Leptons

• particles which are untouched by strong forces and which participates in weak interactions are called leptons.
• All these particles (leptons) also have their corresponding anti particles.
• Unlike baryons and mesons no heavier leptons have been detected.
• Electrons with its anti particle positron along with its associated neutrino and anti nutrino are leptons.
• Muon with their respective neutrino and anti particles of these are also leptons. Another pair of leptons known as tau,τ and its associated neutrino.
• All tau's are charged and decay into electrons, muons or pions along with appropriate neutrino.
• The breakdown of parity conservation in weak interaction has important consequence for leptons.
• The particles are left handed and anti particles are right handed.
• In normal weak interactions the particles (electrons, negative muons, neutrinos) behaves as if they were left handed screws; i.e., observer think they spin clockwise when they are travelling towards him.
• Anti particles are right handed screws; an observer thinks they are spinning counter clockwise as they approaches him.
• Nature of weak interactions, with its voilation of such established symmetries as parity, charge conjunction, isotopic spin and strangeness is physics greatest problems.
• Muons were first discovered in decay of charged pions

  Charged pion decay:

  π⁺→μ⁺+ν_μ

  π^-→μ^-+$\overline{{\nu }_{\mu }}$

  Neutral pion decay:

  π⁰→γ+γ

• Muon decay

  μ⁺→e⁺+ν_e+$\overline{{\nu }_{\mu }}$

  μ^-→e^-+ν_μ+$\overline{{\nu }_e}$

Leptons have (B=0) (fermions) and their properties are

Particle	Symbol	Spin	Le	Lμ	Lτ	Mass (in MeV)	Mean Life
electron	e-	1/2	+1	0	0	0.511	stable
muon	μ-	1/2	0	+1	0	106	2.2×10-6
tau	τ-	1/2	0	0	+1	1784	3.4×10-25
electron neutrino	νe	1/2	+1	0	0	0	stable
μ neutrino	νμ	1/2	0	+1	0	0	stable
τ neutrino	ντ	1/2	0	0	+1	0	stable