2.2.3 Mesons

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 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 interaction. 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 conservation but since it is a virtual particle, it is not directly observed

• The gluon is also an exchange particle of the strong force. The difference between pions and gluons as mediators of this force are:
  • Gluons are responsible for binding quarks together. This is referred to as the strong interaction
  • Pions are responsible for binding nucleons together. This is referred to as the strong nuclear force

• Collectively, these are referred to as the strong force

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

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: