Sept 28-29, Structure of a Lagrangian Example: Consider a spin-1/2 (Dirac) particle ( nucleon ) interacting with a spin-0 (Scalar) object ( meson ) Meson potential Nucleon field Nucleon – meson interaction Lorentz structure: interactions can be implemented using combinations of: S: Scalar currents : 1 P: Pseudoscalar currents :  5 V: Vector currents :   A: Axial vector currents :    5 T: Tensor currents :    Dirac field  Exercise: What are the symmetries of this theory under C, P, CP .
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