AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The proton is the only stable baryon into which other baryons eventually…
Full specification wording
Classification of particles: The proton is the only stable baryon into which other baryons eventually decay.
Baryons heavier than the proton are unstable and decay through a chain of processes until a proton is produced.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 5 of 15
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Baryons heavier than the proton are unstable and decay through a chain of processes until a proton is produced. For example, a free neutron decays by beta-minus decay into a proton, an electron and an antineutrino, with a half-life of about 10 minutes. A lambda-zero baryon can decay into a proton and a pion. Because the proton has the lowest rest energy of all baryons, no further baryon decay is energetically possible, so it is stable in the standard model. This does not mean every decay produces a proton immediately; it means that following any baryon decay chain eventually ends at a proton. The statement is about baryons only, so it does not claim that leptons or mesons are stable.
What earns the marks
- State that the proton is stable and is the lightest baryon.
- Explain that heavier baryons decay in one or more steps until a proton is formed.
- Give a valid example, such as neutron beta-minus decay to a proton, electron and antineutrino.
- Link stability to the proton having the lowest rest energy among baryons, so baryon-number-conserving decay to a lighter baryon is impossible.
- Distinguish baryon stability from the stability of other particles such as electrons or neutrinos.
Common mistakes and how to put them right
Claiming that all baryons are stable.
Put it right by stating that only the proton is stable and other baryons eventually decay to it.
Saying the proton decays into a neutron.
Put it right by noting that a neutron is more massive, so this decay is not energetically possible for a free proton.
Confusing the stability of the proton with the stability of the electron.
Put it right by keeping the statement specific to baryons.
Exam tips
- Use the phrase 'eventually decay' to show you understand that decay may occur through intermediate baryons.
- Support your answer with a named decay equation, such as neutron to proton plus electron plus antineutrino.
- If asked why the proton is stable, refer to it being the lightest baryon and to baryon number conservation.
Try the lesson
The start of the Classification of particles: The proton is the only stable baryon into which other baryons eventually decay. lesson, made from checked teaching notes for this specification point.
Have a go first
Which baryon is the only stable baryon into which other baryons eventually decay?
Listen
Baryons heavier than the proton are unstable and decay through a chain of processes until a proton is produced. For example, a free neutron decays by beta-minus decay into a proton, an electron and an antineutrino, with a half-life of about 10 minutes. A lambda-zero baryon can decay into a proton and a pion. Because the proton has the lowest rest energy of all baryons, no further baryon decay is energetically possible, so it is stable in the standard model. This does not mean every decay produces a proton immediately; it means that following any baryon decay chain eventually ends at a proton. The statement is about baryons only, so it does not claim that leptons or mesons are stable. A strong answer earns marks for these points. State that the proton is stable and is the lightest baryon. Explain that heavier baryons decay in one or more steps until a proton is formed. Give a valid example, such as neutron beta-minus decay to a proton, electron and antineutrino. Link stability to the proton having the lowest rest energy among baryons, so baryon-number-conserving decay to a lighter baryon is impossible. A common slip: Claiming that all baryons are stable. Instead, stating that only the proton is stable and other baryons eventually decay to it. Exam tip: Use the phrase 'eventually decay' to show you understand that decay may occur through intermediate baryons.
Key words
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Particle stability and decay chains
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Rest energy and energetically allowed decays
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Classification of fundamental and composite particles
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See every definition
- Particle stability and decay chains
- Baryons heavier than the proton are unstable and decay through a chain of processes until a proton is produced.
- Rest energy and energetically allowed decays
- The proton has the lowest rest energy of all baryons, so no further baryon decay is energetically possible.
- Classification of fundamental and composite particles
- The statement is about baryons only, so it does not claim that leptons or mesons are stable.
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