Four-nucleon system with $\Delta$-isobar excitation

Understand

The four-nucleon bound state and scattering below three-body breakup threshold are described based on the realistic coupled-channel potential CD Bonn + $\Delta$ which allows the excitation of a single nucleon to a $\Delta$ isobar.

• The Coulomb repulsion between protons is included.
• In the four-nucleon system the two-baryon coupled-channel potential yields effective two-, three- and four-nucleon forces, mediated by the $\Delta$ isobar and consistent with each other and with the underlying two-nucleon force.
• The effect of the four-nucleon force on the studied observables is much smaller than the effect of the three-nucleon force.