AQA A Level Physics

Revision Notes

2.2.2 Baryons

Baryon Number

  • The baryon number, B, is the number of baryons in an interaction
  • B depends on whether the particle is a baryon, anti-baryon or neither
    • Baryons have a baryon number B = +1
    • Anti-baryons have a baryon number B = –1
    • Particles that are not baryons have a baryon number B = 0
  • Baryon number is a quantum number and is conserved in all interactions
    • This is one of the indicators for whether an interaction is able to occur or not


The baryon number of a particle depends if it is a baryon, anti–baryon or neither 

  • The up (u), down (d) and strange (s) quark have a baryon number of 1/3 each
  • This means that the anti–up, anti–down and anti–strange quarks have a baryon number of –1/3 each
  • Note: The baryon number of each quark is provided on the datasheet


  • The implication of this is that baryons are made up of all quarks and anti-baryons are made up of all anti-quarks
  • There are no baryons (yet) that have a combination of quarks and anti-quarks eg. up, anti-down, down
  • The reason being that this would equate to a baryon number that is not a whole number (integer)

Worked Example

Show that the baryon number is conserved in β decay.

2.1.5 Annihilation Worked Example

The Proton as a Baryon

  • The proton is the most stable baryon
  • This means it has the longest half-life of any baryon and is the particle which other baryons eventually decay to
  • It is the most stable baryon because it is also the lightest baryon
    • Radioactive decay occurs when heavier particles decay into lighter particles
    • A decay of the proton would therefore violate the conservation of baryon number
  • It is theorised that the proton has a half-life of around 1032 years and research experiments are still underway that are designed to detect proton decay

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