AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The two classes of hadrons: baryons (proton, neutron) and antibaryons…
Full specification wording
Classification of particles: The two classes of hadrons: baryons (proton, neutron) and antibaryons (antiproton and antineutron) mesons (pion, kaon).
Hadrons are particles that experience the strong interaction and are built from quarks.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 15
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Hadrons are particles that experience the strong interaction and are built from quarks. They divide into baryons and mesons. Baryons contain three quarks, for example the proton (uud) and neutron (udd); antibaryons contain three antiquarks, for example the antiproton and antineutron. Mesons contain one quark and one antiquark, for example the pion and kaon. A practical way to classify an unfamiliar particle is to count its quark content: three quarks or three antiquarks means baryon or antibaryon, while a quark–antiquark pair means meson. Charge, baryon number and strangeness then help identify the specific particle. Recognising these patterns lets you predict which particles can be produced in collisions and which decay routes are allowed.
What earns the marks
- State that hadrons are particles that interact through the strong interaction and are composed of quarks.
- Distinguish baryons as three-quark states, giving proton (uud) and neutron (udd) as examples.
- Distinguish antibaryons as three-antiquark states, giving the antiproton and antineutron as examples.
- Distinguish mesons as quark–antiquark states, giving the pion and kaon as examples.
- Use quark content, charge and baryon number to classify an unfamiliar hadron correctly.
Common mistakes and how to put them right
Treating the antiproton as a meson because it is an antiparticle.
Put it right by noting it contains three antiquarks, so it is an antibaryon.
Assuming all hadrons contain three quarks.
Put it right by recalling that mesons contain one quark and one antiquark.
Confusing leptons with hadrons.
Put it right by stating that leptons do not interact through the strong interaction and are not made of quarks.
Exam tips
- When asked to classify a particle, first count the quarks or antiquarks and state the class before giving its quark composition.
- Quote the quark composition of named baryons and mesons accurately, for example proton uud and neutron udd.
- Link each class to the interaction it experiences, since hadrons feel the strong interaction whereas leptons do not.
Try the lesson
The start of the Classification of particles: The two classes of hadrons: baryons (proton, neutron) and antibaryons (antiproton and antineutron) mesons (pion, kaon). lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following particles is a meson?
Listen
Hadrons are particles that experience the strong interaction and are built from quarks. They divide into baryons and mesons. Baryons contain three quarks, for example the proton (uud) and neutron (udd); antibaryons contain three antiquarks, for example the antiproton and antineutron. Mesons contain one quark and one antiquark, for example the pion and kaon. A practical way to classify an unfamiliar particle is to count its quark content: three quarks or three antiquarks means baryon or antibaryon, while a quark–antiquark pair means meson. Charge, baryon number and strangeness then help identify the specific particle. Recognising these patterns lets you predict which particles can be produced in collisions and which decay routes are allowed. A strong answer earns marks for these points. State that hadrons are particles that interact through the strong interaction and are composed of quarks. Distinguish baryons as three-quark states, giving proton (uud) and neutron (udd) as examples. Distinguish antibaryons as three-antiquark states, giving the antiproton and antineutron as examples. Distinguish mesons as quark–antiquark states, giving the pion and kaon as examples. A common slip: Treating the antiproton as a meson because it is an antiparticle. Instead, noting it contains three antiquarks, so it is an antibaryon. Exam tip: When asked to classify a particle, first count the quarks or antiquarks and state the class before giving its quark composition.
See it
Key words
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Quark model of hadrons
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Baryons and antibaryons
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See every definition
- Quark model of hadrons
- Hadrons are particles that experience the strong interaction and are built from quarks.
- Baryons and antibaryons
- Baryons contain three quarks, for example the proton (uud) and neutron (udd); antibaryons contain three antiquarks, for example the antiproton and antineutron.
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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