Abstract
We present here a method for incorporating electromagnetic interaction into the covariant confined quark model (CCQM) while preserving local \(U(1)\) gauge invariance. We consider nonlocal interpolating quark currents describing heavy quarkonium states, and introduce gauge-invariant phase factors for the quark fields. This approach, previously applied in the nonlocal chiral quark model, is extended here to charmonium states including those with derivative structures in their currents. Electromagnetic interaction terms are derived up to first order in the gauge field, including expressions for nonlocal vertices involving single-photon emission, which are essential for describing radiative transitions. The method ensures consistency with fundamental symmetry principles and provides a solid basis for phenomenological studies of heavy quarkonium decays.
