# Confinement in Nuclei and the Expanding Proton

## TL;DR

This paper investigates how the internal structure of protons is altered when they are bound within nuclei, using Poincare-invariant models and lattice QCD to show that bound protons are larger than free ones, impacting nuclear physics measurements.

## Contribution

The paper introduces simple, Poincare-invariant models respecting the Ward-Takahashi identity to demonstrate that medium modifications of proton structure are inevitable.

## Key findings

- Bound protons are larger than free protons.
- Medium modifications of proton structure are theoretically unavoidable.
- Models confirm the necessity of considering nucleon size changes in nuclear physics.

## Abstract

High-precision knowledge of electromagnetic form factors of nuclei is a subject of much current experimental and theoretical activity in nuclear and atomic physics. Such precision mandates that effects of the non-zero spatial extent of the constituent nucleons be handled in a manner that goes beyond the usual impulse approximation. A series of simple, Poincare-invariant, composite-proton models that respect the Ward-Takahashi identity and in which quarks are confined are used to study the validity of this approximation. The result of all of the models is a general theorem showing that medium modification of proton structure must occur. Combining this result with lattice QCD calculations leads to a conclusion that a bound proton must be larger than a free one.