# The Sea of Quarks and Antiquarks in the Nucleon: a Review

## TL;DR

This review discusses the complex quark and antiquark structure of the proton, highlighting experimental findings, theoretical challenges, and the non-perturbative components of the sea quark distributions that remain not fully understood.

## Contribution

It provides a comprehensive overview of the current experimental and theoretical understanding of the proton's sea quark structure, emphasizing unresolved non-perturbative aspects.

## Key findings

- Non-perturbative sea quark components are not well understood.
- Experimental data show features not fully captured by existing models.
- Future research directions are outlined for better understanding of proton structure.

## Abstract

The quark and gluon structure of the proton has been under intense experimental and theoretical investigation for five decades. Even for the distributions of the well-studied valence quarks, challenges such as the value of the down quark to up quark ratio at high fractional momenta remain. Much of the sea of quark-antiquark pairs emerges from the splitting of gluons and is well described by perturbative evolution in quantum chromodynamics. However, experiments confirm that there is a non-perturbative component to the sea that is not well understood and hitherto has been difficult to calculate with ab initio non-perturbative methods. This non-perturbative structure shows up, perhaps most directly, in the flavor dependence of the sea antiquark distributions. While some of the general trends can be reproduced by models, there are features of the data that do not seem to be well described. This article discusses the experimental situation, the status of calculations and models, and the directions where these studies will progress in the near future.