7.3.3 Hadrons, Baryons & Mesons

• Hadrons are the group of subatomic particles that are made up of quarks
• These may be either a:
  • Baryon (3 quarks)
  • Meson (quark and anti-quark pair)

Hadrons may be either a baryon or a meson

• Quarks have never been discovered on their own, always in pairs or groups of three
• Anti-hadrons can be either
  • Anti-baryons (3 anti-quarks)
  • Anti-meson (quark and anti-quark pair)

Anti-hadrons may be either an anti-baryon or an anti-meson

• Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
• This means quarks in a baryon are either all quarks or all anti-quarks. Combination of quarks and anti-quarks don’t exist in a baryon
  • e.g.

    

• The anti-particle of a meson is still a quark-antiquark pair. The difference being the quark becomes the anti-quark and vice versa

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 data booklet
• 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 discovered) that have a combination of quarks and anti-quarks eg. up, anti-down, down
• The reason is that this would equate to a baryon number that is not a whole number (integer)

Mesons

• Mesons are composed of a quark and an anti-quark pair
• Mesons will have a baryon number of 0
• They can have a charge of -1, 0 or +1
• Types of mesons include pions, kaons, D mesons, B mesons and many other varieties

Pions

• Pions (π–mesons) can be positive (π⁺), negative (π^–) or neutral (π⁰)
• The anti–particle of the positive pion is the negative pion (and vice versa)
  • The neutral pion is its own anti–particle

Pions consist of up and down quarks, and their respective antiparticles, depending on their sign

• Pions are the lightest mesons, making them more stable than other types of meson
• They were originally discovered in cosmic rays and can be observed in a cloud chamber
• The strong nuclear force keeps the protons and neutrons bound together in a nucleus and is one of the four fundamental interactions. Each of these interactions is caused by a particle exchange
• The pion is the exchange particle of the strong nuclear force
• This means that the strong force is transmitted between a proton and neutron by the exchange of a pion
  • Pions are said to mediate (bring about) the strong nuclear force

• The pion created is a temporary violation of energy and mass conversation but since it is a virtual particle, it is not directly observed

Kaons

• Kaons (K–mesons) can also be positive (K⁺), negative (K^–) or neutral (K⁰)
• The anti–particle of the positive kaon is the negative kaon (and vice versa)
  • The neutral kaon is its own anti–particle

• Kaons can be produced by the strong interaction between pions and protons

Kaons consist of up, down and strange quarks, and their respective antiparticles, depending on their sign

Kaon Decay

• Kaons are heavy and unstable and normally decay into pions
• They are known to have unusually long lifetimes
  • This is because kaons contain a strange quark and longer lifetimes are characteristic of particles containing strange quarks

• Kaons decay through the weak interaction
• An example of a kaon decay would be a neutral kaon decaying into a positive pion and negative pion: