A small proton charge radius from an electron-proton scattering experiment.

Area of Science:

• Atomic, Molecular, and Optical Physics
• Nuclear Physics
• Quantum Electrodynamics

Background:

• The proton charge radius (rₚ) is traditionally determined by electron-proton scattering and hydrogen spectroscopy.
• A discrepancy, known as the 'proton radius puzzle,' emerged in 2010 due to results from muonic hydrogen experiments.
• Recent spectroscopic measurements on ordinary hydrogen also show discrepancies, indicating unresolved issues.

Purpose of the Study:

• To precisely measure the proton charge radius using a novel electron-proton scattering method.
• To address the ongoing 'proton radius puzzle' by providing new experimental data.
• To investigate the discrepancy between different experimental and theoretical values of rₚ.

Main Methods:

• Employed a high-precision electron-proton scattering experiment at Jefferson Laboratory (PRad).
• Utilized a magnetic-spectrometer-free method to overcome limitations of previous experiments.
• Implemented a windowless hydrogen gas target for measurements at very small forward-scattering angles.

Main Results:

• Measured the proton charge radius as rₚ = 0.831 ± 0.007_stat ± 0.012_syst femtometers.
• This result is smaller than the most recent high-precision electron-proton scattering measurements.
• The measured value is 2.7 standard deviations smaller than the average of all previous electron-proton scattering results.

Conclusions:

• The new measurement supports the smaller proton charge radius values obtained from muonic hydrogen experiments.
• The findings align with the revised Rydberg constant, a fundamental constant in physics.
• This research contributes to resolving the long-standing 'proton radius puzzle'.