The Proton Radius Puzzle and Discrepancies in Proton Structure Measurements

TL;DR

The proton radius puzzle highlights discrepancies between different measurement methods, revealing limitations in current experimental and theoretical approaches, and necessitating coordinated efforts to resolve fundamental questions about proton structure.

Contribution

This paper reviews recent experimental and theoretical efforts to understand the proton radius discrepancy, emphasizing the need for improved methods and models to resolve the puzzle.

Findings

Recent measurements favor a smaller proton radius (~0.84 fm) consistent with muonic hydrogen.

Persistent discrepancies exist between electron-based and muon-based measurements.

Addressing systematic errors and developing new theoretical models are crucial for resolving the puzzle.

Abstract

The proton radius puzzle remains a key challenge in modern physics, highlighting both the precision and limitations of current experimental and theoretical approaches. Recent studies, such as those by Xiong et al. and Bezginov et al., have consistently found a smaller proton radius of about 0.84 femtometers, in line with muonic hydrogen measurements, but discrepancies with earlier electron-proton scattering and atomic hydrogen spectroscopy persist. These unresolved differences reveal systematic errors or limitations in existing theoretical frameworks, as pointed out by Arrington and Sick. The divergence between results from muonic and electronic hydrogen measurements remains unexplained, with contributions from both experimental uncertainties and theoretical gaps. To address these challenges, a coordinated approach is needed-focused on reducing systematic uncertainties through advanced experimental setups, like improved scattering experiments at Jefferson Lab, and developing new theoretical models, such as those proposed by Alar\'{c}on et al. and Lin et al. Integrating experimental precision with theoretical rigor offers the best path toward resolving this enduring puzzle and providing deeper insights into the fundamental forces of nature.