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Question about the matrix of vacuum to meson

  1. Mar 12, 2013 #1
    I have some questions about the matrix
    1.Pi is a psuedoscalar particle, but the current between vacuum and pi state is an axial one, why not a psuedoscalar one?
    And how do we determine the current appearing in the similar matrix like [tex]<0|j|meson>[/tex]

    2.The r.h.s is propotional to the momentum of the meson pi, but in the vector current case it is propotional to the polarization and mass
    [tex]<0|j^{\mu}|Vector(p)>=if_{V}m_{V}e^{\mu}[/tex], why?
  2. jcsd
  3. Mar 13, 2013 #2
    That matrix element would also be nonzero. When we use an axial-vector current, the LHS is scalar * axial-vector * pseudoscalar and so the RHS is a vector (proportional to p). If we used a pseudoscalar operator, the LHS would be scalar * pseudoscalar * pseudoscalar and the RHS would be some scalar. Since the axial-vector matrix element is (part of) what determines the rate of weak decays of mesons, it seems appropriate to define the "decay constant" via the axial-vector matrix element.

    To define the decay constant of a given meson, you use the axial current with the same flavor quantum numbers as the meson. For example, for the decay constant of the ##K^+##, you use the current ##\bar{s}\gamma^\mu\gamma_5 u##.
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