# Luneburg-lens-like structural Pauli attractive core of nuclear force at short distances

## TL;DR

This paper reveals that the short-distance nuclear force, traditionally viewed as repulsive, actually has a Luneburg-lens-like attractive core structure that mimics repulsion due to Pauli forbidden states.

## Contribution

It introduces the concept of a Luneburg-lens-like structural Pauli attractive core (SPAC) in nuclear forces, challenging the conventional view of a purely repulsive core.

## Key findings

- The nuclear force with an attractive short-distance potential reproduces experimental phase shifts.
- The attractive core has a Luneburg-lens-like structure causing apparent repulsion.
- Apparent repulsion arises from embedded Pauli forbidden states.

## Abstract

The nuclear force has been understood to have a repulsive core at short distances, similar to a molecular force, since Jastrow proposed it in 1951. The existence of the repulsion was experimentally confirmed from the proton-proton scattering 1S_0 phase shift, which becomes negative beyond 230 MeV. This repulsion is essential for preventing the nucleon-nucleon system from collapsing by attraction. The origin of the repulsion has been considered to be due to the Pauli principle, similar to the repulsion originally revealed in alpha-alpha scattering, in many studies including recent lattice QCD calculations. On the other hand, very recently it was shown that an inter-nuclear potential including alpha-alpha interactions has a Luneburg-lens-like attraction at short distances rather than repulsion. We show that the nuclear force with an attractive potential at short distances that reproduces the experimental phase shifts well has a Luneburg-lens-like structural Pauli attractive core (SPAC) at short distances and acts as apparent repulsion. The apparent repulsion is caused by the deeply embedded unobservable Pauli forbidden state similar to nucleus-nucleus potentials.