Baryons (AQA A Level Physics)

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)

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