# The Missing Three-Nucleon Forces: Where Are They?

## TL;DR

This paper reviews the progress and challenges in deriving three-nucleon forces from chiral effective field theory, highlighting the need for N3LO forces and discussing potential implications if they are weak.

## Contribution

It provides an analysis of the current status of chiral three-nucleon forces at N3LO and explores possible scenarios for future developments in nuclear force modeling.

## Key findings

- N3LO three-nucleon forces may be weaker than expected
- Current N2LO forces improve some predictions but leave unresolved issues
- Further research needed to determine the role of N3LO forces

## Abstract

In recent years, there has been substantial progress in the derivation of nuclear forces from chiral effective field theory. Accurate two-nucleon forces (2NF) have been constructed up to next-to-next-to-next-to-leading order (N3LO) of chiral perturbation theory and applied in microscopic nuclear structure calculations with a good degree of success. However, chiral three-nucleon forces (3NF) have been used only at N2LO, improving some miscroscopic predictions, but leaving also several issues, like the "Ay puzzle'" of nucleon-deuteron scattering, unresolved. Thus, the 3NF at N3LO is needed for essentially two reasons: For consistency with the 2NF, and to (hopefully) improve some critical predictions of nuclear structure and reactions. However, there are indications that the 3NF at N3LO (in the so-called Delta-less version of the theory) is rather weak and may not solve any of the outstanding problems. If this suspicion is confirmed, we have to go beyond, which may be similar to opening Pandora's Box. In this talk, I will discuss the various possible scenarios and how to deal with them.