The Proton Radius Puzzle

Abstract

The proton size, specifically its charge radius, was thought known to about 1% accuracy. Now a new method probing the proton with muons instead of electrons finds a radius about 4% smaller, and to boot gives an uncertainty limit of about 0.1%. We review the different measurements, some of the calculations that underlie them, some of the suggestions that have been made to resolve the conflict, and give a brief overview new related experimental initiatives. At present, however, the resolution to the problem remains unknown.

Accepted invited review for Progress in Particle and Nuclear Physics. 28 pages, 15 figures

References in corpus (9)

• High-precision determination of the electric and magnetic form factors of the proton
• The size of the proton - closing in on the radius puzzle
• The RMS Charge Radius of the Proton and Zemach Moments
• New Parity-Violating Muonic Forces
• Proton polarisability contribution to the Lamb shift in muonic hydrogen at fourth order in chiral perturbation theory
• Proton size anomaly
• The Electromagnetic Self-Energy Contribution to M_p - M_n and the Isovector Nucleon Magnetic Polarizability
• Laboratory tests of Lorentz and CPT symmetry with muons
• Proton structure corrections to electronic and muonic hydrogen hyperfine splitting

Cited by in corpus (28)

• Baryons as relativistic three-quark bound states
• Proton and neutron electromagnetic form factors and uncertainties
• Defining the Proton Radius: a Unified Treatment
• On the pi pi continuum in the nucleon form factors and the proton radius puzzle
• High-precision determination of the electric and magnetic radius of the proton
• Deuteron charge radius and Rydberg constant from spectroscopy data in atomic deuterium
• Not Normal: the uncertainties of scientific measurements
• Proton radius from electron-proton scattering and chiral perturbation theory
• Two-photon exchange contribution to elastic $e^{-}$-proton scattering: Full dispersive treatment of $\pi N$ states and comparison with data
• The proton radius (puzzle?) and its relatives
• Two-photon exchange correction to $2S$-$2P$ splitting in muonic helium-3 ions
• Two-photon exchange correction to the hyperfine splitting in muonic hydrogen
• Lepton-Proton Two-Photon Exchange in Chiral Perturbation Theory
• Point-Particle Effective Field Theory III: Relativistic Fermions and the Dirac Equation
• Spectroscopic tests for short-range modifications of Newtonian and post-Newtonian potentials
• Forward doubly-virtual Compton scattering off the nucleon in chiral perturbation theory: the subtraction function and moments of unpolarized structure functions
• High-Resolution Hydrogen Spectroscopy and the Proton Radius Puzzle
• Meson exchange in lepton-nucleon scattering and proton radius puzzle
• Restrained Dark $U(1{)}_d$ at Low Energies
• Interference between two resonant transitions with distinct initial and final states connected by radiative decay
• Leading-order QED radiative corrections to timelike Compton scattering on the proton
• A theory vade mecum for PSI experiments
• Reanalysis of low-energy electron-proton scattering data and proton radius
• On the cancellation of radiative corrections to the cross section of electron-proton scattering
• Light dark matter searches with positrons
• Connecting spatial moments and momentum densities
• Soft-photon radiative corrections to the $e^{-}p\to e^{-}p{l}^{-}{l}^{+}$ process
• Spin polarization of $\Lambda$ hyperons in $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$ at BESIII