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Tuesday, January 17, 2012

Symmetries and conservation Laws: Part 2

Baryon and Lepton numbers

  • The Baryon number B=1 is assigned to all baryons and B=-1 is assigned to all anti baryons: all other particles have B=0.
  • The lepton number Le=1 is assigned to electrons and electron neutrino, Le=-1 to their anti particles ; all other particles have Le=0
  • Lμ=1 for muon and μ-neutrino and Lτ=1 for tau lepton and its neutrino.
  • Significance of these numbers is that , in every process of whwtever kind , the total valuse of B, Le,Lμ,Lτ separately remains constant.
  • Conservation of leptons has a signefence for strond interactions.
  • Another property that is conserved only in strong interactions is isospin.


  • A number of particles were discovered that behsve so unexpectedly that they were called strange particles.
  • They were created in pairs, and decay only in certain ways but not in others that were allowed by existing conservation laws.
  • To clarify the observation Gell-Mann and Nishijina indepensently introduced the strangeness number.
  • For photon , π0 and η0, B, Le,Lμ, Lτ and S are zero . There is no way to distinguish between them and their antiparticles, and they are regarded as their own anti particles.
  • Strangeness number is conserved in all processes mediated by strong and electromagnetic interactions.
  • The multiple creation of particles with S≠0 is the result of this conservation principle
  • S can change in an event mediated by the week interaction. Decays that proceed via a week interaction are relatively slow, a billion tomes slower than the interactions proceeded via strong interactions.
  • Week interactions does not allow S to change by more than ±1 in a decay. For example,

    Ξ- decays in two steps Ξ- →Λ0-→η00


  • There are number of hadron families whose numbers have similar masses but different charges. Thes families are called multiplets. Member of multiplet represents different charged states of a single fundemental entity.
  • Each multiplet according to number of charge states exhibits a number I such that the multiplicity of state is given by 2I+1.
  • Isospin can be represented by vector I in an abstract iso space whose component in any specific direction is governed by the quantum number denoted by I3.
  • Possible values of I3 varies from I, I-1 to -I. The charge of a baryon is related to its baryon numberB, its strangeness number S and the component I3 of its isotopic spin by the formula

    Q = e ( I 3 + B 2 + S 2 )

Conservation of statistics

  • The interchange of identicle particles in a system is a type of symmetry operation which leads to the preservation of the wave
  • Conservation of statistics signifies that no process occuring within an isolated system can change the statistical behaviour of the system.


  • Hypercharge is defined as Y=S+B
  • Classification system for hadrons encompasses many short lived particles as well as relatively stable hadrons
  • This scheme cillects isospin multiplets into submultiplets whose members have the same spin but different in isospin and a quantity called hypercharge.

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