Mesons (AQA A Level Physics): Revision Note

Pions & The Strong Nuclear Force

Pions

• Pions (π–mesons) can be positive $\left({\mathrm{\pi }}^{+}\right)$, negative $\left({\mathrm{\pi }}^{-}\right)$, or neutral $\left({\mathrm{\pi }}^0\right)$

• They contain up and down quarks only, so they have zero strangeness

• The antiparticle of the positive pion is the negative pion (and vice versa)

  • The neutral pion is its own antiparticle

• Pions are the lightest mesons, making them more stable than other types of mesons

  • They were originally discovered in cosmic rays and can be observed in a cloud chamber

Table of pions and their properties

Pions as exchange particles

• The strong nuclear force, one of the four fundamental interactions, keeps the protons and neutrons bound together in a nucleus

• Fundamental interactions are mediated by exchange particles

• The pion is the exchange particle of the strong nuclear force between baryons

• In a nucleus, protons and neutrons exhibit the strong force by exchanging pions

  • 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

Kaon Decay

Kaons

• Kaons (K-mesons) can also be positive $\left(K^{+}\right)$, negative $\left(K^{-}\right)$, or neutral $\left(K^0\right)$

• Kaons have a property known as strangeness

  • This is because they contain a strange quark, which can be paired with either an up or down quark

• The antiparticle of the positive kaon is the negative kaon (and vice versa)

  • Unlike the neutral pion, the neutral kaon is not its own antiparticle

• Kaons are heavy and unstable and normally decay into pions

• They are known to have unusually long lifetimes (compared to other mesons)

  • This is characteristic of particles containing strange quarks

Table of kaons and their properties

Kaon production

• Kaons can be produced in pairs (pair production) via the strong interaction, where strangeness is always conserved

• An example of kaon pair production $\left({\mathrm{K}}^{+}/{\mathrm{K}}^{-}\right)$ is through a high-energy proton-proton collision:

$\mathrm{p}+\mathrm{p}\stackrel{}{\to }\mathrm{p}+\mathrm{p}+K^{+}+K^{-}$

• The strangeness on both sides is zero, so strangeness is conserved

Kaon decay

• Kaons decay through the weak interaction

• An example of a kaon decay would be a neutral kaon decaying into a positive pion and a negative pion:

$K^0\to {\mathrm{\pi }}^{+}+{\mathrm{\pi }}^{-}$

• Pions do not contain strange quarks, therefore, strangeness is not conserved in weak interactions