Off-Shell Longitudinal Photon Light-Cone Wavefunction in the Low-Energy Effective Theory of QCD

Within the framework of the low-energy effective theory arising from the instanton vacuum model of QCD, the longitudinal virtual photon light-cone wavefunction, Ф_γ||(u,P²), corresponding to the nonlocal quark--antiquark vector current is calculated at the low-energy scale. The coupling constant, F_γ(P²) or equivalently f_γ(P²), of the quark--antiquark vector current to the virtual photon state is also obtained by imposing the normalization condition to the photon wavefunction. The behaviour of the coupling constant as well as the obtained photon wavefunction is discussed.