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Tuesday, December 13, 2011


  • 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 ¯

    Neutral pion decay:


  • Muon decay

    μ+→e+e+ ν μ ¯

    μ-→e-μ+ ν 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

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