Exchange effects and SU4 invariance in electromagnetic and weak transitions

Here V and A denote the vector and axial vector hadronic currents, with the upper index referring to isospin and the lower one to Lorentz space (k=l, 2 and 3). Particular emphasis is given to the treatment of muon capture. The equality between the vector and axial vector matrix elements and the connection of the vector matrix elements to the photoabsorption cross section are discussed for this process. Finally we draw some conclusions about many-body effects in electromagnetic and weak nuclear transitions from the current algebra and SU4 invariance for the nuclear Hamiltonian.