Neutral current scattering

• Electron scattering

• $e^+e^-$ annihilation

Neutrino scattering

• Neutral current scattering

• Charged current scattering

Charged pion decay

The charge pion $\pi^+$ composed of $u\overline d$ has the following constraints

• It cannot decay with the strong force since its already the lightest charges meson
• It cannot decay via EM since that conserves quark flavour
• Thus it can decay only using the weak decay

🗒️ Note: pions cannot decay into $\tau \nu$ because there isn't enough energy, $e\nu$ is supressed see later

Lepton decay

• Muons have a lifetime of $2.2\; \rm \mu s$ and decay as follows

  

• Tau have a lifetime of $290\, \rm fm$ and can decay into hadrons or leptons

• 1-prong decay

  

The $W^-$ can decay into more $\pi$ depending on energy and these can decay again

• 3-prong decay (if enough energy)

Lepton universality

The coupling strength of a $W$ and $Z$ boson to a lepton is independent of the lepton flavour

🗒️ Note: these have the same coupling strength and neglecting phase space are equally likely

Lepton-quark symmetry

The coupling strength of a $W$ to quarks is the same as the coupling strength of a $W$ to lepton

🗒️ Note: There an additional factor $3$ for colour