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

TL;DR

This paper models the four-nucleon system including $0$-isobar excitation using a coupled-channel potential, analyzing bound states and scattering with realistic forces and Coulomb effects, but finds limited resolution of experimental discrepancies.

Contribution

It introduces a coupled-channel potential incorporating $0$-isobar excitation for four-nucleon systems and assesses its impact on observables.

Findings

Four-nucleon forces are smaller than three-nucleon forces.

Including $0$-isobar does not fully resolve experimental discrepancies.

The potential yields effective multi-nucleon forces consistent with two-nucleon interactions.

Abstract

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. The inclusion of the $\Delta$ isobar is unable to resolve the existing discrepancies with the experimental data.