# 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.

### 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

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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