄. Mesons contain a quark–antiquark pair: the π⁺ is ud̄, the π⁻ is ūd, the π⁰ is a superposition of uū and dd̄, the K⁺ is us̄, the K⁻ is ūs, the K⁰ is ds̄ and the anti-K⁰ is d̄s. Checking charge, baryon number and strangeness confirms each combination. A strong answer earns marks for these points. Recall that baryons contain three quarks and antibaryons contain three antiquarks, while mesons contain one quark and one antiquark. Give the quark composition of the proton as uud and the neutron as udd. Give the quark composition of the antiproton as ūūd̄ and the antineutron as ūd̄d̄. Give the quark composition of the pion as ud̄, ūd or a uū/dd̄ combination for the neutral pion. Exam tip: Learn the quark compositions of the proton, neutron, antiproton, antineutron, pion and kaon as required, and practise writing them with clear antiquark symbols.
See it
Key words
Say what each word means before you turn the card.
Baryons
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Antibaryons
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Mesons
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See every definition
- Baryons
- Particles made from three quarks, with the proton given as uud and the neutron as udd.
- Antibaryons
- Particles made from three antiquarks, with the antiproton given as ūūd̄ and the antineutron as ūd̄d̄.
- Mesons
- Particles containing one quark and one antiquark, such as the pion and the kaon.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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