Trinucleon photonuclear reactions with $\Delta$-isobar excitation: Processes below pion-production threshold

Radiative nucleon-deuteron capture and two- and three-body photo disintegration of the three-nucleon bound state are described. The description uses the purely nucleonic charge-dependent CD-Bonn potential and its coupled-channel extension CD Bonn + $\Delta$. The $\Delta$-isobar excitation yields an effective three-nucleon force and effective two- and three-nucleon currents besides other $\Delta$-isobar effects; they are mutually consistent. Exact solutions of three-particle equations are employed for the initial and final states of the reactions. The current has one-baryon and two-baryon contributions and couples nucleonic with $\Delta$-isobar channels. $\Delta$-isobar effects on the observables are isolated. Shortcomings of the theoretical description are discussed and their consequence for the calculation of observables is estimated.