High Precision Measurement of the Proton Elastic Form Factor Ratio $\mu_pG_E/G_M$ at low $Q^2$

TL;DR

This study provides a high-precision measurement of the proton elastic form factor ratio at low Q^2, revealing deviations from unity and refining the proton charge and magnetic radii with implications for proton structure understanding.

Contribution

The paper presents the first high-precision measurement of the form factor ratio at low Q^2, showing deviations from previous assumptions and refining proton radius values.

Findings

Form factor ratio deviates from unity at low Q^2.

Proton charge radius is 0.875±0.010 fm.

Proton magnetic radius is 0.867±0.020 fm.

Abstract

We report a new, high-precision measurement of the proton elastic form factor ratio \mu_p G_E/G_M for the four-momentum transfer squared Q^2 = 0.3-0.7 (GeV/c)^2. The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio \mu_p G_E/G_M from unity observed in previous polarization measurements at high Q^2 continues down to the lowest Q^2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q^2 range. We obtain new extractions of the proton electric and magnetic radii, which are <r^2_E>^(1/2)=0.875+/-0.010 fm and <r^2_M>^(1/2)=0.867+/-0.020 fm. The charge radius is consistent with other recent extractions based on the electron-proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.