## Tuesday, December 13, 2011

### 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+μ

π-→μ-+$\overline{{\nu }_{\mu }}$

Neutral pion decay:

π0→γ+γ

• Muon decay

μ+→e+e+$\overline{{\nu }_{\mu }}$

μ-→e-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